The lead chalcogenides with rocksalt structure, $\mathrm{Pb}X$ ($X$ = S, Se, Te), show great promise for thermoelectric applications in various doped and alloyed forms. Part of the explanation for this is their favorable low lattice thermal conductivities whose origin has been heavily debated. Powder x-ray diffraction experiments suggested Pb to be off-centered particularly in PbS. However, as shown here, single crystal x-ray diffraction shows the anharmonicity to be insignificant in PbS from 20 to 300 K, while it is significant from 100 to 622 K in PbTe although without leading to any Pb off-centering. Inelastic neutron scattering and anharmonic ab initio studies previously suggested the soft transverse optical mode to play an important role with respect to lowering the thermal conductivities of the lead chalcogenides. Here, the diffuse x-ray scattering of single crystal PbS is modeled from 20 to 300 K by correlated atomic displacements. The results indicate softening of the zone center transverse optical mode with decreasing temperature. From a chemical bonding perspective, the soft transverse optical mode can be rationalized by invoking the concepts of stereoactive lone pair formation and metavalent/resonant bonding.

We compare the pole structure of the electronic Green's function obtained by cluster dynamical mean-field theory to the results from the fractionalized pair density wave idea. In the superconducting phase, we can consider the system in a state with coexistence of superconducting and charge order. Writing the Green's function in a way analogous to the previously proposed “hidden-fermions” model [Sakai et al., Phys. Rev. Lett. 116, 057003 (2016)] leads to a similar pole structure for the self-energy. The fractionalization of the pair density wave order also describes the pseudogap phase as a superposition of superconducting and charge order fluctuations. Considering a phenomenological lifetime for the particle-particle and particle-hole pairs leads to an electronic spectral function that matches the numerical results.

Superconducting hybrid structures with topological order and induced magnetization offer a promising way to realize fault-tolerant quantum computation. However, the effect of the interplay between magnetization and the property of the topological insulator surface, otherwise known as spin-momentum locking on the superconducting proximity effect, still remains to be investigated. We relied on the quasiclassical self-consistent approach to consider the superconducting transition temperature in the two-dimensional superconductor/topological insulator (S/TI) junction with an in-plane helical magnetization on the TI surface. It has emerged that the presence of the helical magnetization leads to the nonmonotonic dependence of the critical temperature on the TI thickness for both cases when the magnetization evolves along or perpendicular to the interface. The results obtained can be helpful for designing novel superconducting nanodevices and better understanding the nature of superconductivity in S/TI systems with nonuniform magnetization.

We propose an anti-parity-time (anti-$\mathcal{PT}$) symmetric non-Hermitian Su-Schrieffer-Heeger (SSH) model, where the large non-Hermiticity constructively creates nontrivial topology and greatly expands the topological phase. In the anti-$\mathcal{PT}$-symmetric SSH model, the gain and loss are alternatively arranged in pairs under the inversion symmetry. The appearance of the degenerate point at the center of the Brillouin zone determines the topological phase transition, whereas the exceptional points unaffect the band topology. The large non-Hermiticity leads to unbalanced wave-function distribution in the broken anti-$\mathcal{PT}$-symmetric phase and induces the nontrivial topology. Our findings can be verified through introducing dissipations in every other two sites of the standard SSH model even in its trivial phase where the nontrivial topology is solely induced by the dissipations.

Periodic stacking of topologically trivial and nontrivial layers with opposite symmetry of the valence and conduction bands induces topological interface states that, in the strong coupling limit, hybridize both across the topological and normal insulator layers. Using band structure engineering, such superlattices (SLs) can be effectively realized using the IV–VI lead tin chalcogenides. This leads to emergent minibands with a tunable topology, as demonstrated both by theory and experiments. The topological minibands are proven by magneto-optical spectroscopy, revealing Landau level transitions both at the center and edges of the artificial SL mini-Brillouin zone. Their topological character is identified by the topological phase transitions within the minibands observed as a function of temperature. The critical temperature of this transition as well as the miniband gap and miniband width can be precisely controlled by the layer thicknesses and compositions. This witnesses the generation of a fully tunable quasi-three-dimensional topological state that provides a template for realization of magnetic Weyl semimetals and other strongly interacting topological phases.

We have observed commensurability oscillations (CO) in the Hall resistance $R_{yx}$ of a unidirectional lateral superlattice (ULSL). The CO, having small amplitudes $(\sim 1\mathrm{\Omega })$ and being superposed on a roughly three orders of magnitude larger background, are obtained by directly detecting the difference in $R_{yx}$ between the ULSL area and the adjacent unmodulated two-dimensional electron gas area and then extracting the odd part with respect to the magnetic field. The CO thus obtained are compared with a theoretical calculation and turn out to have the amplitude much smaller than the theoretical prediction. The implication of the smaller-than-predicted CO in $R_{yx}$ on the thermoelectric power of ULSL is briefly discussed.

We present a nonperturbative quasiclassical theory of graphene photoconductivity. We consider the influence of low-frequency (microwave, terahertz, midinfrared) radiation on the static conductivity of a uniform graphene layer and calculate its photoconductivity as a function of the frequency, polarization, and strength of the external ac electric field, as well as on the material properties (electron density, scattering time) and temperature. The theory is valid at frequencies $\hslash \omega \lesssim 2E_F$ and at arbitrarily strong ac electric fields. We compare our results with those of the third-order perturbation theory and determine the applicability range of the perturbative solutions.

Quantum dot based parity-to-charge conversion is a promising method for reading out quantum information encoded nonlocally into pairs of Majorana zero modes. To obtain a sizable parity-to-charge visibility, it is crucial to tune the relative phase of the tunnel couplings between the dot and the Majorana modes appropriately. However, in the presence of multiple quasidegenerate dot orbitals, it is in general not experimentally feasible to tune all couplings individually. This paper shows that such configurations could make it difficult to avoid a destructive multiorbital interference effect that substantially reduces the readout visibility. We analyze this effect using a Lindblad quantum master equation. This exposes how the experimentally relevant system parameters enhance or suppress the visibility when strong charging energy, measurement dissipation, and, most importantly, multiorbital interference is accounted for. In particular, we find that an intermediate-time readout could mitigate some of the interference-related visibility reductions affecting the stationary limit.

We study the many-body localization (MBL) transition for interacting fermions subject to quasiperiodic potentials by constructing the local integrals of motion (LIOMs) in the MBL phase as time-averaged local operators. We study numerically how these time-averaged operators evolve across the MBL transition. We find that the norm of such time-averaged operators drops discontinuously to zero across the transition; as we discuss, this implies that LIOMs abruptly become unstable at some critical localization length of order unity. We analyze the LIOMs using hydrodynamic projections and isolating the part of the operator that is associated with interactions. Equipped with these data we perform a finite-size scaling analysis of the quasiperiodic MBL transition. Our results suggest that the quasiperiodic MBL transition occurs at considerably stronger quasiperiodic modulations and has a larger correlation-length critical exponent than previous studies had found.

We consider a model arising in facilitated Rydberg chains with positional disorder which features a Hilbert space with the topology of a $d$-dimensional hypercube. This allows for a straightforward interpretation of the many-body dynamics in terms of a single-particle one on the Hilbert space and provides an explicit link between the many-body and single-particle scars. Exploiting this perspective, we show that an integrability-breaking disorder enhances the scars followed by inhibition of the dynamics due to strong localization of the eigenstates in the large disorder limit. Next, mapping the model to the spin-1/2 XX Heisenberg chain offers a simple geometrical perspective on the recently proposed Onsager scars [Phys. Rev. Lett. 124, 180604 (2020)], which can be identified with the scars on the edge of the Hilbert space. This makes apparent the origin of their insensitivity to certain types of disorder perturbations.

Phonon Hall effect (PHE) has attracted a lot of attention in recent years with many theoretical and experimental explorations published. While experiments work on complicated materials, theoretical studies are still hovering around the phenomenon-based models. Moreover, previous microscopic theory was found unable to explain large thermal Hall conductivity obtained by experiments in strontium titanate (STO). Therefore, in an attempt to bridge this gap, we implement first-principles calculations to explore the PHE in real materials. Our work provides a benchmark of the PHE in sodium chloride (NaCl) under a large external magnetic field. Moreover, we demonstrate our results in barium titanate (BTO) and discuss the results in STO in detail about their deviation from experiments. As a possible future direction, we further propose that the inner electronic Berry curvature and cubic potential plays important roles in the PHE in STO.

We perform a comparative study of the $\mathrm{K}{\mathrm{Cr}}_3{\mathrm{As}}_3$ and the ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ quasi-one-dimensional compounds and show that the strong interplay between the lattice and the spin degrees of freedom promotes a collinear ferrimagnetic ground state within the chains in the presence of intrachain antiferromagnetic couplings. We propose that the interchain antiferromagnetic coupling in $\mathrm{K}{\mathrm{Cr}}_3{\mathrm{As}}_3$ plays a crucial role in the experimentally observed spin-glass phase with low critical temperature. In the same region of the parameter space, we predict ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ to be nonmagnetic but on the verge of magnetism, sustaining interchain ferromagnetic spin fluctuations while the intrachain spin fluctuations are antiferromagnetic.

Spin wave, the collective excitation of magnetic order, is one of the fundamental angular momentum carriers in magnetic systems. Understanding the spin wave propagation in magnetic textures lies in the heart of developing pure magnetic information processing schemes. Here we show that the spin wave propagation across a chiral domain wall follows simple geometric trajectories, similar to the geometric optics. And the geometric behaviors are qualitatively different in normally magnetized film and tangentially magnetized film. We identify the lateral shift, refraction, and total reflection of spin wave across a ferromagnetic domain wall. Moreover, these geometric scattering phenomena become polarization dependent in antiferromagnets, indicating the emergence of spin wave birefringence inside antiferromagnetic domain walls.

In this paper, we demonstrate the magnetization-dependent inverse spin Hall effect (MD-ISHE) in ${\mathrm{Tb}}_x{\mathrm{Co}}_{100-x}/\mathrm{Pt}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ devices using the spin Seebeck effect. It was found that a thermally injected spin current is precessed around the magnetization of the ${\mathrm{Tb}}_x{\mathrm{Co}}_{100-x}$ alloy before it is converted into a charge current. Furthermore, the sign of the MD-ISHE voltage was found to depend on the Co magnetic moment direction, whereas the sign and magnitude of the conventional ISHE voltage are independent of the ${\mathrm{Tb}}_x{\mathrm{Co}}_{100-x}$ magnetization direction. Meanwhile, the spin rotation angle was estimated to be ∼12% in ${\mathrm{Tb}}_x{\mathrm{Co}}_{100-x}/\mathrm{Pt}$. Our findings demonstrate that perpendicularly magnetized ${\mathrm{Tb}}_x{\mathrm{Co}}_{100-x}$ supplies additional symmetry to the ISHE and provides advantages in terms of tailoring the sign of the spin-current polarization through adjusting the alloy composition.

A driven-dissipative nonlinear photonic system (e.g., exciton-polaritons) can operate in a gapped superfluid regime. We demonstrate theoretically that the reflection of a linear wave on this superfluid is an analog of the Andreev reflection of an electron on a superconductor. A normal region surrounded by two superfluids is found to host Andreev-like bound states. These bound states form topological synthetic bands versus the phase difference between the two superfluids. Changing the width of the normal region allows us to invert the band topology and to create “interface” states. Instead of demonstrating a linear crossing, synthetic bands are attracted by the nonlinear non-Hermitian coupling of bosonic systems, which gives rise to a self-amplified strongly occupied topological state.

In situ Seebeck and Hall measurements on FeSe films are carried out to understand the intrinsic evolution of the band structure as superconducting critical temperature $\left(T_c\right)$ is tuned by ionic liquid gating. At the initial state, the nonmonotonous behavior of Hall and Seebeck curves indicates the multiband feature of FeSe film. As $T_c$ increases via gating, the hole band gradually sinks below the Fermi level. Meanwhile, the Fermi energy of the electron band reduces while its carrier density increases, indicating that the electron band is flattening as $T_c$ enhances. When $T_c$ reaches the maximum at the final state, only the electron band exists and the effective electron mass is about $3.4m_e$.

We have studied the epitaxial ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ (111) films grown on ${\mathrm{Al}}_2{\mathrm{O}}_3$ (0001) substrates of different thickness at various temperature and discovered colossal intrinsic exchange bias up to 7 ± 2 kOe. X-ray and electron diffraction clearly indicate an interfacial layer about 2 nm of different crystal structure from the “bulk” part of the ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ film. The thickness dependence of the exchange bias suggests a hidden antiferromagnetic composition in the interfacial layer that couples to the ferrimagnetic “bulk” part of the ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ film as the origin of the exchange bias. Considering the structural, magnetic, and electronic structure, CoO has been identified as the most likely candidate of the antiferromagnetic composition in the interfacial layer. This work suggests a path for enhancing intrinsic exchange bias using combination of film and substrate of large structural differences, highlighting the role of interfacial atomic and electronic reconstructions.

A magnet with precessing magnetization pumps a spin current into adjacent leads. As a special case of this spin pumping, a precessing macrospin (magnetization) can assist electrons in tunneling. In small systems, however, the Coulomb blockade effect can block the transport of electrons. Here, we investigate the competition between macrospin-assisted tunneling and the Coulomb blockade for the simplest system where both effects meet; namely, for a single tunnel junction between a normal metal and a metallic ferromagnet with precessing magnetization. By combining Fermi's golden rule with magnetization dynamics and charging effects, we show that the macrospin-assisted tunneling can soften or even break the Coulomb blockade. The details of these effects—softening and breaking of the Coulomb blockade—depend on the macrospin dynamics. This allows us, for example, to measure the macrospin dynamics via a system's current-voltage characteristics. It also allows us to control a spin current electrically. From a general perspective, our results provide a platform for the interplay between spintronics and electronics on the mesoscopic scale. We expect our work to provide a basis for the study of Coulomb blockade in more complicated spintronic systems.