We propose a new method to generate pure magnon valley currents in ferromagnetic thin films with triangular antidot lattices. We find that inhomogeneous distortions can generate pseudomagnetic fields which can be up to 60 $\mathrm{T}$ for magnons, and give rise to well-defined Landau levels. The pseudomagnetic fields push magnons at different valleys $\mathrm{K}$ and ${\mathrm{K}}^{\prime }$ transversely to opposite directions and accumulate at opposite edges, leading to magnon valley Hall effect and forming pure magnon valley edge currents. We also find that the oppositely directed pseudomagnetic fields result in magnon snake states at the center of the ribbon, forming pure magnon valley currents. Our work might pave a new way to manipulate magnon transport by using artificial designed structures.

The quantum many-body states in the Bardeen-Cooper-Schrieffer–Bose-Einstein condensation (BCS-BEC) crossover regime are of long-lasting interest. Here we report direct spectroscopic evidence of BCS-BEC crossover in real space in a FeSe monolayer thin film by using spatially resolved scanning tunneling spectra. The crossover is driven by the shift of band structure relative to the Fermi level. The theoretical calculation based on a two-band model qualitatively reproduces the measured spectra in the whole crossover range. In addition, the Zeeman splitting of the quasiparticle states is found to be consistent with the characteristics of a condensate. Our work paves the way to study the exotic states of BCS-BEC crossover in a two-dimensional crystalline material at the atomic scale.

Superconducting circuits provide a favorable platform for quantum thermodynamic experiments. An important component for such experiments is a heat valve, i.e., a device which allows one to control the heat power flowing through the system. Here we theoretically study the heat valve based on a superconducting quantum interference device (SQUID) coupled to two heat baths via two resonators. The heat current in such a system can be tuned by magnetic flux. We investigate how the heat current modulation depends on the coupling strength $g$ between the SQUID and the resonators. In the weak coupling regime the heat current modulation grows as $g^2$, but, surprisingly, at the intermediate coupling it can be strongly suppressed. This effect is linked to the resonant nature of the heat transport at weak coupling, where the heat current dependence on the magnetic flux is a periodic set of narrow peaks. At the intermediate coupling the peaks become broader and overlap, thus reducing the heat modulation. At very strong coupling the heat modulation grows again and finally saturates at a constant value.

We provide real-space and Fock-space (FS) characterizations of ergodic, nonergodic extended (NEE) and many-body localized (MBL) phases in an interacting quasiperiodic system, namely, the generalized Aubry-André-Harper model, which possesses a mobility edge in the noninteracting limit. We show that a mobility edge in the single-particle (SP) excitations survives even in the presence of interaction in the NEE phase. In contrast, all single-particle excitations get localized in the MBL phase due to the MBL proximity effect. We give complementary insights into the distinction of the NEE states from the ergodic and MBL states by computing local FS self-energies and decay length associated, respectively, with the local and the nonlocal FS propagators. Based on a finite-size scaling analysis of the typical local self-energy across the NEE to ergodic transition, we show that MBL and NEE states exhibit qualitatively similar multifractal character. However, we find that the NEE and MBL states can be distinguished in terms of the distribution of local self-energy and the decay of the nonlocal propagator in the FS, whereas the typical local FS self-energy cannot tell them apart.

We report the electrical transport of the layered, spin-ladder antiferromagnet ${\mathrm{TaFe}}_{1.25}{\mathrm{Te}}_3$ ($T_N\sim 167$ K) under intense magnetic field (up to 60 T) and high pressure (up to 50 GPa). In high magnetic field, ${\mathrm{TaFe}}_{1.25}{\mathrm{Te}}_3$ undergoes two metamagnetic transitions, manifested in both the magnetoresistance and the Hall resistivity. Synchrotron x-ray diffraction reveals no obvious structural transition up to $\sim 50$ GPa. However, the application of a small pressure ($\sim 0.7$ GPa) fosters a sizable anomalous Hall conductivity and a negative magnetoresistance, possibly arising from the competing ferromagnetic exchange interaction in this antiferromagnet. Both the anomalous Hall conductivity and the negative magnetoresistance culminate at a pressure of $\sim 4$ GPa. With further increase of pressure, both physical quantities diminish. The Hall coefficient and the magnetoresistance change sign above $\sim 20$ GPa, presumably due to the cessation of ferromagnetic spin fluctuations in this high pressure regime.

We have investigated the origin of the negative-thermal-expansion (NTE) phenomena observed in various strongly correlated $\mathit{f}\text{−}$electron systems, including Ce, Sm, and Yb rare-earth compounds and actinide element Pu. We have thoroughly surveyed existing nonmagnetic $\mathit{f}\text{−}$electron NTE materials and identified that the temperature ($T$)-induced valence transition plays an essential role in realizing the NTE in Sm- and Yb-based mixed-valence (MV) systems. We have discussed the contrasting thermal-expansion behaviors between (Sm, Yb)-based MV systems and their electron-hole symmetric counterparts (Ce, Eu)-based MV systems that exhibit positive-thermal-expansion (PTE) phenomena. We have also clarified the origin of intriguing thermal expansion behavior of Pu, which, upon cooling, reveals not only the NTE, but also the strong PTE. Finally, we have predicted the possible existence of an additional NTE feature in topological-Kondo-insulator (TKI) candidates of ${\mathrm{SmB}}_6$, g-SmS, and ${\mathrm{YbB}}_{12}$ in the low-$T$ regime where the TKI nature is expected to emerge.

We present systematic theoretical results on thermoelectric effects in semimetals based on the variational method of the linearized Boltzmann equation. Inelastic electron-hole scattering is known to play an important role in the unusual transport of semimetals, including the broad $T^2$ temperature dependence of the electrical resistivity and the strong violation of the Wiedemann-Franz law. By treating the inelastic electron-hole scattering more precisely beyond the relaxation-time approximation, we show that the Seebeck coefficient when compensated depends on the screening length of the Coulomb interaction as well as the Lorenz ratio (the ratio of thermal to electric conductivity due to electrons divided by temperature). It is found that deviations from the compensation condition significantly increase the Seebeck coefficient, along with crucial suppressions of the Lorenz ratio. The result indicates that uncompensated semimetals with the electron-hole scattering have high thermoelectric efficiency when the phonon contribution to thermal conductivity is suppressed.

Squashed entanglement and its universal upper bound, the quantum conditional mutual information, are faithful measures of bipartite quantum correlations defined in terms of multipartitions. As such, they are sensitive to the fine-grain structure of quantum systems. Building on this observation, we introduce the concept of quantum conditional mutual information between the edges of quantum many-body systems. We show that this quantity characterizes unambiguously one-dimensional topological insulators and superconductors, being equal to Bell-state entanglement in the former and to half Bell-state entanglement in the latter, mirroring the different statistics of the edge modes in the two systems. The edge-to-edge quantum conditional mutual information is robust in the presence of disorder or local perturbations, converges exponentially with the system size to a quantized topological invariant, even in the presence of interactions, and vanishes in the trivial phase. We thus conjecture that it coincides with the edge-to-edge squashed entanglement in the entire ground-state phase diagram of symmetry-protected topological systems, and we provide some analytical evidence supporting the claim. By comparing them with the entanglement negativity, we collect further indications that the quantum conditional mutual information and the squashed entanglement provide a very accurate characterization of nonlocal correlation patterns in one-dimensional quantum matter.

A series of recent papers claimed that intranode scattering alone can contribute to positive longitudinal magnetoconductance (LMC) due to the chiral anomaly (CA) in Weyl semimetals (WSMs) in the quasiclassical limit. We revisit the problem of CA-induced LMC in WSMs in the quasiclassical limit and show that intranode scattering, by itself, does not result in enhancement of LMC. In the limit of zero internode scattering, chiral charge must remain conserved, which is shown to actually decrease LMC. Only in the presence of a finite nonzero internode scattering does one obtain a positive LMC due to nonconservation of the chiral charge. Even weak internode scattering suffices in generating positive LMC since it redistributes charges across both the nodes, although on a timescale larger than that of the intranode scattering. Our work is fundamental to correctly interpret the recent experiments on magnetoconductance in Weyl semimetals that claim to have observed chiral anomaly. Furthermore, our calculations reveal that, in contrast to recent works, in inhomogeneous WSMs strain-induced axial magnetic field $B_5$, by itself, leads to negative longitudinal magnetoconductance and a negative planar Hall conductance.

The current-induced spin polarization (CISP) in a system without the inversion symmetry is known to efficiently generate the spin current. In this paper we propose another approach, generating the spin current from the CISP locally arising in a system with the inversion symmetry in the form of the antiparallel CISP in sublattice structure. In this approach the local CISP is extracted from one of sublattices by a selective contact between the sublattice and an electrode. As the simplest system with such antiparallel CISP, we consider a symmetric double-quantum-well structure (DQWS) and calculate the antiparallel CISP and the spin current to a metallic layer in parallel contact with one well of the DQWS using the Boltzmann equation in the relaxation time approximation. When the Fermi energy is large enough that the first-excited sub-band is occupied, the magnitude of the spin current, which is proportional to that of the antiparallel CISP, increases with the interwell tunnel coupling and reaches twice the value of the decoupled quantum well with the broken inversion symmetry. Such an estimate suggests that inversion-symmetric systems can be useful in generating the spin current if the inversion symmetry is locally broken.

Two-dimensional (2D) ferroelectric materials have attracted great attention in recent years due to their thin thickness, high stability, and switchable polarization states. In particular, ferroelectric tunnel junctions (FTJs) constructed from 2D ferroelectric materials have been shown to have very high tunnel electroresistance (TER) ratios. In this work, we design a ferroelectric tunnel junction composed of ${\mathrm{Sc}}_2{\mathrm{CO}}_2/{\mathrm{In}}_2{\mathrm{Se}}_3$ vertical van der Waals (vdW) heterostructure based on two different 2D ferroelectric materials with out-of-plane polarization. Through density functional calculations combined with a nonequilibrium Green's function technique, it is found that the TER ratio as high as ${10}^7\%$ can be achieved. Analysis shows that it originates from the difference in the work functions of the contact surfaces of the two ferroelectric materials, which makes charge transfer occur or not occur between them and further leads to a metalinsulator switching of the ferroelectric vdW heterostructure upon the reversion of the applied electrical field. The results suggest the importance of ferroelectric vdW heterostructure in the design of FTJs and a feasible design scheme characterized by proper choice of the work functions and band gaps of the component materials.

Elemental calcium (Ca), a simple metal at ambient conditions, has attracted huge interest because of its unusual high-pressure behavior in structural, electrical, and melting properties whose origins remain unsolved. Here, using a theoretical framework appropriate for describing electride phase formation, i.e., the presence of anionic electrons, we establish electride formation in Ca at a pressure as low as 8 GPa. Our analysis shows that under pressure the valence electrons of Ca localize at octahedral holes and exhibit anionic character which is responsible for its strange pressure behavior. Our calculated enthalpy and electrical resistance indicate that Ca will directly transform from an fcc-electride phase to a simple cubic sc-electride phase near 30 GPa, thereby avoiding the intermediate bcc phase. These findings are not limited to Ca but might hold a key to the understanding of host-guest type structures which occur in other elemental solids, though at much higher pressures.

Condensed matter realization of a single Dirac cone of fermions in two dimensions is a long-standing issue. Here we report the discovery of a single gapless Dirac cone of half-quantized Hall conductance in a magnetically doped topological insulator heterostructure. It demonstrates that the Hall conductance is half-quantized in the unit $e^2/h$ when the parity symmetry is preserved near the Fermi surface. The gapless Dirac point is stable and protected by the local parity symmetry and the topologically nontrivial band structure of the topological insulator. The one-half Hall conductance observed in a recent experiment [M. Mogi et al., Nat. Phys. 18, 390 (2022)] is attributed to the existence of the gapless Dirac cone and the parity invariance of the Fermi surface. The results suggest a condensed matter realization of a topological phase with a one-half topological invariant.

Motivated by recent interest in fracton topological phases, we explore the interplay between gapped 2D ${\mathbb{Z}}_N$ topological phases which admit fractional excitations with restricted mobility and geometry of the lattice on which such phases are placed. We investigate the properties of the phases in a geometric context—graph theory. By placing the phases on a 2D lattice consisting of two arbitrary connected graphs, $G_x⊠G_y$, we study the behavior of fractional excitations of the phases. We derive the formula of the ground-state degeneracy of the phases, which depends on invariant factors of the Laplacian.

In this paper, we present a unifying analytical framework for identifying conditions for transport effects such as reflectionless and transparent transport, lasing, and coherent perfect absorption in non-Hermitian nonreciprocal systems using a generalized transfer matrix method. This provides a universal approach to studying the transport of tight-binding platforms, including higher-dimensional models and those with an internal degree of freedom going beyond the previously studied case of one-dimensional chains with nearest-neighbor couplings. For a specific class of tight-binding models, the relevant transport conditions and their signatures of non-Hermitian, nonreciprocal, and topological behavior are analytically tractable from a general perspective. We investigate this class and illustrate our formalism in a paradigmatic ladder model where the system's parameters can be tuned to adjust the transport effect and topological phases.