We consider observation of Aharonov-Bohm oscillations in clean systems based on the flow of topologically protected currents in silicene and bilayer graphene. The chiral channels in these materials are defined by the flips of the vertical electric field. The line of the flip confines chiral currents flowing along it in the direction determined by the valley. We present an electric field profile that forms a crossed ring to which four terminals can be attached, and find that the conductance matrix elements oscillate in the perpendicular magnetic field in spite of the absence of backscattering. We propose a four-probe resistance measurement setup, and demonstrate that the resistance oscillations have large visibility provided that the system is prepared in such a way that a direct transfer of the chiral carriers between the current probes is forbidden.

Inspired by the breakthroughs of twisted bilayer graphene, stacking-dependent phenomena are recently emerging as fascinating objects of condensed matter research, harboring a variety of new physics. However, current studies are restricted to electronic, superconducting and magnetic properties. Here, using first-principles calculations, we reveal the connection between stacking order and topological property in bilayer $M\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4\left(M=\mathrm{Ge},\mathrm{Sn},\mathrm{Pb}\right)$. We show the stacking order determines the topological property. Upon changing the stacking order, one can achieve topological phase transition between trivial and nontrivial states. We unveil that this stacking-dependent topological property is attributed to the interlayer $\mathrm{Te}-p_z$ orbitals coupling, which is closely related to the stacking order. Our work not only expands the scope of stacking-dependent properties but also provides a promising experimental platform to study this novel stacking-dependent topological property.

A triangular lattice with onsite Coulomb interaction $U$ present only on one sublattice is periodically driven by electromagnetic field with a frequency $\mathrm{\Omega }\gg (t,U)$ at half filling. In this high frequency limit, the electromagnetic vector potential, with an amplitude $A$, modifies the bare hopping and generates new next nearest neighbor hopping parameters. For $U=0$, the driving acts like an emergent intrinsic spin-orbit coupling term and stabilizes three dispersive bands with the lower and upper bands having nonzero Chern numbers. Within a slave rotor mean field theory, we show that while $U$ freezes out charge fluctuations on the interacting sublattice, it does not open up a charge gap without the external drive. In the presence of the drive, and small $U$, the system exhibits repeated metal-insulator transitions as a function of the amplitude $A$. For large $U$, we establish that the freezing of charge fluctuations on the interacting sublattice stabilizes an emergent, low energy half filled noninteracting Kane-Mele model, whose band gaps can be tuned by varying $A$. In this limit, we show that the external drive provides a handle to engineer periodic band inversions at specific values of $A$ accompanied by topological phase transitions that are characterized by swapping of band Chern numbers.

Non-Hermitian topological systems exhibit properties very different from those of their Hermitian counterparts. An important puzzling issue for non-Hermitian topological systems is the existence of defective edge states beyond the usual bulk-boundary correspondence (BBC). In this Rapid Communication, to understand the existence of these defective edge states, the number-anomalous bulk-boundary correspondence (NA-BBC) theory, which distinguishes the non-Bloch BBC, is developed. With the one-dimensional non-Hermitian Su-Schrieffer-Heeger model taken as an example, the underlying physics of the defective edge states is explored. The defective edge states are a consequence of non-Hermitian coalescence from the anomalous edge Hamiltonian. In addition, with the help of a theorem, the number anomaly of the edge states in non-Hermitian topological systems becomes a mathematical problem in quantitative calculations when identifying the normal/non-normal non-Hermitian condition for the edge Hamiltonian and verifying the deviation of the BBC ratio from 1. In the future, NA-BBC theory can be generalized to higher-dimensional non-Hermitian topological systems (for example, the two-dimensional Chern insulator).

We study relaxation dynamics in a strongly interacting two-site Fermi-Hubbard model that is induced by coupling each site to a local fermionic bath. To derive the proper form of the Lindblad operators that enter an effective description of the system-bath coupling in different temperature regimes, we employ a diagrammatic real-time technique for the time evolution of the reduced density matrix. In spite of a local coupling to the baths, the found Lindblad operators are nonlocal in space. We compare with the local approximation, where those nonlocal effects are neglected. Furthermore, we propose an improvement on the commonly used secular approximation (rotating-wave approximation), referred to as coherent approximation, which turns out superior in all studied parameter regimes (and equivalent otherwise). We look at the relaxation dynamics for several important observables and compare the methods for early and late times in various temperature regimes.

Nematicity, where rotational symmetry is broken while translational symmetry is conserved, is prevalent in high-temperature superconductors. In particular, nematic quantum critical point has been universally found near the optimum doping of the superconducting dome of several iron-based superconductor families. In such a regime, evidence for strong nematic fluctuations have been observed. As the precursor to this order, nematic fluctuations emerge before nematicity, providing favorable ground to study how nematic order modifies the electronic structure in the absence of structural distortion. Here we use spatially resolved angle-resolved photoemission spectroscopy to investigate the correlation between the onset of nematic fluctuations and electronic structure in an optimally doped ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ ($x\sim 0.3$) superconductor. We reveal a strong spatially varying anisotropy of the Fermi surface on a length scale of tens of microns with strong correlation between the changes in the hole and electron Fermi pockets, consistent with the variations expected in the presence of fluctuating nematic order. These results provide direct evidence for spatial nematic fluctuations in the optimal doping regime of iron-based superconductors.

$\left(\mathrm{N}{\mathrm{a}}_{1/2}\mathrm{B}{\mathrm{i}}_{1/2}\right)\mathrm{Ti}{\mathrm{O}}_3\text{−}x\mathrm{BaTi}{\mathrm{O}}_3\left(\mathrm{NBT}\text{−}x\mathrm{BT}\right)$ single crystal, as one of the most promising candidates to replace lead-based ferroelectrics, is a typical and unique lead-free system with morphotropic phase boundary (MPB) by far. In this paper, the phase transition process of NBT-0.06BT crystal in a wide temperature range of $20\sim 550{}^{\circ }\mathrm{C}$ was analyzed at atomic scale by theoretical fitting to the measured synchrotron radiation x-ray absorption fine structure spectra quantitatively. Detailed analysis revealed a gradually transition of local structure at the point of phase transition from rhombohedral to tetragonal phase in NBT-0.06BT crystal on the bond lengths of Bi-O and Bi-Ti. The continuous evolution of local structure with no obvious barrier to overcome is one of the intrinsic characteristics of MPB at atomic scale and is responsible for the high piezoelectric properties. These results could promote the understanding of the origin of the high piezoelectric performance around the MPB and meanwhile guide new lead-free material designs.

The valley degree of freedom is a sought-after quantum number in monolayer transition-metal dichalcogenides. Similar to optical spin orientation in semiconductors, the helicity of absorbed photons can be relayed to the valley (pseudospin) quantum number of photoexcited electrons and holes. Also similar to the quantum-mechanical spin, the valley quantum number is not a conserved quantity. Valley depolarization of excitons in monolayer transition-metal dichalcogenides due to long-range electron-hole exchange typically takes a few ps at low temperatures. Exceptions to this behavior are monolayers ${\mathrm{MoSe}}_2$ and ${\mathrm{MoTe}}_2$ wherein the depolarization is much faster. We elucidate the enigmatic anomaly of these materials, finding that it originates from Rashba-induced coupling of the dark and bright exciton branches next to their degeneracy point. When photoexcited excitons scatter during their energy relaxation between states next to the degeneracy region, they reach the light cone after losing the initial helicity. The valley depolarization is not as fast in monolayers ${\mathrm{WSe}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{MoS}}_2$, wherein the degeneracy is absent resulting in negligible Rashba-induced coupling between bright and dark excitons.

We study the poloidal modes (modes whose variations are strictly along the direction of the torus axis) of toroidal nanoparticles. We show that the modes may be understood in terms of the symmetry breaking that occurs when an infinite cylinder is folded to form a torus. This symmetry breaking results in the splitting of the transverse cylinder modes into two distinct sets of modes on a torus. One set of these modes was known to exist already. We show the existence of the second set and also analytically derive the surface charge structure for that set of modes. We also consolidate recent advances made in studying the modal structure on torus particles and complete our understanding of the plasmonic modes of a torus.

We introduce a topological theory of perfect isolation: perfect transmission from one side and total reflection from the other side simultaneously. The theory provides an efficient approach for determining whether such a perfect-isolation point exists within a finite parameter space. Herein, we demonstrate the theory using an example of a Lorentz nonreciprocal metasurface composed of dimer unit cells. Our theory also suggests that perfect-isolation points can annihilate each other through the coalescence of opposite topological charges. Our findings could lead to novel designs for high-performance optical isolators.

Using ab initio molecular-dynamics simulations combined with linear-response theory, we studied the x-ray absorption near-edge spectra (XANES) of a two-temperature dense copper plasma. As the temperature increases, XANES spectra exhibit a pre-edge structure balanced by a reduction of the absorption just behind the edge. By performing systematic simulations for various thermodynamic conditions, we establish a formulation to deduce the electronic temperature $T_e$ directly from the spectral integral of the pre-edge that can be used for various thermodynamic conditions encountered in a femtosecond heating experiment where thermal nonequilibrium and expanded states have to be considered.

We determine the shear viscosity and the dc electrical conductivity of interacting three-dimensional Luttinger semimetals, which have a quadratic band touching point in the energy spectrum, in the hydrodynamic regime. It is well known that when the chemical potential is right at the band touching point, the long-range Coulomb interaction induces the Luttinger-Abrikosov-Beneslavskii (LAB) phase at $T=0$, which is an interacting, scale-invariant, non-Fermi-liquid state of electrons. Upon combining the renormalization-group (RG) analysis near the upper critical spatial dimension of 4 with the Boltzmann kinetic equation, we determine the universal ratio of viscosity over entropy and the electrical dc conductivity of the system at the interacting LAB fixed point of the RG flow. The projection of the Coulomb interaction on the eigenstates of the system is found to play an important quantitative role for the scattering amplitude in the collision integral, and the so-called Auger processes make a large numerical contribution to the inverse scattering time in the transport quantities. The obtained leading-order result suggests that the universal ratio of the viscosity over entropy, when extrapolated to the physical three spatial dimensions, is above, but could be rather close to, the Kovtun-Son-Starinets lower bound.

Interaction of dipolar polaritons can be efficiently tuned by means of a shape resonance in their excitonic component. Provided the resonance width is large, a squeezed population of strongly interacting polaritons may persist on the repulsive side of the resonance. We derive an analytical expression for the polariton coupling constant, and we estimate the degree of squeezing of the emitted light. The squeezing may approach $100\%$ in typical experimental conditions. Our arguments hold promises for implementation of strong correlations in quantum photonics.

We solve for the finite temperature collective mode dynamics in the Holstein-double-exchange problem, using coupled Langevin equations for the phonon and spin variables. We present results in a strongly anharmonic regime, close to a polaronic instability. For our parameter choice, the system transits from an undistorted ferromagnetic metal at low temperature to a structurally distorted paramagnetic insulator at high temperature, through a short-range charge-ordered (CO) phase near the ferromagnetic crossover at $T_{\text{FM}}$. The small amplitude harmonic phonons at low temperature cross over to large amplitude dynamics around $0.5T_{\text{FM}}$ as spin disorder promotes formation of short-range correlated polarons. The CO background in turn leads to the softening of phonons near momentum $\mathbf{q}=(\pi ,\pi )$ with temperature, while strong anharmonicity close to the magnetic transition leads to unusually large damping. The most remarkable phonon feature, however, arises from the rare thermal tunneling of CO domains, generating a momentum selective spectral weight at very low energy. The spin dynamics is similar to that of a nearest-neighbor Heisenberg model with sharp spin waves at low $T$, transiting to diffusive modes near $T_{\text{F}\text{M}}$. We compare our results to inelastic neutron data in the manganites and suggest how the singular low-energy features can be probed.

We study a model of Rydberg atoms in a nearest-neighbor Rydberg blockaded regime, introduced by Lesanovsky [Phys. Rev. Lett. 108, 105301 (2012)]. This many-body model (which has one parameter $z$) has an exactly known gapped liquid ground state, and two exactly known low-lying excitations. We discover two exact low-lying eigenstates. We also discuss behavior of the model at small parameter $z$ and its proximity to an integrable model. Lastly, we discuss connections between the Lesanovsky model at intermediate $z$ and the so-called PXP model. The PXP model describes a recent experiment that observed unusual revivals from a charge-density-wave initial state, which are attributed to a set of many-body “scar states” which do not obey the eigenstate thermalization hypothesis. We discuss the possibility of approximate scar states in the Lesanovsky model and present two approximations for them.

We investigate the thermoelectric properties of Fe/MgO/Fe(001) magnetic tunnel junctions (MTJs) by means of the linear-response theory combined with a first-principles-based Landauer-Büttiker approach. We find that the Seebeck coefficient of Fe/MgO/Fe(001) MTJs strongly depends on the barrier thickness and the tetragonal distortion. A compressive tetragonal distortion of the in-plane lattice parameter in the MTJs provides interface resonant states just above the Fermi energy. This causes resonant tunneling in the MTJs and significantly enhances the Seebeck coefficient when the thickness of the MgO barrier is around 1 nm (four or five atomic layers of MgO). Moreover, an extensive tetragonal distortion of the in-plane lattice parameter pushes the interface states away from the Fermi energy, leading to a reduction of the Seebeck coefficient. Furthermore, we find that the interface resonant tunneling enhances the power factor of the MTJs for the compressive distortion. These results indicate that control of the barrier thickness and the tetragonal distortion will be effective for maximizing the thermoelectric properties of MTJs.

Inelastic neutron scattering has been used to study the in-plane Cu-O bond-stretching mode in oxygen-doped ${\mathrm{La}}_{1.94}{\mathrm{Sr}}_{0.06}{\mathrm{CuO}}_{4.035}$ $\left(T_c=38\text{K}\right)$ and ${\mathrm{La}}_2{\mathrm{CuO}}_{4+\delta }$ $\left(T_c=43\text{K}\right)$. Similar to results from optimally doped ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_4$ $\left(T_c=35\text{K}\right)$, we observe anomalous features in the dispersion of this half-breathing mode in the form of a softening halfway through the Brillouin zone. Considering the differences in electronic structure and local environment between the oxygen- and strontium-doped compounds with similar $T_{\text{c}}$, we rule out a connection between the phonon anomaly and structural instabilities related to the specific dopant type. We interpret the phonon anomaly as a signature of correlated charge fluctuations ubiquitous in optimally doped superconductors.

As a member of the Ruddlesden-Popper $L{n}_{\mathrm{n}+1}\mathrm{N}{\mathrm{i}}_{\mathrm{n}}{\mathrm{O}}_{3\mathrm{n}+1}$ series rare-earth-nickelates, $\mathrm{P}{\mathrm{r}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$ consists of infinite quasi-two-dimensional perovskite-like Ni-O based layers. Although a metal-to-metal phase transition at $T_{pt}\approx 157\mathrm{K}$ has been revealed by previous studies, a comprehensive study of physical properties associated with this transition has not yet been performed. We have grown single crystals of $\mathrm{P}{\mathrm{r}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$ at high oxygen pressure, and report on the physical properties around that phase transition, such as heat-capacity, electric-transport, and magnetization. We observe a distinctly anisotropic behavior between in-plane and out-of-plane properties: a metal-to-metal transition at $T_{pt}$ within the a-b plane, and a metal-to-insulator-like transition along the $c$ axis with decreasing temperature. Moreover, an anisotropic and anomalous negative magnetoresistance is observed at $T_{pt}$ that we attribute to a slight suppression of the first-order transition with magnetic field. The magnetic susceptibility can be well described by a Curie-Weiss law, with different Curie constants and Pauli-spin susceptibilities between the high-temperature and the low-temperature phases. The single crystal x-ray diffraction measurements show a shape variation of the different $\mathrm{Ni}{\mathrm{O}}_6$ octahedra from the high-temperature phase to the low-temperature phase. This subtle change of the environment of the Ni sites is likely responsible for the different physical properties at high and low temperatures.

Cluster perturbation theory is applied to the two-dimensional Hubbard $t-t^{\prime }-t^{\prime \prime }-U$ model to obtain doping and temperature-dependent electronic spectral function with $4\times 4$ and 12-site clusters. It is shown that evolution of the pseudogap and electronic dispersion with doping and temperature is similar and in both cases it is significantly influenced by spin-spin short-range correlations. When short-range magnetic order is weakened by doping or temperature and Hubbard-I-like electronic dispersion becomes more pronounced, the Fermi arc turns into a large Fermi surface and the pseudogap closes. It is demonstrated how static spin correlations impact the overall dispersion's shape and how accounting for dynamic contributions leads to momentum-dependent spectral weight at the Fermi surface and broadening effects.