We derive the general continuum model for a bilayer system of staggered-flux square lattices, with arbitrary elastic deformation in each layer. Applying this general continuum model to the case where the two layers are rigidly rotated relative to each other by a small angle, we obtain the band structure of the twisted bilayer staggered-flux square lattice. We show that this band structure exhibits a magic continuum in the sense that an exponential reduction of the Dirac velocity and bandwidths occurs in a large parameter regime. We show that the continuum model of the twisted bilayer system effectively describes a massless Dirac fermion in a spatially modulating magnetic field, whose renormalized Dirac velocity can be exactly calculated. We further give an intuitive argument for the emergence of flattened bands near half filling in the magic continuum and provide an estimation of the large number of associated nearly zero-energy states. We also show that the entire band structure of the twisted bilayer system is free of band gaps due to symmetry constraints.

Describing dynamics of quantum many-body systems is a formidable challenge due to rapid generation of quantum entanglement between remote degrees of freedom. A promising approach to tackle this challenge, which has been proposed recently, is to characterize the quantum dynamics of a many-body system and its properties as a bath via the Feynman-Vernon influence matrix (IM), which is an operator in the space of time trajectories of local degrees of freedom. Physical understanding of the general scaling of the IM's temporal entanglement and its relation to basic dynamical properties is highly incomplete to the present day. In this paper, we analytically compute the exact IM for a family of integrable Floquet models—the transverse-field kicked Ising chain—finding a Bardeen-Cooper-Schrieffer-like “wave function” on the Schwinger-Keldysh contour with algebraically decaying correlations. We demonstrate that the IM exhibits area-law temporal entanglement scaling for all parameter values. Furthermore, the entanglement pattern of the IM reveals the system's phase diagram, exhibiting jumps across transitions between distinct Floquet phases. Near criticality, a nontrivial scaling behavior of temporal entanglement is found. The area-law temporal entanglement allows us to efficiently describe the effects of sizable integrability-breaking perturbations for long evolution times by using matrix-product-state methods. This work shows that tensor-network methods are efficient in describing the effect of noninteracting baths on open quantum systems and provides an approach to studying quantum many-body systems with weakly broken integrability.

${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$ (SCOC) is a model undoped cuprate with $I4/mmm$ crystallographic symmetry, and a simple magnetic space group $C_{A}mca$ with associated magnetic point group $mmm{1}^{\prime }$. However, recent second harmonic spectroscopy in the antiferromagnetic phase has challenged this picture, suggesting instead a magnetic point group $4/m{m}^{\prime }{m}^{\prime }$ that coexists with the antiferromagnetism and breaks the two orthogonal mirror planes containing the tetragonal $c$ axis. Here, we analyze the symmetry of SCOC in light of the second harmonic results, and discuss possible ground states that are consistent with the data.

We construct a continuum model of twisted trilayer graphene using ab initio density-functional-theory calculations, and we apply it to address twisted trilayer electronic structure. Our model accounts for moiré variation in site energies, hopping between outside layers, and hopping within layers. We focus on the role of a mirror symmetry present in ABA graphene trilayers with a middle layer twist. The mirror symmetry is lost intentionally when a displacement field is applied between layers, and unintentionally when the top layer is shifted laterally relative to the bottom layer. We calculate two band-structure characteristics that are directly relevant to transport measurements, namely the Drude weight and the weak-field Hall conductivity. We relate them via the Hall density to assess the influence of the accidental lateral stacking shifts currently present in all experimental devices on electronic properties. Finally, we comment on the role of the possible importance of accidental lateral stacking shifts for superconductivity in twisted trilayers.

We consider the transport properties of a model of fermions scattered by a critical bosonic mode. The mode is overdamped and scattering is mainly in the forward direction. Such a mode appears at the quantum critical point for an electronic nematic phase transition and in gauge theories for a U(1) spin liquid. It leads to a short fermion lifetime, violating Landau's criterion for a Fermi liquid. In spite of this, transport can be described by a Boltzmann equation. We include momentum relaxation by umklapp scattering, supplemented by weak impurity scattering. We find that above a very low temperature which scales with ${\mathrm{\Delta }}_q^3$, where ${\mathrm{\Delta }}_q$ is the minimum umklapp scattering vector, the resistivity is linear in $T$ with a coefficient which is independent of the amount of disorder. This picture holds until the temperature dependent part of the resistivity exceeds that due to impurity scattering. We compare the relaxation time approximation with an exact numerical solution of the Boltzmann equation. Surprisingly we find that, unlike the resistivity, the Hall coefficient strongly deviates from the relaxation time approximation and shows a strong reduction with increasing temperature. We comment on possible comparisons with experiments on high $T_c$ cuprates.

A sulfur-oxygen complex in CdTe characterized by IR absorption lines at 1097 and $1108 {\mathrm{cm}}^{-1}$ is studied by means of infrared absorption spectroscopy and first principles theory. Temperature-sensitive measurements performed on samples with different ${}^{16}\mathrm{O}:{}^{18}\mathrm{O}$ isotope ratios show that these lines are due to a split ${\nu }_3$ stretch vibrational mode of a disturbed sulfate ion $\left({\mathrm{SO}}_4^{2-}\right)$ transformed under the $A_1$ $(1097 {\mathrm{cm}}^{-1})$ and $E$ $(1108 {\mathrm{cm}}^{-1})$ representations of the $C_{3v}$ point group. The ${\nu }_1$ symmetric stretch, ${\nu }_4$ bend, and ${\nu }_1+{\nu }_3$ combination modes of the ion are also identified. An axially disturbed ${\mathrm{SO}}_4$ located at the Cd vacancy site is proposed as a microscopic model of the complex.

${\mathrm{Co}}_{1-x}{\mathrm{Fe}}_x$ bilayer nanomagnets formed on a Cu(111) substrate with the alloy composition of $x=0$, 0.2, 0.5, 0.8, and 1 were investigated using scanning tunneling microscopy (STM). The islands with $x=0$, 0.2, and 0.5 were further studied using spin-polarized STM. With $x=0.2$, the nanoislands exhibit triangular shapes similar to the pure Co islands with an upward shift in the peak energy of the minority $d$ state. On the other hand, at $x=0.5$, the appearance of the nanoislands evolve to a round shape, and the peak energy is distributed irregularly within the islands. A hexagonal shape and inhomogeneous electronic states similar to pure Fe islands are observed on the $x=0.8$ islands. The magnetic field dependence of the spin-sensitive signal revealed that the perpendicular anisotropy of the ${\mathrm{Co}}_{1-x}{\mathrm{Fe}}_x$ alloy islands is persistent up to half of the Fe alloy composition.

We study the formation of topological defects (quantum vortices) during the formation of a two-dimensional (2D) polariton condensate at the $\mathrm{\Gamma }$ point of a honeycomb lattice via the Kibble-Zurek mechanism. The lattice modifies the single-particle dispersion. The typical interaction energies at the quench time correspond to the linear part of the dispersion. The resulting scaling exponent for the density of topological defects is numerically found as $0.95\pm 0.05$. This value differs from the one expected for 2D massive particles (1/2), but is indeed compatible with the one expected for a linear dispersion. We moreover demonstrate that the vortices can be pinned to the lattice, which prevents their recombination and could facilitate their observation and counting in continuous wave experiments.

Recently, topological phases in non-Hermitian systems have attracted much attention because non-Hermiticity sometimes gives rise to unique phases with no Hermitian counterparts. Non-Hermitian Bloch Hamiltonians can always be mapped to doubled Hermitianized Hamiltonians with chiral symmetry, which enables us to utilize the existing framework for Hermitian systems to classify non-Hermitian topological phases. While this strategy succeeded in the topological classification of non-Hermitian Bloch Hamiltonians in the presence of internal symmetries, the generalization of symmetry indicators—a way to efficiently diagnose topological phases—to non-Hermitian systems is still elusive. In this work, we study a theory of symmetry indicators for non-Hermitian systems. We define space group symmetries of non-Hermitian Bloch Hamiltonians as the ones of the doubled Hermitianized Hamiltonians. Consequently, symmetry indicator groups for chiral symmetric Hermitian systems are equivalent to those for non-Hermitian systems. Based on this equivalence, we list symmetry indicator groups for non-Hermitian systems in the presence of space group symmetries. We also discuss the physical implications of symmetry indicators for some symmetry classes. Furthermore, explicit formulas of symmetry indicators for spinful electronic systems are included in appendices.

There is broad consensus that magnetite (${\mathrm{Fe}}_3{\mathrm{O}}_4$) is a promising material for spintronics applications due to its high degree of spin polarization at the Fermi level. However, previous attempts to measure the spin polarization by spin-resolved photoemission spectroscopy have been hampered by the use of low photon energies resulting in high surface sensitivity. The surfaces of magnetite, though, tend to reconstruct due to their polar nature, and thus their magnetic and electronic properties may strongly deviate from each other and from the bulk, dependent on their orientation and specific preparation. In this study, we determine the intrinsic—i.e., bulk—spin polarization of magnetite by spin-resolved photoelectron spectroscopy on (111)-oriented thin films, epitaxially grown on ZnO(0001), with hard x-rays, making it a truly bulk-sensitive probe. This becomes possible by using a novel, specially adapted momentum microscope, featuring time-of-flight energy recording and an imaging spin-filter.

Temperature-dependent reflectivity measurements on the kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ in a broad frequency range of $50-20000{\mathrm{cm}}^{-1}$ down to $T$=10 K are reported. The charge-density wave (CDW) formed below $T_{\mathrm{CDW}}$ = 94 K manifests itself in a prominent spectral-weight transfer from low to higher energy regions. The CDW gap of 60–75 meV is observed at the lowest temperature and shows significant deviations from an isotropic BCS-type mean-field behavior. Absorption peaks appear at frequencies as low as 200 ${\mathrm{cm}}^{-1}$ and can be identified with interband transitions according to density-functional calculations. The change in the interband absorption compared to ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ reflects the inversion of band saddle points between the K and Cs compounds. Additionally, a broader and strongly temperature-dependent absorption feature is observed below 1000 ${\mathrm{cm}}^{-1}$ and assigned to a displaced Drude peak. It reflects localization effects on charge carriers.

The giant negative magnetoresistance (GNMR), observed in two-dimensional electron gases of high mobility, is studied by controlled definition of additional short-range scatterers in form of two-dimensional Lorentz arrays of varying obstacle density, as well as in the presence of edge scattering. The results support models, which ascribe the temperature-independent regime of the GNMR to strong, classical scattering and the temperature-dependent regime to electron-electron interactions under the influence of mixed disorder. The threshold magnetic field, which separates the two regimes, is in rough agreement with the lower percolation transition of the Lorentz array. At large obstacle densities, interaction corrections are suppressed and memory effects become more relevant. Shape, amplitude, and width of the GNMR depend sensitively on the time scales of the contributing scattering mechanisms. This can lead to qualitatively similar shapes for quite different parameter values.

Photoluminescence and multiple ${\mathrm{Mn}}^{2+}$ spin-flip Raman scattering (SFRS) are studied in the (Cd,Mn)Te/(Cd,Mg)Te quantum well with excess resident hole concentration under resonance continuous-wave photoexcitation in the localized states of excitons and trions at $T=1.5$ K. A magnetic polaron shift of both exciton and trion photoluminescence is observed to be increasing with an increase in the magnetic field up to 6 T applied perpendicularly to the sample growth axis (Voigt geometry). Exciton magnetic polaron shift in magnetic field is attributed to the adiabatic energy transfer from the exchange reservoir to the Zeeman reservoir during the precession of the ${\mathrm{Mn}}^{2+}$ ions in the summary field of external magnetic and exchange hole fields. A mechanism of the magnetic polaron shift of trion photoluminescence is suggested. It is related to direct photoexcitation of the triplet trion state, which is stable in the conditions which are provided by the resident hole magnetic polarons. In these conditions, the ${\mathrm{Mn}}^{2+}$ magnetic moment experiences additional magnetization in the double exchange field provided by two holes in the triplet trion state. It is observed that the maximum efficiency of multiple ${\mathrm{Mn}}^{2+}$ spin-flip Raman scattering falls into the energy range corresponding to the states of localized excitons where the exciton magnetic polaron shift is observed. From this we conclude that the magnetic polaron state could be an intermediate state of the multiple ${\mathrm{Mn}}^{2+}$ SFRS process.

We calculate the rates of phonon-assisted hyperfine spin flips during electron and hole tunneling between quantum dots in a self-assembled quantum dot molecule. We show that the hyperfine process dominates over the spin-orbit-induced spin relaxation in magnetic fields up to a few teslas for electrons, while for holes this crossover takes place at field magnitudes of a fraction of a tesla, upon the assumption of a large $d$-shell admixture to the valence band state, resulting in a strong transverse hyperfine coupling. The interplay of the two spin-flip mechanisms leads to a minimum of the spin-flip probability, which is, in principle, experimentally measurable and can be used as a test for the presence of substantial transverse hyperfine couplings in the valence band.

The ill-defined Chern number and disappearing topological edge states make it difficult to characterize the topological properties of a metal. In this work, we investigate the abundant topological phases of monolayer graphene, which is sandwiched by asymmetric ferromagnets and irradiated by off-resonant light. In this system, there exist some rarely noticed metal phases, which are spin-valley polarized metal, topological spin metal (TSM), spin half metal, and topological spin half metal (TSHM). Particularly, for the TSM, the subband with spin up or spin down is topological, but the whole state becomes a metal. For the TSHM, a subband with one spin is topological, while the other subband with the opposite spin is insulated, and the whole state is a spin half metal. As a consequence, one topological protected spin current flows on the edge and the other spin propagates in the bulk. Further calculations indicate that the Berry curvatures for the metal phases are nonzero. We propose to probe the topological properties of the metal states with the anomalous Nernst effect. For these topological phases, spin and valley splitting and flip can be obtained by modulating the Fermi level. An electrically or magnetically controlled switch is designed by a two-terminal TSHM junction. It is expected that these topological metal phases can broaden the band engineering in monolayer graphene and support a promising platform for the studies of spin and valley caloritronics.

Obtaining dynamics of an interacting quantum many-body system connected to multiple baths initially at different, finite, temperatures and chemical potentials is a challenging problem. This is due to a combination of the prevalence of strong correlations in the system, the infinite nature of the baths and the long time to reach steady state. In this paper, we develop a general formalism that allows access to the full non-Markovian dynamics of such open quantum many-body systems up to the nonequilibrium steady state, provided its uniqueness. Specifically, we show how finite-time evolution in the presence of finite-sized baths, whose opportune size is determined by their original spectral density, can be recursively used to faithfully reconstruct the exact dynamics without requiring any small parameter. Such a reconstruction is possible even in parameter regimes which would otherwise be inaccessible by current state-of-the-art techniques. We specifically demonstrate this by obtaining the full numerically exact non-Markovian dynamics of interacting fermionic chains in two terminal setups with finite temperature and voltage biases, a problem which previously remained outstanding despite its relevance in a wide range of contexts, for example, quantum heat engines and refrigerators.

We study the subgap transport through the antiferromagnet/superconductor (AF/S) and antiferromagnet/superconductor/antiferromagnet (AF/S/AF) junctions controlled by electric field in a generic buckled honeycomb system, such as silicene, germanene, and stanene. In the presence of electric field and antiferromagnetic exchange field, the spin-valley polarized half-metallic phase can be achieved in the honeycomb system due to the spin-orbit coupling, which affords an opportunity to generate the pure crossed Andreev reflection (CAR). It is found that the pure CAR can be generated without local Andreev reflection (AR) and elastic cotunneling (EC) over a wide range of electric field. A spin-valley switch effect can be realized between the pure CAR and the pure EC by adjusting the electric field. The properties of AR process and CAR process strongly depend on the spin-valley polarized states. Our results suggest that the device can implement an electrical measurement of the CAR process and spin-valley switch.

Hybrid semiconducting nanowire devices combining epitaxial superconductor and ferromagnetic insulator layers have been recently explored experimentally as an alternative platform for topological superconductivity at zero applied magnetic field. In this proof-of-principle work we show that the topological regime can be reached in actual devices depending on some geometrical constraints. To this end, we perform numerical simulations of InAs wires in which we explicitly include the superconducting Al and magnetic EuS shells, as well as the interaction with the electrostatic environment at a self-consistent mean-field level. Our calculations show that both the magnetic and the superconducting proximity effects on the nanowire can be tuned by nearby gates thanks to their ability to move the wavefunction across the wire section. We find that the topological phase is achieved in significant portions of the phase diagram only in configurations where the Al and EuS layers overlap on some wire facet, due to the rather local direct induced spin polarization and the appearance of an extra indirect exchange field through the superconductor. While of obvious relevance for the explanation of recent experiments, tunable proximity effects are of interest in the broader field of superconducting spintronics.

We clarify universal critical properties of delocalization-localization transitions in three-dimensional unitary and orthogonal classes with particle-hole and/or chiral symmetries (classes AIII, BDI, D, C, and CI). We first introduce tight-binding models on cubic lattice that belong to these five nonstandard symmetry classes, respectively. Unlike the Bogoliubov-de Gennes Hamiltonian for superconductors, all the five models have finite areas of Fermi surfaces in the momentum space in the clean limit. Thereby, the scaling theory of the Anderson transition guarantees the presence of the delocalization-localization transitions at finite disorder strength in these models. Based on this expectation, we carry out extensive transfer matrix calculations of the Lyapunov exponents for zero-energy eigenstates of the disordered tight-binding models with quasi-one-dimensional geometry. Near the Anderson transition point, the correlation length diverges with a universal critical exponent $\nu$. Using finite-size scaling analysis of the localization length, we determine the critical exponent of the transitions and scaling function of the (normalized) localization length in the three nonstandard unitary symmetry classes: ${\nu }_{\text{AIII}}=1.06\pm 0.02$, ${\nu }_{\text{D}}=0.87\pm 0.03$, and ${\nu }_{\text{C}}=0.996\pm 0.012$. Our result of the class C is consistent with a previous study of classical network model [M. Ortuño et al., Phys. Rev. Lett. 102, 070603 (2009)]. The critical exponents of the two nonstandard orthogonal classes are estimated as ${\nu }_{\text{CI}}=1.17\pm 0.02$ and ${\nu }_{\text{BDI}}=1.12\pm 0.06$. Our result of the class CI is consistent with another previous study of a lattice model, while the exponent of the class BDI is at variance with the previous evaluation of nodal Dirac ring model [X. Luo et al., Phys. Rev. B 101, 020202(R) (2020)].

We show that carbon nanopeapods are an ideal artificial superlattice to induce band modulation of thermal phonons. Based on spectral energy density analysis, we found that a periodic deformation in carbon nanotubes induced by fullerene encapsulation leads to zone folding of phonon dispersions up to near the maximum frequency. The zone-folding effect gives rise to phonon band gaps caused either by Bragg reflection or mode hybridization, and this reduces group velocity. This was quantified to play the leading role in reduction of thermal conductivity by the fullerene encapsulation. The strain modulation of thermal phonon bands opens a possibility to control material thermal conductivity.