Condensed Matter

New submissions

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New submissions for Fri, 27 Mar 20

[1]  arXiv:2003.11553 [pdf, other]
Title: Absolute anomalies in (2+1)D symmetry-enriched topological states and exact (3+1)D constructions
Comments: 39+18 pages, 20+13 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th); Quantum Algebra (math.QA); Quantum Physics (quant-ph)

Certain patterns of symmetry fractionalization in (2+1)D topologically ordered phases of matter can be anomalous, which means that they possess an obstruction to being realized in purely (2+1)D. In this paper we demonstrate how to compute the anomaly for symmetry-enriched topological (SET) states of bosons in complete generality. We demonstrate how, given any unitary modular tensor category (UMTC) and symmetry fractionalization class for a global symmetry group $G$, one can define a (3+1)D topologically invariant path integral in terms of a state sum for a $G$ symmetry-protected topological (SPT) state. We present an exactly solvable Hamiltonian for the system and demonstrate explicitly a (2+1)D $G$ symmetric surface termination that hosts deconfined anyon excitations described by the given UMTC and symmetry fractionalization class. We present concrete algorithms that can be used to compute anomaly indicators in general. Our approach applies to general symmetry groups, including anyon-permuting and anti-unitary symmetries. In addition to providing a general way to compute the anomaly, our result also shows, by explicit construction, that every symmetry fractionalization class for any UMTC can be realized at the surface of a (3+1)D SPT state. As a byproduct, this construction also provides a way of explicitly seeing how the algebraic data that defines symmetry fractionalization in general arises in the context of exactly solvable models. In the case of unitary orientation-preserving symmetries, our results can also be viewed as providing a method to compute the $\mathcal{H}^4(G, U(1))$ obstruction that arises in the theory of $G$-crossed braided tensor categories, for which no general method has been presented to date.

[2]  arXiv:2003.11559 [pdf, other]
Title: Excitonic fractional quantum Hall hierarchy in Moiré heterostructures
Comments: 5+4 pages, 1+3 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We consider fractional quantum Hall states in systems where two flat Chern number $C=\pm 1$ bands are labeled by an approximately conserved 'valley' index and interchanged by time reversal symmetry. At filling factor $\nu=1$ this setting admits an unusual hierarchy of correlated phases of excitons, neutral particle-hole pair excitations of a fully valley-polarized `orbital ferromagnet' parent state where all electrons occupy a single valley. Excitons experience an effective magnetic field due to the Chern numbers of the underlying bands. This obstructs their condensation in favor of a variety of crystalline orders and gapped and gapless liquid states. All these have the same quantized charge Hall response and are electrically incompressible, but differ in their edge structure, orbital magnetization, and hence valley and thermal responses. We explore the relevance of this scenario for Moir\'e heterostructures of bilayer graphene on a hexagonal boron nitride substrate.

[3]  arXiv:2003.11560 [pdf, other]
Title: Exciton band topology in spontaneous quantum anomalous Hall insulators: applications to twisted bilayer graphene
Comments: 5+11 pages, 2+2 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We uncover topological features of neutral particle-hole pair excitations of correlated quantum anomalous Hall (QAH) insulators whose approximately flat conduction and valence bands have equal and opposite non-zero Chern number. Using an exactly solvable model we show that the underlying band topology affects both the center-of-mass and relative motion of particle-hole bound states. This leads to the formation of topological exciton bands whose features are robust to nonuniformity of both the dispersion and the Berry curvature. We apply these ideas to recently-reported broken-symmetry spontaneous QAH insulators in substrate aligned magic-angle twisted bilayer graphene.

[4]  arXiv:2003.11564 [pdf, other]
Title: Environment assisted and environment hampered efficiency at maximum power in a molecular photo cell
Comments: submitted for publication, comments are welcome
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Other Condensed Matter (cond-mat.other); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)

The molecular photo cell, i.e., a single molecule donor-acceptor complex, beside being technologically important, is a paradigmatic example of a many-body system operating in strong non-equilibrium. The quantum transport and the photo-voltaic energy conversion efficiency of the photocell, attached to two external leads, are investigated within the open quantum system approach by solving the Lindblad master equation. The interplay of the vibrational degrees of freedom corresponding to the molecules (via the electron-phonon interaction) and the environment (via dephasing) shows its signature in the efficiency at maximum power. We find vibration assisted electron transport in the medium to strong electron-phonon coupling regime when the system does not suffer dephasing. Exposure to dephasing hampers such a vibration assisted electron transport in a specific range of dephasing rate.

[5]  arXiv:2003.11569 [pdf]
Title: Crystal and magnetic structure of antiferromagnetic Mn$_{2}$PtPd
Subjects: Materials Science (cond-mat.mtrl-sci)

We have investigated the crystal and magnetic structure of Mn${}_{2}$PtPd alloy using powder x-ray and neutron diffraction experiments. This compound is believed to belong to the Heusler family having crystal symmetry $\mathit{I}$4/$\mathit{mmm}$ (TiAl${}_{3}$-type). However, in this work we found that the Pd and Pt atoms are disordered and thus Mn${}_{2}$PtPd crystallizes in the $\mathit{L}$1${}_{0}$ structure having $\mathit{P}$4/$\mathit{mmm}$ symmetry (CuAu-I type) like MnPt and MnPd binary alloys. The lattice constants are $\mathit{a}$ = 2.86 \r{A} and $\mathit{c}$ = 3.62 \r{A} at room temperature. Mn${}_{2}$PtPd has a collinear antiferromagnetic spin structure below the N\'{e}el temperature $\mathit{T}$${}_{N}$ = 866 K, where Mn moments of $\mathrm{\sim}$4 $\mu$${}_{B}$ lie in the $\mathit{ab}$-plane. We observed a strong change in the lattice parameters near $\mathit{T}$${}_{N}$. The sample exhibits metallic behaviour, where electrical resistivity and carrier concentration are of the order of 10${}^{-5}$ $\Omega$ cm and 10${}^{21}$ cm${}^{-3}$, respectively.

[6]  arXiv:2003.11573 [pdf, other]
Title: Extracting non-Fermi liquid fermionic self-energy at $T=0$ from quantum Monte Carlo data
Comments: 9 pages, 8 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Statistical Mechanics (cond-mat.stat-mech); Superconductivity (cond-mat.supr-con)

Quantum Monte Carlo (QMC) simulations of correlated electron systems provide unbiased information about system behavior at a quantum critical point (QCP) and can verify or disprove the existing theories of non-Fermi liquid (NFL) behavior at a QCP. However, simulations are carried out at a finite temperature, where quantum-critical features are masked by finite temperature effects. Here we present a method to rigorously separate thermal and quantum effects and extract the information about NFL physics at $T=0$. We demonstrate our method for a specific example of 2D fermions near a Ising-ferromagnetic QCP. We show that one can accurately extract from QMC data the zero-temperature form of fermionic self-energy $\Sigma (\omega)$ even though the leading contribution to the self-energy comes from thermal effects. We find that the frequency dependence of $\Sigma (\omega)$ agrees well with the analytic form obtained within the Eliashberg theory of dynamical quantum criticality, and obeys $\omega^{2/3}$ scaling at low frequencies. Our results open up a new avenue for QMC studies of quantum-critical metals.

[7]  arXiv:2003.11581 [pdf, other]
Title: Magnon Trap by Chiral Spin Pumping
Comments: 6 pages, 3 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Chiral spin pumping is the generation of a unidirectional spin current in ferromagnetic films by dynamic dipolar stray fields from close-by nanomagnets. We formulate the theory of long-range chiral interactions between magnetic nanowires mediated by unidirectional spin waves in a magnetic film. We predict that two magnetic nanowires, of which one is actuated by microwaves, can trap spin waves. When both nanomagnets are excited by a uniform microwave, the interaction induced by the film magnons creates an imbalance in their magnon numbers.

[8]  arXiv:2003.11584 [pdf, ps, other]
Title: Experimental Determination of the Critical Spin Glass Correlation Length
Comments: 5 pages, 5 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Disordered Systems and Neural Networks (cond-mat.dis-nn)

Measuring ThermoRemanent Magnetization (TRM) decays on a single crystal CuMn(6$\%$) spin glass sample, we have systematically mapped the rapid decrease of the characteristic timescale $tw_{eff}$ near $T_g$. Using $tw_{eff}$ to determine the length scale of the growth of correlations during the waiting time, $\xi_{TRM}$, (observed in both numerical studies and experiment), we observe both growth of $\xi_{TRM}$ in the spin glass phase and then a rapid reduction very close to $T_g$. We interpret this reduction in $\xi_{TRM}$, for all waiting times, as being governed by the critical correlation length scale $\xi_{crit}=a(T-T_c)^{-\nu}$.

[9]  arXiv:2003.11600 [pdf, other]
Title: Birefringent Graphene Oxide Liquid Crystals in Micro-channels for Optical Switch
Comments: 5 pages, 4 figures
Journal-ref: ACS Applied Nano Materials, Articles ASAP (Letter), February 24, 2020
Subjects: Soft Condensed Matter (cond-mat.soft); Applied Physics (; Optics (physics.optics)

We propose a mechanical-hydrodynamical experimental setup in which the microfluidic motion manipulates the optical birefringence of levitated graphene oxide liquid crystal. The birefringence of the sample is changed by flowing graphene oxide liquid crystal in the micro-channel. By measuring the ordinary and extraordinary refractive indices at five flow rates, one can determine the value of the birefringence of the samples, precisely. Our results demonstrate that, by adjusting the concentration and flow rate of dispersion of the graphene oxide nano flakes, the induced birefringence can be controlled. It is also shown that this approach can be used as an optical switch.

[10]  arXiv:2003.11601 [pdf, other]
Title: Dualism of the 4f electrons and high-temperature antiferromagnetism of the heavy-fermion compound YbCoC$_{2}$
Comments: 25 pages, 8 figures, includes supplement
Journal-ref: Physical Review B 101, 100406(R) (2020)
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Materials Science (cond-mat.mtrl-sci)

We report on the first study of the noncentrosymmetric ternary carbide YbCoC$_{2}$. Our magnetization, specific heat, resistivity and neutron diffraction measurements consistently show that the system behaves as a heavy-fermion compound, displaying an amplitude-modulated magnetic structure below the N\'eel temperature reaching $T_{N}$ = 33 K under pressure. Such a large value, being the highest among the Yb-based systems, is explained in the light of our ab initio calculations, which show that the 4f electronic states of Yb have a dual nature -- i.e., due to their strong hybridization with the 3d states of Co, 4f states expose both localized and itinerant properties.

[11]  arXiv:2003.11610 [pdf, other]
Title: Detecting chiral pairing and topological superfluidity using circular dichroism
Comments: 7 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)

Realising and probing topological superfluids is a key goal for fundamental science, with exciting technological promises. Here, we show that chiral px + ipy pairing in a two-dimensional topological superfluid can be detected through circular dichroism, namely, as a difference in the excitation rates induced by a clockwise and counter-clockwise circular drive. For weak pairing, this difference is to a very good approximation determined by the Chern number of the superfluid, whereas there is a non-topological contribution scaling as the superfluid gap squared that becomes significant for stronger pairing. This gives rise to a competition between the experimentally driven goal to maximise the critical temperature of the superfluid, and observing a signal given by the underlying topology. Using a combination of strong coupling Eliashberg and Berezinskii-Kosterlitz-Thouless theory, we analyse this tension for an atomic Bose-Fermi gas, which represents a promising platform for realising a chiral superfluid. We identify a wide range of system parameters where both the critical temperature is high and the topological contribution to the dichroic signal is dominant.

[12]  arXiv:2003.11621 [pdf, other]
Title: How Circular Dichroism in time- and angle-resolved photoemission can be used to spectroscopically detect transient topological states in graphene
Comments: 16 pages, 8 figures
Subjects: Materials Science (cond-mat.mtrl-sci); Strongly Correlated Electrons (cond-mat.str-el)

Pumping graphene with circularly polarized light is the archetype of light-tailoring topological bands. Realizing the induced Floquet-Chern insulator state and tracing clear experimental manifestions has been a challenge, and it has become clear that scattering effects play a crucial role. We tackle this gap between theory and experiment by employing microscopic quantum kinetic calculations including realistic electron-electron and electron-phonon scattering. Our theory provides a direct link to the build-up of the Floquet-Chern insulator state in light-driven graphene and its detection in time- and angle-resolved photoemission spectroscopy (ARPES). This allows us to study the stability of the Floquet features due to dephasing and thermalization effects. We also discuss the ultrafast Hall response in the laser-heated state. Furthermore, the induced pseudospin texture and the associated Berry curvature gives rise to momentum-dependent orbital magnetization, which is reflected in circular dichroism in ARPES (CD-ARPES). Combining our nonequilibrium calculations with an accurate one-step theory of photoemission allows us to establish a direct link between the build-up of the topological state and the dichroic pump-probe photoemission signal. The characteristic features in CD-ARPES are further corroborated to be stable against heating and dephasing effects. Thus, tracing circular dichroism in time-resolve photoemission provides new insights into transient topological properties.

[13]  arXiv:2003.11629 [pdf, other]
Title: Predicting plasticity in disordered solids from structural indicators
Subjects: Soft Condensed Matter (cond-mat.soft)

Amorphous solids lack long-range order. Therefore identifying structural defects --- akin to dislocations in crystalline solids --- that carry plastic flow in these systems remains a daunting challenge. By comparing many different structural indicators in computational models of glasses, under a variety of conditions we carefully assess which of these indicators are able to robustly identify the structural defects responsible for plastic flow in amorphous solids. We further demonstrate that the density of defects changes as a function of material preparation and strain in a manner that is highly correlated with the macroscopic material response. Our work represents an important step towards predicting how and when an amorphous solid will fail from its microscopic structure.

[14]  arXiv:2003.11633 [pdf]
Title: Universal Gelation of Metal Oxide Nanocrystals via Depletion Attractions
Subjects: Soft Condensed Matter (cond-mat.soft)

Nanocrystal gelation provides a powerful framework to translate nanoscale properties into bulk materials and to engineer emergent properties through the assembled microstructure. However, many established gelation strategies rely on chemical reactions and specific interactions, e.g., stabilizing ligands or ions on the surface of the nanocrystals, and are therefore not easily transferrable. Here, we report a general gelation strategy via non-specific and purely entropic depletion attractions applied to three types of metal oxide nanocrystals. The gelation thresholds of two compositionally distinct spherical nanocrystals agree quantitatively, demonstrating the adaptability of the approach for different chemistries. Consistent with theoretical phase behavior predictions, nanocrystal cubes form gels at a lower polymer concentration than nanocrystal spheres, allowing shape to serve as a handle to control gelation. These results suggest that the fundamental underpinnings of depletion-driven assembly, traditionally associated with larger colloidal particles, are also applicable at the nanoscale.

[15]  arXiv:2003.11649 [pdf, other]
Title: Nonreciprocity of spin waves in noncollinear magnets due to the Dzyaloshinskii-Moriya interaction
Comments: 12 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Broken inversion symmetry in combination with the spin-orbit interaction generates a finite Dzyaloshinskii-Moriya interaction (DMI), which can induce noncollinear spin textures of chiral nature. The DMI is characterized by an interaction vector whose magnitude, direction and symmetries are crucial to determine the stability of various spin textures, such as skyrmions and spin spirals. The DMI can be measured from the nonreciprocity of spin waves in ferromagnets, which can be probed via inelastic scattering experiments. In a ferromagnet, the DMI can modify the spin-wave dispersion, moving its minimum away from the $\Gamma$ point. Spin waves propagating with opposite wavevectors are then characterized by different group velocities, energies and lifetimes, defining their nonreciprocity. Here, we address the case of complex spin textures, where the manifestation of DMI-induced chiral asymmetries remains to be explored. We discuss such nonreciprocal effects and propose ways of accessing the magnitude and direction of the DMI vectors in the context of spin-polarized or spin-resolved inelastic scattering experiments. We show that only when a periodic magnetic system has finite net magnetization, that is, when the vector sum of all magnetic moments is nonzero, can it present a total nonreciprocal spin-wave spectrum. However, even zero-net-magnetization systems, such as collinear antiferromagnets and cycloidal spin spirals, can have spin-wave modes that are individually nonreciprocal, while the total spectrum remains reciprocal.

[16]  arXiv:2003.11659 [pdf, other]
Title: Hartree-Fock-Bogoliubov theory of trapped one-dimensional imbalanced Fermi systems
Comments: 11 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)

Ground state Hartree-Fock-Bogoliubov (HFB) theory is applied to imbalanced spin-1/2 one-dimensional Fermi systems that are spatially confined by either a harmonic or a hard-wall trapping potential. It has been hoped that such systems, which can be realized using ultracold atomic gases, would exhibit the long-sought-after Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid phase. The HFB formalism generalizes the standard Bogoliubov quasi-particle transformation, by allowing for Cooper pairing to exist between all possible single-particle states, and accounts for the effects of the inhomogeneous trapping potential as well as the mean-field Hartree potential. This provides an unbiased framework to describe inhomgenous densities and pairing correlations in the FFLO state of a confined 1D gas. In a harmonic trap, numerical minimization of the HFB ground state energy yields a spatially oscillating order parameter reminiscent of the FFLO state. However, we find that this state has almost no imprint in the local fermion densities (consistent with experiments that found no evidence of the FFLO phase). In contrast, for a hard-wall geometry, we find a strong signature of the spatial oscillations of the FFLO pairing amplitude reflected in the local in situ densities. In the hard wall case, the excess spins are strongly localized near regions where there is a node in the pairing amplitude, creating an unmistakeable crystalline modulation of the density.

[17]  arXiv:2003.11669 [pdf, other]
Title: Measuring total density correlations in a Fermi-Hubbard gas via bilayer microscopy
Comments: 8 pages, 8 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We report on the single atom and single site-resolved detection of the total density in a cold atom realization of the 2D Fermi-Hubbard model. Fluorescence imaging of doublons is achieved by splitting each lattice site into a double well, thereby separating atom pairs. Full density readout yields a direct measurement of the equation of state, including direct thermometry via the fluctuation-dissipation theorem. Site-resolved density correlations reveal the Pauli hole at low filling, and strong doublon-hole correlations near half filling. These are shown to account for the difference between local and non-local density fluctuations in the Mott insulator. Our technique enables the study of atom-resolved charge transport in the Fermi-Hubbard model, the site-resolved observation of molecules, and the creation of bilayer Fermi-Hubbard systems.

[18]  arXiv:2003.11678 [pdf, other]
Title: Magnetic-field-induced FM-AFM metamagnetic transition and strong negative magnetoresistance in Mn$_{1/4}$NbS$_2$ under pressure
Subjects: Materials Science (cond-mat.mtrl-sci)

Transition metal dichalcogenides (TMDC) stand out with their high chemical stability and the possibility to incorporate a wide range of magnetic species between the layers. The behavior of conduction electrons in such materials intercalated by 3d-elements is closely related to their magnetic properties and can be sensitively controlled by external magnetic fields. Here, we study the magnetotransport properties of NbS$_2$ intercalated with Mn, Mn$_{1/4}$NbS$_2$, demonstrating a complex behavior of the magnetoresistance and of the ordinary and anomalous Hall resistivities. Application of pressure as tuning parameter leads to the drastic changes of the magnetotransport properties of Mn$_{1/4}$NbS$_2$ exhibiting large negative magnetoresistance up to $65 \%$ at 7.1 GPa. First-principles electronic structure calculations indicates pressure-induced transition from ferromagnetic to antiferromagnetic state. Theoretical calculations accounting for the finite temperature magnetic properties of Mn$_{1/4}$NbS$_2$ suggest a field-induced metamagnetic ferromagnetic-antiferromagnetic transition as an origin of the large negative magentoresistance. These results inspire the development of materials for spintronic applications based on intercalated TMDC with a well controllable metamagnetic transition.

[19]  arXiv:2003.11680 [pdf, other]
Title: Phase transition for parameter learning of Hidden Markov Models
Comments: 9 pages, 9 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Biological Physics (; Computational Physics (physics.comp-ph); Data Analysis, Statistics and Probability (

We study a phase transition in parameter learning of Hidden Markov Models (HMMs). We do this by generating sequences of observed symbols from given discrete HMMs with uniformly distributed transition probabilities and a noise level encoded in the output probabilities. By using the Baum-Welch (BW) algorithm, an Expectation-Maximization algorithm from the field of Machine Learning, we then try to estimate the parameters of each investigated realization of an HMM. We study HMMs with n=4, 8 and 16 states. By changing the amount of accessible learning data and the noise level, we observe a phase-transition-like change in the performance of the learning algorithm. For bigger HMMs and more learning data, the learning behavior improves tremendously below a certain threshold in the noise strength. For a noise level above the threshold, learning is not possible. Furthermore, we use an overlap parameter applied to the results of a maximum-a-posteriori (Viterbi) algorithm to investigate the accuracy of the hidden state estimation around the phase transition.

[20]  arXiv:2003.11697 [pdf, other]
Title: T-square resistivity without Umklapp scattering in dilute metallic Bi$_2$O$_2$Se
Comments: 7 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

The electrical resistivity of Fermi liquids (FLs) displays a quadratic temperature ($T$) dependence because of electron-electron (e-e) scattering. For such collisions to decay the charge current, there are two known mechanisms: inter-band scattering (identified by Baber) and Umklapp events. However, dilute metallic strontium titanate (STO) was found to display $T^2$ resistivity in absence of either of these two mechanisms. The presence of soft phonons and their possible role as scattering centers raised the suspicion that $T$-square resistivity in STO is not due to e-e scattering. Here, we present the case of Bi$_2$O$_2$Se, a layered semiconductor with hard phonons, which becomes a dilute metal with a small single-component Fermi surface upon doping. It displays $T$-square resistivity well below the degeneracy temperature where neither Umklapp nor interband scattering is conceivable. We observe a universal scaling between the prefactor of $T^2$ resistivity and the Fermi energy, which is an extension of the Kadowaki-Woods plot to dilute metals. Our results imply the absence of a satisfactory theoretical basis for the ubiquity of e-e driven $T$-square resistivity in Fermi liquids.

[21]  arXiv:2003.11710 [pdf, other]
Title: Fermi polaron revisited: polaron-molecule transition and coexistence
Authors: Xiaoling Cui
Comments: 5 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We revisit the polaron-molecule transition in ultracold Fermi gases using the well-established variational approach. It is found that the molecule is in fact an asymptotic limit of a finite-momentum polaron in the strong coupling regime, which can be continuously connected to the weak coupling polaronic state in the same momentum sector. The polaron-molecule transition can therefore be reinterpreted as a first-order transition between Fermi polarons with different momenta. Within certain interaction window near their transition, both states appear as local minima in the dispersion curve, indicating they can coexist in a realistic system. We have further confirmed the polaron-molecule coexistence in the presence of a finite impurity concentration and at low temperature, which well explains the recent experimental observations of smooth polaron-molecule transition in a three-dimensional ultracold Fermi gas. Our results have provided an unambiguous physical picture for the competition and conversion between polaron and molecule, and also shed light on Fermi polaron properties in low dimensions.

[22]  arXiv:2003.11731 [pdf]
Title: Giant linear non-reciprocal charge transport in one-dimensional channels with a magnetic proximity effect
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci)

In non-centrosymmetric conducting channels, the electrical resistance $R$ differs when the current flows in forward ($R^+$) and backward ($R^-$) directions under an external magnetic field $B$, which results from breaking of both the spatial inversion symmetry (SIS) and time reversal symmetry (TRS). Thus far, this nonreciprocal charge transport has been observed only in the non-linear transport regime by using second harmonic measurements, where the resistance difference $R^+ - R^-$ is proportional to both the current intensity $I$ and magnetic field $B$, and these $R^+ - R^-$ values were minuscule, ~0.01$\%$ of the total resistance. Here, we report a new non-reciprocal charge transport phenomenon observed in the linear response regime, which is independent of $I$, in chiral one-dimensional (1D) edge channels of an InAs thin film interfaced with a ferromagnetic semiconductor (FMS) (Ga,Fe)Sb layer.By switching the $B$ direction, the resistance change between $\pm$ 10 T of the 1D channels is as large as 27$\%$ and is clearly detected by DC transport measurements. Our theoretical analysis using Boltzmann's equation suggests that the relativistic spin-orbit interaction (SOI) in the 1D channels and the spin-dependent scattering at the InAs/(Ga,Fe)Sb interface are the origins of this phenomenon. These findings suggest that utilizing the magnetic proximity effect (MPE) is a revolutionary way to study non-reciprocal electrical transport, which is directly related to fundamental issues in condensed matter physics such as symmetries.

[23]  arXiv:2003.11742 [pdf, ps, other]
Title: Possible occurrence of superconductivity by the $π$-flux Dirac string formation due to spin-twisting itinerant motion of electrons
Authors: Hiroyasu Koizumi
Subjects: Superconductivity (cond-mat.supr-con)

We show that the Rashba spin-orbit interaction causes spin-twisting itinerant motion of electrons in metals and realizes the quantized cyclotron motion of conduction electrons without an external magnetic field. From the view point of the Berry connection, the cause of this {quantized} motion is the appearance of a non-trivial Berry connection ${\bf A}^{\rm fic}=-{\hbar \over {2e}}\nabla \chi$ ($\chi$ is an angular variable with period $2\pi$ that generates $\pi$ flux (in the units of $\hbar=1, e=1,c=1$) inside the nodal singularities of the wave function (a "Dirac string") along the centers of spin-twisting.
Since it has been shown in our previous work[Ref.1]that the collective mode of $\nabla \chi$ is stabilized by the electron-pairing and generates supercurrent, the $\pi$-flux Dirac string created by the spin-twisting itinerant motion will be stabilized by the electron-pairing and produce supercurrent.

[24]  arXiv:2003.11752 [pdf, other]
Title: Topological superconductor from superconducting topological surface states and fault-tolerant quantum computing
Comments: 6 pages, 5 figures
Subjects: Superconductivity (cond-mat.supr-con); Quantum Physics (quant-ph)

The chiral $p$-wave superconductor/superfluid in two dimensions (2D) is the simplest and most robust system for topological quantum computation . Candidates for such topological superconductors/superfluids in nature are very rare. A widely believed chiral $p$-wave superfluid is the Moore-Read state in the $\nu=\frac{5}2$ fractional quantum Hall effect, although experimental evidence are not yet conclusive. Experimental realizations of chiral $p$-wave superconductors using quantum anomalous Hall insulator-superconductor hybrid structures have been controversial. Here we report a new mechanism for realizing 2D chiral $p$-wave superconductors on the surface of 3D $s$-wave superconductors that have a topological band structure and support superconducting topological surface states (SC-TSS), such as the iron-based superconductor Fe(Te,Se). We find that tunneling and pairing between the SC-TSS on the top and bottom surfaces in a thin film or between two opposing surfaces of two such superconductors can produce an emergent 2D time-reversal symmetry breaking chiral topological superconductor. The topologically protected anyonic vortices with Majorana zero modes as well as the chiral Majorana fermion edge modes can be used as a platform for more advantageous non-abelian braiding operations. We propose a novel device for the CNOT gate with six chiral Majorana fermion edge modes, which paves the way for fault-tolerant universal quantum computing.

[25]  arXiv:2003.11762 [pdf]
Title: Moire supercells through SrTiO$_3$ nanolayer relaxation
Comments: 21 pages, 4 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el)

The interface of complex oxide heterostructures sets the stage for various electronic and magnetic phenomena. Many of these collective effects originate from the precise structural arrangement at the interface that in turn governs local spin- and charge interactions. Currently, interfacial straining, so the naturally evolving compressive or tensile strain by mismatch of the neighboring lattices, is the most common route towards engineering collective material properties -- yet, significant progress might require exploration of entirely new approaches towards interface correlations. In this work, we turn the page by looking at the interface of a perfectly relaxed, unstrained heterostructure, where we identify a highly ordered Moire lattice at an inherently disordered SrTiO$_3$ (STO) - LSAT interface. Using high-resolution reciprocal space mapping via synchrotron based X-Ray diffraction, we find long-ranged ordered supercells of 106/107 unit cells of STO/LSAT, caused by lattice relaxation through high-temperature annealing. Model calculations confirm the experimentally observed scattering phenomena, showing that cross-interfacial bonding is locally different at the Moire-overlap points. Notably, the presence of such super-ordered structures in the family of 2D electron gas systems sets the ideal conditions for Moire-motif tuned plasmonic responses and ferroelectric super-crystallinity and opens up the possibility to novel interface functionalities in these simple perovskites.

[26]  arXiv:2003.11765 [pdf, ps, other]
Title: Fingerprints of spin-current physics on magnetoelectric response in the spin-$1/2$ magnet Ba$_2$CuGe$_2$O$_7$
Comments: 15 pages, 8 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Materials Science (cond-mat.mtrl-sci)

The single-site anisotropy vanishes for the spin-1/2 as a consequence of Kramers degeneracy. We argue that similar property holds for the magnetically induced electric polarization P, which should depend only on the relative orientation of spins in the bonds but not on the direction of each individual spin. Thus, for insulating multiferroic compounds, P can be decomposed in terms of pairwise isotropic, antisymmetric, and anisotropic contributions, which can be rigorously derived in the framework of the superexchange (SE) theory, in an analogy with the spin Hamiltonian. The SE theory also allows us to identify the microscopic mechanism, which stands behind each contribution. The most controversial and intriguing one is antisymmetric or spin-current mechanism. In this work, we propose that the disputed magnetoelectric (ME) properties of Ba2CuGe2O7 can be explained solely by the spin-current mechanism, while other contributions are either small or forbidden by symmetry. First we explicitly show how the cycloidal spin order induces the experimentally observed P in the direction perpendicular to the xy plane, which can be naturally explained by the spin-current mechanism operating in the out-of-plane bonds. Then, we unveil previously overlooked ME effect, where the application of the magnetic field perpendicular to the plane not only causes the incommensurate-commensurate transition, but also flips P into the plane due to the spin-current mechanism operating in the neighboring bonds within this plane. In both cases, the magnitude and direction of P can be controlled by rotating the spin pattern in the xy plane. Our analysis is based on a realistic spin model, which was rigorously derived from the first-principles calculations and supplemented with the new algorithm for the construction of localized Wannier functions obeying the crystallographic symmetry of Ba2CuGe2O7.

[27]  arXiv:2003.11767 [pdf, other]
Title: Study of classical and quantum phase transitions on non-Euclidean geometries in higher dimensions
Comments: accepted in Acta Physica Slovaca (a review based on Michal Daniska's PhD thesis)
Subjects: Statistical Mechanics (cond-mat.stat-mech)

The investigation of the behavior of both classical and quantum systems on non-Euclidean surfaces near the phase transition point represents an interesting research area of modern physics. In the case of classical spin systems, a generalization of the Corner Transfer Matrix Renormalization Group algorithm has been developed and successfully applied to spin models on infinitely many regular hyperbolic lattices. In this work, we extend these studies to specific types of lattices. It is important to say that no suitable algorithms for numerical analysis of ground-states of quantum systems in similar conditions have been implemented yet. In this work, we offer a particular solution by proposing a variational numerical algorithm Tensor Product Variational Formulation, which assumes a quantum ground-state written in the form of a low-dimensional uniform tensor product state. We apply the Tensor Product Variational Formulation to three typical quantum models on a variety of regular hyperbolic lattices. The main outcomes are the following: (1) We propose an algorithm for calculation and classification of the thermodynamic properties of the Ising model on triangular-tiled hyperbolic lattices. In addition, we investigate the origin of the mean-field universality on a series of weakly curved lattices. (2) We develop the Tensor Product Variational Formulation algorithm for the numerical analysis of the ground-state of the quantum systems on the hyperbolic lattices. (3) We study quantum phase transition phenomena for the three selected spin models on various types of the hyperbolic lattices including the Bethe lattice.

[28]  arXiv:2003.11800 [pdf]
Title: Distinguishing strain, charge and molecular orbital induced effects on the electronic structure: graphene/ammonia system
Subjects: Materials Science (cond-mat.mtrl-sci)

Molecular adsorption at the surface of a 2D material poses numerous questions regarding the modification to the band structure and interfacial states, which of course deserve full attention. In line with this, first-principle density functional theory is employed on the graphene/ammonia system. We identify the effects on the band structure due to strain, charge transfer and presence of molecular orbitals (MOs) of NH3 for six adsorption configurations. The induced strain upon ammonia-adsorption opens the bandgap (Eg) of graphene due to the breaking of translational symmetry and shifts the equilibrium Fermi energy (EF). The Eg and EF values and charge density distribution are dependent on the adsorption configuration, where the MO structure of NH3 plays a crucial role. The presence of MOs of N or H -originated pushes the unoccupied states of graphene towards EF. NH3 forms an interfacial occupied state originating from N2p below the EF within 1.6 - 2.2 eV for all configurations. These findings enhance the fundamental understanding of the graphene/NH3 system.

[29]  arXiv:2003.11804 [pdf, other]
Title: Active Learning Approach to Optimization of Experimental Control
Subjects: Quantum Gases (cond-mat.quant-gas); Machine Learning (cs.LG); Quantum Physics (quant-ph)

In this work we present a general machine learning based scheme to optimize experimental control. The method utilizes the neural network to learn the relation between the control parameters and the control goal, with which the optimal control parameters can be obtained. The main challenge of this approach is that the labeled data obtained from experiments are not abundant. The central idea of our scheme is to use the active learning to overcome this difficulty. As a demonstration example, we apply our method to control evaporative cooling experiments in cold atoms. We have first tested our method with simulated data and then applied our method to real experiments. We demonstrate that our method can successfully reach the best performance within hundreds of experimental runs. Our method does not require knowledge of the experimental system as a prior and is universal for experimental control in different systems.

[30]  arXiv:2003.11805 [pdf, other]
Title: Chiral Domain Wall Injector Driven by Spin-orbit Torques
Journal-ref: Nano Lett. 2019, 19, 9, 5930-5937
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci)

Memory and logic devices that encode information in magnetic domains rely on the controlled injection of domain walls to reach their full potential. In this work, we exploit the chiral coupling induced by the Dzyaloshinskii-Moriya interaction between in-plane and out-of-plane magnetized regions of a Pt/Co/AlO\textsubscript{x} trilayer in combination with current-driven spin-orbit torques to control the injection of domain walls into magnetic conduits. We demonstrate that the current-induced domain nucleation is strongly inhibited for magnetic configurations stabilized by the chiral coupling and promoted for those that have the opposite chirality. These configurations allow for efficient domain wall injection using current densities of the order of $4\times$\SI{e11}{A m^{-2}}, which are lower than those used in other injection schemes. Furthermore, by setting the orientation of the in-plane magnetization using an external field, we demonstrate the use of a chiral domain wall injector to create a controlled sequence of alternating domains in a racetrack structure driven by a steady stream of unipolar current pulses.

[31]  arXiv:2003.11809 [pdf, other]
Title: Theory of absorption lineshape in monolayers of transition metal dichalcogenides
Comments: to be published in Physical Review B
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

The linear absorption spectra in monolayers of transition metal dichalcogenides show pronounced signatures of the exceptionally strong exciton-phonon interaction in these materials. To account for both exciton and phonon physics in such optical signals, we compare different theoretical methods to calculate the absorption spectra using the example of $\mathrm{MoSe_2}$. In this paper, we derive the equations of motion for the polarization either using a correlation expansion up to 4th Born approximation or a time convolutionless master equation. We show that the Born approximation might become problematic when not treated in high enough order, especially at high temperatures. In contrast, the time convolutionless formulation gives surprisingly good results despite its simplicity when compared to higher-order corrrelation expansion and therefore provides a powerful tool to calculate the lineshape of linear absorption spectra in the very popular monolayer materials.

[32]  arXiv:2003.11825 [pdf, other]
Title: Multiparticle collision dynamics for fluid interfaces with near-contact interactions
Subjects: Soft Condensed Matter (cond-mat.soft)

We present an extension of the multiparticle collision dynamics method for flows with complex interfaces, including supramolecular near-contact interactions mimicking the effect of surfactants. The new method is demonstrated for the case of (i) short range repulsion of droplets in close contact, (ii) arrested phase separation and (iii) different pattern formation during spinodal decomposition of binary mixtures.

[33]  arXiv:2003.11829 [pdf, other]
Title: Experimental realization of spin-tensor momentum coupling in ultracold Fermi gases
Comments: 6 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We experimentally realize the spin-tensor momentum coupling (STMC) using the three ground Zeeman states coupled by three Raman laser beams in ultracold atomic system of $^{40}$K Fermi atoms. This new type of STMC consists of two bright-state bands as a regular spin-orbit coupled spin-1/2 system and one dark-state middle band. Using radio-frequency spin-injection spectroscopy, we investigate the energy band of STMC. It is demonstrated that the middle state is a dark state in the STMC system. The realized energy band of STMC may open the door for further exploring exotic quantum matters.

[34]  arXiv:2003.11833 [pdf, other]
Title: Quantum corrections to the classical field approximation for 1D quantum many-body systems in equilibrium
Comments: 16 pages, 6 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas)

We present a semiclassical treatment of one dimensional many-body quantum systems in equilibrium, where quantum corrections to the classical field approximation are systematically included by a renormalization of the classical field parameters. Our semiclassical approximation is reliable in the limit of weak interactions and high temperatures. As a specific example, we apply our method to the interacting Bose gas and study experimentally observable quantities, such as correlation functions of bosonic fields and the full counting statistics of the number of particles in an interval. Where possible, our method is checked against exact results derived from integrability, showing excellent agreement.

[35]  arXiv:2003.11841 [pdf]
Title: A QM/MD coupling method to model the ion-induced polarization of graphene
Comments: 19 pages, 6 figures
Subjects: Materials Science (cond-mat.mtrl-sci)

We report a new Quantum Mechanical/Molecular Dynamics (QM/MD) simulation loop to model the coupling between the electron and atom dynamics in solid/liquid interfacial systems. The method can describe simultaneously both the quantum mechanical surface polarizability emerging from the proximity to the electrolyte, and the electrolyte structure and dynamics. In the current set up Density Functional Tight Binding calculations for the electronic structure calculations of the surface are coupled with classical molecular dynamics to simulate the electrolyte solution. The reduced computational cost of the QM part makes the coupling with a classical simulation engine computationally feasible and allows simulation of large systems for hundreds of nanoseconds. We tested the method by simulating a non-charged graphene flake immersed in an NaCl electrolyte solution at varying concentrations. We found that ions preferentially remained in solution and only cations are mildly attracted to the surface of the graphene. This behaviour is found to originate from the relatively small adsorption energy compared to the value of the ion hydration energy and rules out any possible ions/surface charge transfer.

[36]  arXiv:2003.11847 [pdf, other]
Title: The Many-Body localization transition in the Hilbert space
Authors: Marco Tarzia
Comments: 15 pages, 10 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Disordered Systems and Neural Networks (cond-mat.dis-nn)

In this paper we propose a new perspective to analyze the many-body localization (MBL) transition when recast in terms of a single-particle tight-binding model in the space of many-body configurations. We compute the distribution of tunneling rates between many-body states separated by an extensive number of spin flips at the lowest order in perturbation theory starting from the insulator, and determine the scaling of their typical amplitude with the number of accessible states in the Hilbert space. By using an analogy with the Rosenzweig-Porter random matrix ensemble, we propose an ergodicity breaking criterion for the MBL transition based on the Fermi Golden Rule. According to this criterion, in the MBL phase many resonances are formed at large distance from an infinite temperature initial state, but they are not enough for the quantum dynamics to decorrelate from it in a finite time. This implies that, differently from Anderson localized states, in the insulating phase many-body eigenstates are multifractal in the Hilbert space, as they occupy a large but subexponential part of the total volume, in agreement with recent numerical results, perturbative calculations, and intuitive arguments. Possible limitations and implications of our interpretation are discussed in the conclusions.

[37]  arXiv:2003.11849 [pdf]
Title: Electron-phonon coupling and a resonant-like optical observation of a band inversion in topological crystalline insulator Pb$_{1-x}$Sn$_x$Se
Comments: 12 pages, 5 figures
Subjects: Materials Science (cond-mat.mtrl-sci)

The optical reflectivity of Pb$_{0.865}$Sn$_{0.135}$Se and Pb$_{0.75}$Sn$_{0.25}$Se solid solutions was measured in the THz spectral region energetically corresponding to bulk optical phonon excitations and in the temperature range from 40 K to 280 K. The analysis of Pb$_{0.75}$Sn$_{0.25}$Se data performed within the dynamic dielectric function formalism revealed a new effect due to the electron-phonon coupling resulting in resonant changes of LO phonon frequency for energy gap equal to zero or to LO phonon energy. This effect is absent for Pb$_{0.865}$Sn$_{0.135}$Se that exhibits an open energy gap with trivial band ordering at all temperatures. These results show that reflectivity in the THz range constitute a versatile experimental method for precise determination of band inversion in narrow-gap topological materials. For Pb$_{0.75}$Sn$_{0.25}$Se the transition from trivial insulator to topological crystalline insulator phase takes place at temperature T$_0$ = (172 $\pm$ 2) K.

[38]  arXiv:2003.11866 [pdf, other]
Title: Griffiths-like phase close to the Mott transition
Comments: 5 pages, 3 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Materials Science (cond-mat.mtrl-sci); Other Condensed Matter (cond-mat.other)

We explore the coexistence region in the vicinity of the Mott critical end point employing a coupled energy-volume spin-$1/2$ Ising-like model. We analyze the case for the spin-liquid candidate $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$, where close to the Mott critical end point metallic puddles coexist with an insulating ferroelectric phase. Our results are fourfold: $i$) a universal divergent-like behavior of the Gr\"uneisen parameter upon crossing the spinodal line; $ii$) based on scaling arguments, we show that in the coexistence region, for $any$ system close to the critical point, the relaxation time is entropy-dependent; $iii$) we propose the electric Gr\"uneisen parameter $\Gamma_E$, which quantifies the electrocaloric effect; $iv$) we identify the metallic/insulating coexistence region as a Griffiths phase. Our findings suggest that $\Gamma_E$ governs the dielectric response close to the critical point and that a Griffiths-like phase emerges in the coexistence region of any system.

[39]  arXiv:2003.11873 [pdf, other]
Title: Unveiling the Physics of the Mutual Interactions in Paramagnets
Comments: 21 pages, 8 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Materials Science (cond-mat.mtrl-sci); Other Condensed Matter (cond-mat.other); Statistical Mechanics (cond-mat.stat-mech)

In real paramagnets, there is always a subtle many-body contribution to the system's energy, which can be regarded as a small effective local magnetic field $B_{loc}$. Usually, it is neglected, since it is very small when compared with thermal fluctuations and/or external magnetic fields $B$. Nevertheless, as both the temperature $T \rightarrow$ 0K and $B \rightarrow$ 0T, such many-body contributions become ubiquitous. Here, employing the magnetic Gr\"uneisen parameter $\Gamma_{mag}$ and entropy arguments, we report on the pivotal role played by the mutual interactions in the regime of ultra-low-$T$ and vanishing $B$. Our key results are: $i$) absence of a genuine zero-field quantum phase transition due to the presence of $B_{loc}$; $ii$) connection between the canonical definition of temperature and $\Gamma_{mag}$; and $iii$) possibility of performing adiabatic magnetization by only manipulating the mutual interactions. Our findings unveil unprecedented aspects emerging from the mutual interactions.

[40]  arXiv:2003.11876 [pdf, other]
Title: Emergence of a nematic paramagnet via quantum order-by-disorder and pseudo-Goldstone modes in Kitaev magnets
Comments: 14 pages, 9 figure
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

The appearance of nontrivial phases in Kitaev materials exposed to an external magnetic field has recently been a subject of intensive studies. Here, we elucidate the relation between the field-induced ground states of the classical and quantum spin models proposed for such materials, by using the infinite density matrix renormalization group (iDMRG) and the linear spin wave theory (LSWT). We consider the $K \Gamma \Gamma'$ model, where $\Gamma$ and $\Gamma'$ are off-diagonal spin exchanges on top of the dominant Kitaev interaction $K$. We show that a nematic paramagnet, which breaks the lattice-rotational symmetry, emerges in the quantum model. This phenomenon can be understood as the effect of quantum order-by-disorder in the frustrated ferromagnet discovered in the corresponding classical model. In addition, various classical ordering patterns with large unit cells are replaced by the nematic paramagnet in the quantum model. We compute the dynamical spin structure factors using a matrix operator based time evolution and compare them with the predictions from LSWT. We point out the existence of a pseudo-Goldstone mode, which results from the lifting of a continuous degeneracy via quantum fluctuations. We also discuss these results in the light of inelastic neutron scattering experiments.

[41]  arXiv:2003.11879 [pdf, other]
Title: Pressure and electric field dependence of quasicrystalline electronic states in 30$^{\circ}$ twisted bilayer graphene
Comments: 7 pages, 8 figures, 1 table
Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

30$^{\circ}$ twisted bilayer graphene demonstrates the quasicrystalline electronic states with 12-fold symmetry. These states are however far away from the Fermi level, which makes conventional Dirac fermion behavior dominating the low energy spectrum in this system. By using tight-binding approximation, we study the effect of external pressure and electric field on the quasicrystalline electronic states. Our results show that by applying the pressure perpendicular to graphene plane one can push the quasicrystalline electronic states towards the Fermi level. Then, the electron or hole doping of the order of $\sim$ $4\times10^{14}$ $cm^{-2}$ is sufficient for the coincidence of the Fermi level with these quasicrystalline states. Moreover, our study indicates that applying the electric field perpendicular to the graphene plane can destroy the 12-fold symmetry of these states and break the energy degeneracy of the 12-wave states, and it is easier to reach this in the conduction band than in the valence band. Importantly, the application of the pressure can recover the 12-fold symmetry of these states to some extent against the electric field. We propose a hybridization picture which can explain all these phenomena.

[42]  arXiv:2003.11898 [pdf, other]
Title: A simple approach to bulk bioinspired tough ceramics
Comments: 7 pages, 4 figures
Subjects: Materials Science (cond-mat.mtrl-sci); Applied Physics (

The development of damage-resistant structural materials that can withstand harsh environments is a major issue in materials science and engineering. Bioinspired brick-and-mortar designs have recently demonstrated a range of interesting mechanical properties in proof-of-concept studies. However, reproducibility and scalability issues associated with the actual processing routes have impeded further developments and industrialization of such materials. Here we demonstrate a simple approach based on uniaxial pressing and field assisted sintering of commercially available raw materials to process bioinspired ceramic/ceramic composites of larger thickness than previous approaches, with a sample thickness up to 1 cm. The ceramic composite retains the strength typical of dense alumina ($430~\pm 30MPa$) while keeping the excellent damage resistance demonstrated previously at the millimeter scale with a crack initiation toughness of $6.6MPa.m^{1/2}$ and fracture toughness up to $17.6 MPa.m^{1/2}$. These results validate the potential of these all-ceramic composites, previously demonstrated at lab scale only, and could enable their optimization, scale-up, and industrialization.

[43]  arXiv:2003.11905 [pdf]
Title: Fermi liquid behavior and colossal magnetoresistance in layered MoOCl2
Subjects: Materials Science (cond-mat.mtrl-sci)

A characteristic of a Fermi liquid is the T^2 dependence of its resistivity, sometimes referred to as the Baber law. However, for most metals, this behavior is only restricted to very low temperatures, usually below 20 K. Here, we experimentally demonstrate that for the single-crystal van der Waals layered material MoOCl2, the Baber law holds in a wide temperature range up to ~120 K, indicating that the electron-electron scattering plays a dominant role in this material. Combining with the specific heat measurement, we find that the modified Kadowaki-Woods ratio of the material agrees well with many other strongly correlated metals. Furthermore, in the magneto-transport measurement, a colossal magneto-resistance is observed, which reaches ~350% at 9 T and displays no sign of saturation. With the help of first-principles calculations, we attribute this behavior to the presence of open orbits on the Fermi surface. We also suggest that the dominance of electron-electron scattering is related to an incipient charge density wave state of the material. Our results establish MoOCl2 as a strongly correlated metal and shed light on the underlying physical mechanism, which may open a new path for exploring the effects of electron-electron interaction in van der Waals layered structures.

[44]  arXiv:2003.11935 [pdf, ps, other]
Title: Full counting statistics and fluctuation-dissipation relation for periodically driven systems
Comments: 11 pages, 7 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We derive the fluctuation theorem for a stochastic and periodically driven system coupled to two reservoirs with the aid of a master equation. We write down the cumulant generating functions for both the current and entropy production in closed compact forms so as to treat the adiabatic and nonadiabatic contributions systematically. We derive the fluctuation theorem by taking into account the property that the instantaneous currents flowing into the left and the right reservoir are not equal. It is found that the fluctuation-dissipation relation derived from the fluctuation theorem involves an expansion with respect to the time derivative of the affinity in addition to the standard contribution.

[45]  arXiv:2003.11957 [pdf]
Title: Large Nernst Power Factor over a Broad Temperature Range in Polycrystalline Weyl Semimetal NbP
Journal-ref: Energy Environ. Sci. 11 (2018) 2813-2820
Subjects: Materials Science (cond-mat.mtrl-sci)

The discovery of topological materials has provided new opportunities to exploit advanced materials for heat-to-electricity energy conversion as they share many common characteristics with thermoelectric materials. In this work, we report the magneto-thermoelectric properties and Nernst effect of the topological Weyl semimetal NbP. We find that polycrystalline, bulk NbP shows a significantly larger Nernst thermopower than its conventional thermopower under magnetic field. As a result, a maximum Nernst power factor of ~ 35*10-4 Wm-1K-2 is achieved at 9 T and 136 K, which is 4 times higher than its conventional power factor and is also comparable to that of state-of-the-art thermoelectrics. Moreover, the Nernst power factor maintains relatively large value over a broad temperature range. These results highlight that the enhancement of thermoelectric performance can be achieved in topological semimetals based on the Nernst effect and transverse transport.

[46]  arXiv:2003.11966 [pdf, other]
Title: Van der Waals materials for energy-efficient spin-orbit torque devices
Comments: Comments are welcome
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci)

Spin-orbit torques (SOTs), which rely on spin current generation from charge current in a nonmagnetic material, promise an energy-efficient scheme for manipulating magnetization in magnetic devices. A critical topic for spintronic devices using SOTs is to enhance the charge to spin conversion efficiency. Besides, the current-induced spin polarization is usually limited to in-plane, whereas out of plane spin polarization could be favored for efficient perpendicular magnetization switching. Recent advances in utilizing two important classes of van der Waals materials$-$topological insulators and transition-metal dichalcogenides$-$as spin source to generate SOT shed light on addressing these challenges. Topological insulators such as bismuth selenide have shown a giant SOT efficiency, which is larger than those from three-dimensional heavy metals by at least one order of magnitude. Transition-metal dichalcogenides such as tungsten telluride have shown a current-induced out of plane spin polarization, which is allowed by the reduced symmetry. In this review, we use symmetry arguments to predict and analyze SOTs in van der Waal material-based heterostructures. We summarize the recent progress of SOT studies based on topological insulators and transition-metal dichalcogenides and show how these results are in line with the symmetry arguments. At last, we identify unsolved issues in the current studies and suggest three potential research directions in this field.

[47]  arXiv:2003.11968 [pdf, other]
Title: Efficient Calculation of Excitonic Effects in Solids Including Approximated Quasiparticle Energies
Subjects: Materials Science (cond-mat.mtrl-sci)

In this work we present a new procedure to compute optical spectra including excitonic effects and approximated quasiparticle corrections with reduced computational effort. The excitonic effects on optical spectra are included by solving the Bethe-Salpeter equation, considering quasiparticle eigenenergies and respective wavefunctions obtained within DFT-1/2 method. The electron-hole ladder diagrams are approximated by the screened exchange. To prove the capability of the procedure, we compare the calculated imaginary part of the dielectric functions of Si, Ge, GaAs, GaP, GaSb, InAs, InP, and InSb with experimental data. The energy position of the absorption peaks are correctly described. The good agreement with experimental results together with the very significant reduction of computational effort makes the procedure suitable on the investigation of optical spectra of more complex systems.

[48]  arXiv:2003.11977 [pdf, other]
Title: High frequency mechanical excitation of a silicon nanostring with piezoelectric aluminum nitride layers
Subjects: Applied Physics (; Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

A strong trend for quantum based technologies and applications follows the avenue of combining different platforms to exploit their complementary technological and functional advantages. Micro and nano-mechanical devices are particularly suitable for hybrid integration due to the easiness of fabrication at multi-scales and their pervasive coupling with electrons and photons. Here, we report on a nanomechanical technological platform where a silicon chip is combined with an aluminum nitride layer. Exploiting the AlN piezoelectricity, Surface Acoustic Waves are injected in the Si layer where the material has been localy patterned and etched to form a suspended nanostring. Characterizing the nanostring vertical displacement induced by the SAW, we found an external excitation peak efficiency in excess of 500 pm/V at 1 GHz mechanical frequency. Exploiting the long term expertise in silicon photonic and electronic devices as well as the SAW robustness and versatility, our technological platform represents a strong candidate for hybrid quantum systems.

[49]  arXiv:2003.12024 [pdf, other]
Title: Sub-THz momentum drag and violation of Matthiessen's rule in an ultraclean ferromagnetic SrRuO$_3$ metallic thin film
Comments: main text 5 pages, 3 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Materials Science (cond-mat.mtrl-sci)

SrRuO$_3$, a ferromagnet with an approximately 160\,K Curie temperature, exhibits a $T^2$ dependent dc resistivity below $\approx$ 30 K. Nevertheless, previous optical studies in the infrared and terahertz range show non-Drude dynamics at low temperatures which seem to contradict a Fermi-liquid picture with long-lived quasiparticles. In this work, we measure the low-frequency THz range response of thin films with residual resistivity ratios, $\rho_{300K}/ \rho_{4K} \approx$ 74. Such low disorder samples allow an unprecedented look at the effects of electron-electron interactions on low-frequency transport. At temperatures below 30 K, we found both a very sharp zero-frequency mode which has a width narrower than $k_BT/\hbar$ as well as a broader zero frequency Lorentzian that has at least an order of magnitude larger scattering rate. Both features have temperature dependencies consistent with a Fermi-liquid with the wider feature explicitly showing a T$^2$ scaling. Such two -Drude transport sheds light on previous reports of the violation of Mathielssen's rule and extreme sensitivity to disorder in metallic ruthenates. We consider a number of possibilities for the origin of the two feature optical conductivity including multiband effects that arise from momentum conserving interband scattering and the approximate conservation of a pseudo-momentum that arises from quasi-1D Fermi surfaces.

[50]  arXiv:2003.12026 [pdf]
Title: Diversity of anisotropy effects in the breakup of metallic FCC nanowires into ordered nanodroplet chains
Comments: 18 pages, 6 figures
Journal-ref: CrystEngComm, March 2020
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We have analyzed the expressed manifestation of the anisotropy of surface energy density in the dynamics of ultrathin nanowires, which break up into disjointed clusters when annealed below their melting temperature. The breakup process is studied for different temperatures and orientations of the nanowire axis relative to its internal crystal structure using the Monte Carlo kinetic method. We have also presented an approximate analytical model of the instability of nanowires. Generally, the interpretation of experimental results refers to the theoretical model developed by Nichols and Mullins, which is based on conceptions about the Rayleigh instability of liquid jets. In both cases, the theories - which do not take into account the anisotropy of surface energy density - predict the breakup of a nanowire/liquid jet with radius r into fragments with an average length {\Lambda}=9r. However, the observed value, {\Lambda}/r, often deviates from 9 either to lower values or to substantially greater ones (up to 24-30). Our results explain various observed features of the breakup and the significant variations in the values of its parameter {\Lambda}/r depending on experimental conditions. In particular, the ambiguous role of exchange by atoms of the surface of a nanowire with the surrounding layer of free atoms formed as a result of their rather intensive sublimation, which occurs in a number of cases, has been investigated. We have shown that this exchange can lead both to a decrease, and to a significant increase, in the parameter {\Lambda}/r. The obtained results could be potentially useful in applications such as the development of optical waveguides based on ordered nanoparticles chains.

[51]  arXiv:2003.12036 [pdf, other]
Title: AnDi: The Anomalous Diffusion Challenge
Comments: 6 pages, 2 figure - AnDi Challenge webpage:
Subjects: Statistical Mechanics (cond-mat.stat-mech); Computational Physics (physics.comp-ph); Data Analysis, Statistics and Probability (; Quantitative Methods (q-bio.QM)

The deviation from pure Brownian motion generally referred to as anomalous diffusion has received large attention in the scientific literature to describe many physical scenarios. Several methods, based on classical statistics and machine learning approaches, have been developed to characterize anomalous diffusion from experimental data, which are usually acquired as particle trajectories. With the aim to assess and compare the available methods to characterize anomalous diffusion, we have organized the Anomalous Diffusion (AnDi) Challenge (\url{}). Specifically, the AnDi Challenge will address three different aspects of anomalous diffusion characterization, namely: (i) Inference of the anomalous diffusion exponent. (ii) Identification of the underlying diffusion model. (iii) Segmentation of trajectories. Each problem includes sub-tasks for different number of dimensions (1D, 2D and 3D). In order to compare the various methods, we have developed a dedicated open-source framework for the simulation of the anomalous diffusion trajectories that are used for the training and test datasets. The challenge was launched on March 1, 2020, and consists of three phases. Currently, the participation to the first phase is open. Submissions will be automatically evaluated and the performance of the top-scoring methods will be thoroughly analyzed and compared in an upcoming article.

[52]  arXiv:2003.12054 [pdf, ps, other]
Title: Thermodynamic properties of the one-dimensional Ising model with magnetoelastic interaction
Subjects: Statistical Mechanics (cond-mat.stat-mech); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

The Ising one-dimensional (1D) chain with spin $S=1/2$ is studied with the lattice contribution included in the form of elastic interaction and thermal vibrations simultaneously taken into account. The magnetic energy term and the elastic (static) energy term based on the Morse potential are calculated exactly. The vibrational energy is calculated in the Debye approximation, in which the anharmonicity is introduced by the Gr{\"u}neisen parameter. The total Gibbs potential, including both the magnetic field, as well as the external force term, is constructed and from its minimum the equation of state is derived.
From the Gibbs energy all the thermodynamic properties are calculated in a self-consistent manner. The comprehensive numerical calculations are performed in a full temperature range, i.e., from zero temperature up to the vicinity of melting. In particular, a role of magneto-elastic coupling is emphasized and examined. The numerical results are illustrated in figures and discussed.

Cross-lists for Fri, 27 Mar 20

[53]  arXiv:2003.11550 (cross-list from hep-th) [pdf, ps, other]
Title: Anomaly inflow and $p$-form gauge theories
Comments: 123 pages
Subjects: High Energy Physics - Theory (hep-th); Strongly Correlated Electrons (cond-mat.str-el)

Chiral and non-chiral $p$-form gauge fields have gravitational anomalies and anomalies of Green-Schwarz type. This means that they are most naturally realized as the boundary modes of bulk topological phases in one higher dimensions. We give a systematic description of the total bulk-boundary system which is analogous to the realization of a chiral fermion on the boundary of a massive fermion. The anomaly of the boundary theory is given by the partition function of the bulk theory, which we explicitly compute in terms of the Atiyah-Patodi-Singer $\eta$-invariant. We use our formalism to determine the $\mathrm{SL}(2,{\mathbb Z})$ anomaly of the 4d Maxwell theory. We also apply it to study the worldvolume theories of a single D-brane and an M5-brane in the presence of orientifolds, orbifolds, and S-folds in string, M, and F theories. In an appendix we also describe a simple class of non-unitary invertible topological theories whose partition function is not a bordism invariant, illustrating the necessity of the unitarity condition in the cobordism classification of the invertible phases.

[54]  arXiv:2003.11556 (cross-list from quant-ph) [pdf, other]
Title: Thermodynamics of ultrastrongly coupled light-matter systems
Comments: 15 pages, 7 figures
Subjects: Quantum Physics (quant-ph); Other Condensed Matter (cond-mat.other)

We study the thermodynamic properties of a system of two-level dipoles that are coupled ultrastrongly to a single cavity mode. By using exact numerical and approximate analytical methods, we evaluate the free energy of this system at arbitrary interaction strengths and discuss strong-coupling modifications of derivative quantities such as the specific heat or the electric susceptibility. From this analysis we identify the lowest-order cavity-induced corrections to those quantities in the collective ultrastrong coupling regime and show that for even stronger interactions the presence of a single cavity mode can strongly modify extensive thermodynamic quantities of a large ensemble of dipoles. In this non-perturbative coupling regime we also observe a significant shift of the ferroelectric phase transition temperature and a characteristic broadening and collapse of the black-body spectrum of the cavity mode. Apart from a purely fundamental interest, these general insights will be important for identifying potential applications of ultrastrong-coupling effects, for example, in the field of quantum chemistry or for realizing quantum thermal machines.

[55]  arXiv:2003.11665 (cross-list from [pdf]
Title: Accurate Determination of Semiconductor Diffusion Coefficient Using Confocal Microscopy
Comments: Main Text: 12 pages and 5 figures. Supporting Information: 7 pages and 6 figures
Subjects: Applied Physics (; Materials Science (cond-mat.mtrl-sci)

Confocal microscopy is a versatile tool capable of directly monitoring photoexcited carrier transport and recombination in thin film and single crystal samples. The diffusion coefficient, an important material property for designing efficient optoelectronic devices, is often determined by fitting the evolution of the photoexcited carrier population to a simplified Gaussian function. Although this model has gained widespread adoption, its application to different material systems and its sensitivity to various experimental conditions has not been explored. Here, we simulate the diffusive processes in metal halide perovskites and find that the diffusion coefficient can be inaccurately fit when higher-order processes, such as bimolecular and Auger recombination, dominate. Significant fitting error (> 5%) is introduced if the initial photoexcited carrier density exceeds 1x10$^1$$^7$ cm$^-$$^3$ and if the material diffusion coefficient is less than ~ 1 cm$^2$ s$^-$$^1$, both conditions commonly encountered in confocal microscopy measurements of perovskites. In addition, we find that grain size and grain boundaries present in polycrystalline thin films impact the carrier density temporal profiles, introducing more error in the diffusion coefficient fits. This analysis highlights important considerations in the interpretation of confocal microscopy data and provides critical steps towards the development of more robust diffusion models.

[56]  arXiv:2003.11745 (cross-list from hep-th) [pdf, other]
Title: Quantum quench, large N, and symmetry restoration
Comments: 13 pages + appendices
Subjects: High Energy Physics - Theory (hep-th); Statistical Mechanics (cond-mat.stat-mech)

We globally quench the theory of two dimensional massless fermions (many flavours) with quartic interactions by making the quartic coupling a smooth function of time. Working in a derivative expansion we show that the discrete Z2 symmetry in case of the Gross-Neveu model, and the U(1) symmetry in case of the Nambu-Jona-Lasinio model, are restored during the zero-temperature quench. For the Gross-Neveu model we show that this can be understood as an effective thermalization. The time of symmetry restoration shows scaling with the quench rate. We identify this with the Kibble-Zurek scaling in the problem. In a suitable double scaling limit, the symmetry restoration may be understood in terms of Liouville quantum mechanics.

[57]  arXiv:2003.11776 (cross-list from [pdf, other]
Title: Bipolar spin Hall nano-oscillators
Subjects: Applied Physics (; Materials Science (cond-mat.mtrl-sci)

We demonstrate a novel type of spin Hall nano-oscillator (SHNO) that allows for efficient tuning of magnetic auto-oscillations over an extended range of gigahertz frequencies, using bipolar direct currents at constant magnetic fields. This is achieved by stacking two distinct ferromagnetic layers with a platinum interlayer. In this device, the orientation of the spin polarised electrons accumulated at the top and bottom interfaces of the platinum layer is switched upon changing the polarity of the direct current. As a result, the effective anti-damping required to drive large amplitude auto-oscillations can appear either at the top or bottom magnetic layer. Tuning of the auto-oscillation frequencies by several gigahertz can be obtained by combining two materials with sufficiently different saturation magnetization. Here we show that the combination of NiFe and CoFeB can result in 3 GHz shifts in the auto-oscillation frequencies. Bipolar SHNOs as such may bring enhanced synchronisation capabilities to neuromorphic computing applications.

[58]  arXiv:2003.11845 (cross-list from quant-ph) [pdf, other]
Title: Going beyond Local and Global approaches for localized thermal dissipation
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Identifying which master equation is preferable for the description of a multipartite open quantum system is not trivial and has led in the recent years to the local vs. global debate in the context of Markovian dissipation. We treat here a paradigmatic scenario in which the system is composed of two interacting harmonic oscillators A and B, with only A interacting with a thermal bath - collection of other harmonic oscillators - and we study the equilibration process of the system initially in the ground state with the bath finite temperature. We show that the completely positive version of the Redfield equation obtained using coarse-grain and an appropriate time-dependent convex mixture of the local and global solutions give rise to the most accurate semigroup approximations of the whole exact system dynamics, i.e. both at short and at long time scales, outperforming the local and global approaches.

[59]  arXiv:2003.11850 (cross-list from physics.optics) [pdf, other]
Title: The critical role of shell in enhanced fluorescence of metal-dielectric core-shell nanoparticles
Subjects: Optics (physics.optics); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Applied Physics (; Chemical Physics (physics.chem-ph)

Large scale simulations are performed by means of the transfer-matrix method to reveal optimal conditions for metal-dielectric core-shell particles to induce the largest fluorescence on their surfaces. With commonly used plasmonic cores (Au and Ag) and dielectric shells (SiO2, Al2O3, ZnO), optimal core and shell radii are determined to reach maximum fluorescence enhancement for each wavelength within 550~850 nm (Au core) and 390~500 nm (Ag core) bands, in both air and aqueous hosts. The peak value of the maximum achievable fluorescence enhancement factors of core-shell nanoparticles, taken over entire wavelength interval, increases with the shell refractive index and can reach values up to 9 and 70 for Au and Ag cores, within 600~700 nm and 400~450 nm wavelength ranges, respectively, which is much larger than that for corresponding homogeneous metal nanoparticles. Replacing air by an aqueous host has a dramatic effect of nearly halving the sizes of optimal core-shell configurations at the peak value of the maximum achievable fluorescence. In the case of Au cores,the fluorescence enhancements for wavelengths within the first near-infrared biological window (NIR-I) between 700 and 900 nm can be improved twofold compared to homogeneous Au particle when the shell refractive index ns > 2. As a rule of thumb, the wavelength region of optimal fluorescence (maximal nonradiative decay) turns out to be red-shifted (blue-shifted) by as much as 50 nm relative to the localized surface plasmon resonance wavelength of corresponding optimized core-shell particle. Our results provide important design rules and general guidelines for enabling versatile platforms for imaging, light source, and biological applications.

[60]  arXiv:2003.11860 (cross-list from q-bio.PE) [pdf, other]
Title: Attacking the Covid-19 with the Ising-model and the Fermi-Dirac Distribution Function
Comments: 15 pages, 2 figures, 1 table
Subjects: Populations and Evolution (q-bio.PE); Statistical Mechanics (cond-mat.stat-mech); Physics and Society (physics.soc-ph)

We employ a spin $S$ = 1/2 Ising-like model and a Fermi-Dirac-like function to describe the spread of Covid-19. Our analysis, using the available official infections rate data reveals: $i$) that the epidemic curves, i.e., the number of reported cases $versus$ time, is well-described by a Gaussian function; $ii$) that the temporal evolution of the cumulative number of infected people follows a distorted Fermi-Dirac-like distribution function; $iii$) the key role played by the quarantine in the prevention of the spread of Covid-19 in terms of an $interacting$ parameter, which emulates the contact between infected and non-infected people. An analysis of the epidemic curves for Ebola, SARS, and Influenza A/H1N1 is also presented and described by a Gaussian function as well. Our findings demonstrate the universal character of well-established concepts in condensed matter Physics and their applications in different areas.

[61]  arXiv:2003.11874 (cross-list from cond-mat.mtrl-sci) [pdf, ps, other]
Title: Electron spin relaxations of phosphorus donors in bulk silicon under large electric field
Comments: 7 pages, 5 figures
Journal-ref: Scientific Reports 9:2951 (2019)
Subjects: Materials Science (cond-mat.mtrl-sci); Applied Physics (

Modulation of donor electron wavefunction via electric fields is vital to quantum computing architectures based on donor spins in silicon. For practical and scalable applications, the donor-based qubits must retain sufficiently long coherence times in any realistic experimental conditions. Here, we present pulsed electron spin resonance studies on the longitudinal $(T_1)$ and transverse $(T_2)$ relaxation times of phosphorus donors in bulk silicon with various electric field strengths up to near avalanche breakdown in high magnetic fields of about 1.2 T and low temperatures of about 8 K. We find that the $T_1$ relaxation time is significantly reduced under large electric fields due to electric current, and $T_2$ is affected as the $T_1$ process can dominate decoherence. Furthermore, we show that the magnetoresistance effect in silicon can be exploited as a means to combat the reduction in the coherence times. While qubit coherence times must be much longer than quantum gate times, electrically accelerated $T_1$ can be found useful when qubit state initialization relies on thermal equilibration.

[62]  arXiv:2003.11887 (cross-list from quant-ph) [pdf, other]
Title: Probabilistic Hysteresis in an Isolated Quantum System: The Microscopic Onset of Irreversibility from a Quantum Perspective
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)

Recently probabilistic hysteresis in isolated Hamiltonian systems of ultracold atoms has been studied in the limit of large particle numbers, where a semiclassical treatment is adequate. The origin of irreversibility in these sweep experiments, where a control parameter is slowly (adiabatically) tuned back and forth, turned out to be a passage blue back and forth across a separatrix (integrable case) or a passage in and out of a chaotic sea in phase space (chaotic case). Here we focus on the full quantum mechanical description of the integrable system and show how the semiclassical results emerge in the limit of large particle numbers. Instead of the crossing of a separatrix in phase space, where classical adiabaticity fails, the origin of irreversibility in the quantum system is a series of avoided crossings of the adiabatic energy levels: they become so close that already for modest particle numbers the change of the external parameter has to be unrealistically slow to reach the quantum adiabatic limit of perfectly reversible evolution. For a slow but finite sweep rate we find a broad regime where the quantum results agree with the semiclassical results, but only if besides the limit $N\to \infty$ an initial ensemble of states is considered, with sufficient initial energy width. For a single initial energy eigenstate we find in contrast that the backward sweep reveals strong quantum effects even for very large particle numbers.

[63]  arXiv:2003.11897 (cross-list from [pdf]
Title: Optical and electronic properties of colloidal CdSe Quantum Rings
Comments: 38 pages, 10 figures
Subjects: Applied Physics (; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci); Chemical Physics (physics.chem-ph); Quantum Physics (quant-ph)

Luminescent colloidal CdSe nanorings are a new type of semiconductor structure that have attracted interest due to the potential for unique physics arising from their non-trivial toroidal shape. However, the exciton properties and dynamics of these materials with complex topology are not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and single particle measurements to study these materials. We find that on transformation of CdSe nanoplatelets to nanorings, by perforating the center of platelets, the emission lifetime decreases and the emission spectrum broadens due to ensemble variations in the ring size and thickness. The reduced PL quantum yield of nanorings (~10%) compared to platelets (~30%) is attributed to an enhanced coupling between: (i) excitons and CdSe LO-phonons at 200 cm-1 and (ii) negatively charged selenium-rich traps which give nanorings a high surface charge (~-50 mV). Population of these weakly emissive trap sites dominates the emission properties with an increased trap emission at low temperatures relative to excitonic emission. Our results provide a detailed picture of the nature of excitons in nanorings and the influence of phonons and surface charge in explaining the broad shape of the PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that the excitonic properties of nanorings are not solely a consequence of the toroidal shape but are also a result of traps introduced by puncturing the platelet center.

[64]  arXiv:2003.11925 (cross-list from quant-ph) [pdf, ps, other]
Title: Probabilistic magnetometry with two-spin system in diamond
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Solid-state magnetometers like the Nitrogen-Vacancy center in diamond have been of paramount importance for the development of quantum sensing with nanoscale spatial resolution. The basic protocol is a Ramsey sequence, that imprints an external static magnetic field into phase of the quantum sensor, which is subsequently readout. In this work we show that the hyperfine coupling between the Nitrogen-Vacancy and a nearby Carbon-13 can be used to set a post-selection protocol that leads to an enhancement of the sensitivity under realistic experimental conditions. We found that for an isotopically purified sample the detection of weak magnetic fields in the $\mu$T range can be achieved with a sensitivity of few nTHz$^{-1/2}$ at cryogenic temperature ($4$ K), and $0.1$ $\mu$THz$^{-1/2}$ at room temperature.

[65]  arXiv:2003.11933 (cross-list from physics.comp-ph) [pdf, other]
Title: Spin textures in chiral magnetic monolayers with suppressed nearest-neighbor exchange
Subjects: Computational Physics (physics.comp-ph); Strongly Correlated Electrons (cond-mat.str-el)

High tunability of two dimensional magnetic materials (by strain, gating, heterostructuring or otherwise) provides unique conditions for studying versatile magnetic properties and controlling emergent magnetic phases. Expanding the scope of achievable magnetic phenomena in such materials is important for both fundamental and technological advances. Here we perform atomistic spin-dynamics simulations to explore the (chiral) magnetic phases of atomic monolayers in the limit of suppressed first-neighbors exchange interaction. We report the rich phase diagram of exotic magnetic configurations, obtained for both square and honeycomb lattice symmetries, comprising coexistence of ferromagnetic and antiferromagnetic spin-cycloids, as well as multiple types of magnetic skyrmions. We perform a minimum-energy path analysis for the skyrmion collapse to evaluate the stability of such topological objects, and reveal that magnetic monolayers could be good candidates to host the antiferromagnetic skyrmions that are experimentally evasive to date.

[66]  arXiv:2003.11996 (cross-list from cs.NE) [pdf, other]
Title: Accelerated Analog Neuromorphic Computing
Subjects: Neural and Evolutionary Computing (cs.NE); Disordered Systems and Neural Networks (cond-mat.dis-nn); Neurons and Cognition (q-bio.NC)

This paper presents the concepts behind the BrainScales (BSS) accelerated analog neuromorphic computing architecture. It describes the second-generation BrainScales-2 (BSS-2) version and its most recent in-silico realization, the HICANN-X Application Specific Integrated Circuit (ASIC), as it has been developed as part of the neuromorphic computing activities within the European Human Brain Project (HBP). While the first generation is implemented in an 180nm process, the second generation uses 65nm technology. This allows the integration of a digital plasticity processing unit, a highly-parallel micro processor specially built for the computational needs of learning in an accelerated analog neuromorphic systems. The presented architecture is based upon a continuous-time, analog, physical model implementation of neurons and synapses, resembling an analog neuromorphic accelerator attached to build-in digital compute cores. While the analog part emulates the spike-based dynamics of the neural network in continuous-time, the latter simulates biological processes happening on a slower time-scale, like structural and parameter changes. Compared to biological time-scales, the emulation is highly accelerated, i.e. all time-constants are several orders of magnitude smaller than in biology. Programmable ion channel emulation and inter-compartmental conductances allow the modeling of nonlinear dendrites, back-propagating action-potentials as well as NMDA and Calcium plateau potentials. To extend the usability of the analog accelerator, it also supports vector-matrix multiplication. Thereby, BSS-2 supports inference of deep convolutional networks as well as local-learning with complex ensembles of spiking neurons within the same substrate.

[67]  arXiv:2003.12051 (cross-list from physics.atom-ph) [pdf, ps, other]
Title: Accurate ab initio properties of the NaLi molecule in the $a^3Σ^+$ electronic state
Comments: 7 pages, 2 figures, 2 tables
Subjects: Atomic Physics (physics.atom-ph); Quantum Gases (cond-mat.quant-gas); Chemical Physics (physics.chem-ph); Quantum Physics (quant-ph)

We calculate the electronic and rovibrational structure of the 14-electron ${}^{23}$Na${}^6$Li molecule in the $a^3\Sigma^+$ state with spectroscopic accuracy ($<0.5\,$cm$^{-1}$) using state-of-the-art ab initio methods of quantum chemistry. We show that the inclusion of higher-level excitations, core-electron correlation, relativistic, QED, and adiabatic corrections is necessary to reproduce accurately scattering and spectroscopic properties of alkali-metal systems. We obtain the well depth, $D_e=229.9(5)\,$cm$^{-1}$, the dissociation energy, $D_0=208.2(5)\,$cm$^{-1}$, and the scattering length, $a_s=-84^{+25}_{-41}\,$bohr, in good agreement with recent experimental measurements. These values are obtained without any adjustment to experimental data, showing that quantum chemistry methods are capable of predicting scattering properties of many-electron systems, provided relatively weak interaction and small reduced mass of the system.

[68]  arXiv:2003.12055 (cross-list from q-bio.PE) [pdf, other]
Title: Age-structured impact of social distancing on the COVID-19 epidemic in India
Comments: Code and updates at 9 pages, 5 figures, and 2 tables
Subjects: Populations and Evolution (q-bio.PE); Statistical Mechanics (cond-mat.stat-mech)

The outbreak of the novel coronavirus, COVID-19, has been declared a pandemic by the WHO. The structures of social contact critically determine the spread of the infection and, in the absence of vaccines, the control of these structures through large-scale social distancing measures appears to be the most effective means of mitigation. Here we use an age-structured SIR model with social contact matrices obtained from surveys and Bayesian imputation to study the progress of the COVID-19 epidemic in India. The basic reproductive ratio R0 and its time-dependent generalization are computed based on case data, age distribution and social contact structure. The impact of social distancing measures - workplace non-attendance, school closure, lockdown - and their efficacy with durations are then investigated. A three-week lockdown is found insufficient to prevent a resurgence and, instead, protocols of sustained lockdown with periodic relaxation are suggested. Forecasts are provided for the reduction in age-structured morbidity and mortality as a result of these measures. Our study underlines the importance of age and social contact structures in assessing the country-specific impact of mitigatory social distancing.

Replacements for Fri, 27 Mar 20

[69]  arXiv:1703.07358 (replaced) [pdf, other]
Title: High-density quantum sensing with dissipative first order transitions
Comments: 6+3 pages, 6+3 figures
Journal-ref: Phys. Rev. Lett. 120, 150501 (2018)
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Statistical Mechanics (cond-mat.stat-mech)
[70]  arXiv:1810.02446 (replaced) [pdf, other]
Title: Physical Origin of the One-Quarter Exact Exchange in Density Functional Theory
Authors: Marco Bernardi
Subjects: Materials Science (cond-mat.mtrl-sci); Chemical Physics (physics.chem-ph)
[71]  arXiv:1811.00555 (replaced) [pdf, other]
Title: Floquet Higher-Order Topological Insulators with Anomalous Dynamical Polarization
Comments: 4+2+18 pages, major update for a new anomalous dynamical polarization theory
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas)
[72]  arXiv:1901.02019 (replaced) [pdf, other]
Title: Initialization of quantum simulators by sympathetic cooling
Comments: 8+4 pages, 6+5 figures
Journal-ref: Sci. Adv. 6, eaaw9268 (2020)
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Atomic Physics (physics.atom-ph)
[73]  arXiv:1902.07759 (replaced) [pdf, other]
Title: Generalized wave-packet model for studying coherence of matter-wave interferometers
Authors: Yonathan Japha
Comments: 24 pages, 8 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
[74]  arXiv:1905.02173 (replaced) [pdf, other]
Title: Assisted concentration of Gaussian resources
Comments: 21 pages, 3 figures. In v2 we changed the title and added the new Figures 1 and 2, illustrating the one-way and two-way Gaussian collaboration protocols, respectively
Subjects: Quantum Physics (quant-ph); Other Condensed Matter (cond-mat.other); Mathematical Physics (math-ph)
[75]  arXiv:1905.03090 (replaced) [pdf, other]
Title: Kondo-induced giant isotropic negative thermal expansion
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
[76]  arXiv:1906.06317 (replaced) [pdf, other]
Title: freud: A Software Suite for High Throughput Analysis of Particle Simulation Data
Comments: GitHub repository: ; Documentation: ; Accepted for publication in Computer Physics Communciation
Subjects: Computational Physics (physics.comp-ph); Materials Science (cond-mat.mtrl-sci); Computational Engineering, Finance, and Science (cs.CE)
[77]  arXiv:1907.01111 (replaced) [pdf, other]
Title: How trapped particles interact with and sample superfluid vortex excitations
Comments: 13 pages, 7 figures
Subjects: Other Condensed Matter (cond-mat.other); Fluid Dynamics (physics.flu-dyn)
[78]  arXiv:1907.07079 (replaced) [pdf, other]
Title: Simulation methods for open quantum many-body systems
Comments: revised version
Subjects: Quantum Physics (quant-ph); Strongly Correlated Electrons (cond-mat.str-el); Computational Physics (physics.comp-ph)
[79]  arXiv:1907.11610 (replaced) [pdf, other]
Title: Collapse of layer dimerization in the photo-induced hidden state of 1T-TaS2
Journal-ref: Nat Commun 11, 1247 (2020)
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
[80]  arXiv:1908.06977 (replaced) [pdf, ps, other]
Title: Superfluids as Higher-form Anomalies
Comments: 29 pages; v3 corrected Eq. (1.25), published version
Subjects: High Energy Physics - Theory (hep-th); Strongly Correlated Electrons (cond-mat.str-el)
[81]  arXiv:1909.05281 (replaced) [pdf]
Title: Chemical bonding in chalcogenides: the concept of multi-centre hyperbonding
Comments: 4 figures
Subjects: Materials Science (cond-mat.mtrl-sci)
[82]  arXiv:1909.05802 (replaced) [pdf, other]
Title: Ecological communities from random generalised Lotka-Volterra dynamics with non-linear feedback
Comments: 48 pages, 11 figures
Journal-ref: Phys. Rev. E 101, 032101 (2020)
Subjects: Populations and Evolution (q-bio.PE); Disordered Systems and Neural Networks (cond-mat.dis-nn); Statistical Mechanics (cond-mat.stat-mech)
[83]  arXiv:1910.01134 (replaced) [pdf, ps, other]
Title: Unifying Lattice Models, Links and Quantum Geometric Langlands via Branes in String Theory
Comments: 31 pages. Typo corrected
Subjects: High Energy Physics - Theory (hep-th); Statistical Mechanics (cond-mat.stat-mech); Geometric Topology (math.GT); Quantum Algebra (math.QA); Representation Theory (math.RT)
[84]  arXiv:1910.07759 (replaced) [pdf, ps, other]
Title: Unusual slow magnetic fluctuations and critical slowing down in Sr$_{2}$Ir$_{1-x}$Rh$_{x}$O$_{4}$
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
[85]  arXiv:1911.04187 (replaced) [pdf, other]
Title: Signatures of unconventional pairing in spin-imbalanced one-dimensional few-fermion systems
Journal-ref: Physical Review Research 2, 012077(R) (2020)
Subjects: Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con); Quantum Physics (quant-ph)
[86]  arXiv:1911.12340 (replaced) [pdf, other]
Title: Elastoviscoplastic rheology and ageing in a simplified soft glassy constitutive model
Comments: 15 pages, 9 figures; version accepted, Journal of Rheology
Subjects: Soft Condensed Matter (cond-mat.soft)
[87]  arXiv:1912.01207 (replaced) [pdf, ps, other]
Title: Strain-induced enhancement of the Seebeck effect in magnetic tunneling junctions via interface resonant tunneling: Ab-initio study
Comments: 9 pages, 10 figures, published version
Journal-ref: Phys. Rev. B 101, 094430 (2020)
Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[88]  arXiv:1912.05230 (replaced) [pdf, other]
Title: First Principles Heisenberg Models of 2D magnetic materials: The Importance of Quantum Corrections to the Exchange Coupling
Comments: Version 3. Modified expression for single-ion anisotropy to make the sign of the spin-wave consistent with the sign of the magnetic anisotropy. Numbers in tables changed slightly according to the modification
Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[89]  arXiv:1912.06799 (replaced) [pdf]
Title: Spin-mediated charge-to-heat current conversion phenomena in ferromagnetic binary alloys
Comments: 21 pages, 14 figures
Journal-ref: Physical Review Materials 4, 034409 (2020)
Subjects: Materials Science (cond-mat.mtrl-sci)
[90]  arXiv:2001.07419 (replaced) [pdf, other]
Title: Progress in cooling nanoelectronic devices to ultra-low temperatures
Comments: 19 pages, 13 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[91]  arXiv:2001.08461 (replaced) [pdf, other]
Title: Exfoliated hexagonal BN as gate dielectric for InSb nanowire quantum dots with improved gate hysteresis and charge noise
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[92]  arXiv:2001.09194 (replaced) [pdf, ps, other]
Title: Ab initio many-body GW correlations in the electronic structure of LaNiO$_2$
Comments: 6 pages, 6 figures, 2 tables
Subjects: Superconductivity (cond-mat.supr-con); Materials Science (cond-mat.mtrl-sci)
[93]  arXiv:2002.03771 (replaced) [pdf, ps, other]
Title: Two universal laws for plastic flows and the consistent thermodynamic dislocation theory
Authors: Khanh Chau Le
Comments: 8 pages, 2 figures
Subjects: Materials Science (cond-mat.mtrl-sci)
[94]  arXiv:2002.06192 (replaced) [pdf, other]
Title: Signatures of topological ground state degeneracy in Majorana islands
Comments: 27 pages, 6 figures. Submitted version
Subjects: Superconductivity (cond-mat.supr-con); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[95]  arXiv:2002.08589 (replaced) [pdf]
Title: Intense Dark Exciton Emission from Strongly Quantum Confined CsPbBr$_3$ Nanocrystals
Subjects: Applied Physics (; Materials Science (cond-mat.mtrl-sci)
[96]  arXiv:2003.02423 (replaced) [pdf]
Title: Gate-tunable van der Waals heterostructure for reconfigurable neural network vision sensor
Comments: 20 pages, 4 figures
Subjects: Materials Science (cond-mat.mtrl-sci); Disordered Systems and Neural Networks (cond-mat.dis-nn); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Applied Physics (
[97]  arXiv:2003.03555 (replaced) [pdf]
Title: Honeycomb Layered Oxides: Structure, Energy Storage, Transport, Topology and Relevant Insights
Comments: 55 pages, 8 figures, 1 cover art, Review Manuscript
Subjects: Materials Science (cond-mat.mtrl-sci)
[98]  arXiv:2003.04885 (replaced) [pdf, other]
Title: Flux noise in superconducting qubits and the gap states continuum
Comments: 6 pages, 2 figures
Subjects: Superconductivity (cond-mat.supr-con)
[99]  arXiv:2003.05566 (replaced) [pdf, ps, other]
Title: Ion Modes in Dense Ionized Plasmas through Non-Adiabatic Molecular Dynamics
Comments: 5 pages and 4 figures in the main manuscript, 6 pages and 9 figures in the supplementary information; typos corrected; corrected grammar, references fixed, author name adjusted
Subjects: Plasma Physics (physics.plasm-ph); Statistical Mechanics (cond-mat.stat-mech); Computational Physics (physics.comp-ph)
[100]  arXiv:2003.07261 (replaced) [pdf, other]
Title: Protected helical transport in magnetically doped quantum wires: beyond the 1D paradigm
Comments: Slightly polished version, improved bibliography
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[101]  arXiv:2003.07834 (replaced) [pdf, other]
Title: Fractional viscoelastic models for power-law materials
Comments: 28 pages, 12 figures
Subjects: Soft Condensed Matter (cond-mat.soft); Biological Physics (; Quantitative Methods (q-bio.QM); Tissues and Organs (q-bio.TO)
[102]  arXiv:2003.10364 (replaced) [pdf]
Title: Optical-Field Driven Charge-Transfer Modulations near Composite Nanostructures
Comments: 8 pages, 6 figures
Subjects: Materials Science (cond-mat.mtrl-sci); Soft Condensed Matter (cond-mat.soft); Chemical Physics (physics.chem-ph)
[103]  arXiv:2003.11130 (replaced) [pdf, other]
Title: Thermal Hall effect in the pseudogap phase of cuprates
Authors: Chandra M. Varma
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
[104]  arXiv:2003.11168 (replaced) [pdf, other]
Title: Measurement-based cooling of a nonlinear mechanical resonator
Comments: 8 pages, 5 figures
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[105]  arXiv:2003.11532 (replaced) [pdf, other]
Title: Managing Flow of Liquid Light
Comments: 5 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Pattern Formation and Solitons (nlin.PS); Optics (physics.optics)
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