Semi-Dirac cones, characterized by linear-parabolic dispersions, endow photonic crystals with many fascinating properties, such as topological transitions and anisotropic electromagnetic responses. While most preceding investigations concentrated on two-dimensional systems, our exploration of three-dimensional photonic crystals comprising a cubic lattice of core-shell spheroids unveils unusual dispersions in three-dimensional systems—dual semi-Dirac cones. The dual semi-Dirac cones arising from a pair of coexisting triply degenerate modes can be analyzed by effective-Hamiltonian and effective-medium theory, accompanied by topological transitions in equal-frequency surfaces and significant changes in electromagnetic responses. We find that the photonic crystal exhibits highly anisotropic wave transport properties, i.e., drastically different transport properties for waves of different wave-vector directions, within the frequency region between the two semi-Dirac frequencies. However, at the frequencies of semi-Dirac points, the photonic crystal behaves as an effective double-zero medium for two propagation directions, but as a single-zero medium for the remaining direction, specifically for orthogonal polarizations. Our findings contribute valuable insights into three-dimensional artificial materials, presenting features absent in two-dimensional systems.

Using a pure electric current to control kagome noncollinear antiferromagnets is promising in information storage and processing, but a full description is still lacking, in particular, on intrinsic (i.e., no external magnetic fields or external spin currents) spin-orbit torques. In this work, we self-consistently describe the relations among the electronic structure, magnetic structure, spin accumulations, and intrinsic spin-orbit torques in the magnetic dynamics of a noncollinear antiferromagnet driven by a pure electric current. Our calculation can yield a critical current density comparable with those in the experiments, when considering the boost from the out-of-plane magnetic dynamics induced by the current-driven spin accumulation on individual magnetic moments. We stress the parity symmetry breaking in deterministic switching among magnetic structures. This work will be helpful for future applications of noncollinear antiferromagnets.

Polaritons are quasiparticles describing the coupling between a photon and a material excitation, which can carry large momentum and confine electromagnetic fields to small dimensions, enabling strong light-matter interactions. In the visible (VIS) to near-infrared (NIR) spectral ranges, the intraband response of metals gives rise to surface plasmon polaritons (SPPs), which have practically governed polaritonic response and its utilization in nanophotonics. Recently, the concept of interband-based VIS/NIR in-plane exciton polaritons has been introduced in two-dimensional materials, such as transition-metal dichalcogenides (TMDs), thus providing an excitonic alternative to plasmonic systems. Here, we compare the properties of such in-plane exciton polaritons supported by monolayer TMDs to the equivalent configuration of SPPs supported by thin metallic films, known as short-range SPPs (SRSPPs). Taking into account both excitonic and plasmonic nonlocal corrections, which play a major role in large momentum modes, we find that in-plane exciton polaritons provide confinement factors that are an order of magnitude larger than those of SRSPPs, and with six times lower propagation losses. In addition, we show that unlike SPPs, in-plane exciton polaritons are coupled to the TMD's valley degree of freedom, leading to directional propagation that depends on the exciton's valley. These properties make in-plane exciton polaritons promising candidates for VIS/NIR nanophotonics and strong light-matter interactions.

The charge density wave (CDW) system $1T\text{−}{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$ was investigated using neutron scattering to determine the nature of the in-plane and out-of-plane disorder. From the local structure analysis, it was deduced that the in-plane star of David motifs are distorted with Se doping just like it was previously observed in $1T\text{−}{\mathrm{TaS}}_2$. This is due to random Ta displacements away from the ideal trigonal symmetry regardless of whether the CDW state is commensurate, nearly commensurate, metallic, or insulating. In the chalcogen layer, the atoms are also distorted with displacements in the perpendicular direction to the planes. The chalcogen distortions are most pronounced in TaSSe that superconducts below 3.6 K in comparison to the distortions observed in ${\mathrm{TaS}}_2$ and ${\mathrm{TaSe}}_2$. The implications of the distortions on superconductivity and the effects on the electron pocket proposed to lead to the superconducting state are not understood at present. Unlike in $1T\text{−}{\mathrm{TaS}}_2$ in which both 13c and 3c layer stacking orders were previously reported, in the nearly commensurate metallic state of $1T$-TaSSe and the commensurate CDW metallic state of ${\mathrm{TaSe}}_2$, no clear evidence for a stacking order is observed down to 2 K.

We discuss the generic slowing down of quantum dynamics in low energy density states of spatially local Hamiltonians. Beginning with quantum walks of a single particle, we prove that for certain classes of Hamiltonians (deformations of lattice-regularized $H\propto p^{2k}$), the “butterfly velocity” of particle motion at low energies has an upper bound that must scale as $E^{(2k-1)/2k}$, as expected from dimensional analysis. We generalize these results to obtain bounds on the typical velocities of particles in many-body systems with repulsive interactions, where for certain families of Hubbard-like models we obtain similar scaling.

We develop a high-throughput computational scheme based on cluster multipole theory to identify new functional antiferromagnets (AFMs). This approach is applied to 228 magnetic compounds listed in the AtomWork-Adv database, known for their elevated Néel temperatures. We conduct systematic investigations of both stable and metastable magnetic configurations of these materials. Our findings reveal that 34 of these compounds exhibit AFM structures with zero propagation vectors and magnetic symmetries identical to conventional ferromagnets, rendering them potentially invaluable for spintronics applications. By cross-referencing our predictions with the existing MAGNDATA database and published literature, we verify the reliability of our findings for 26 out of 28 compounds with partially or fully elucidated magnetic structures in the experiments. These results not only affirm the reliability of our scheme but also point to its potential for broader applicability in the ongoing quest for the discovery of functional magnets.

We investigate through numerical simulations how a two-dimensional crystal yields and flows under an applied shear. We focus on a range that allows us to both address the response in the limit of an infinitesimal shear rate and describe the phase behavior of the system at a finite shear rate. In doing so, we carefully discuss the role of the topological defects and the finite-size effects. We map out the whole phase diagram of the flowing steady state in the plane formed by temperature and shear rate. Shear-induced melting of the two-dimensional crystal is found to proceed in two steps: first, the solid loses long-range bond-orientational order and flows, even for an infinitesimal shear rate (in the thermodynamic limit). The resulting flowing hexatic phase then melts to a flowing, rather isotropic, liquid at a finite shear rate that depends on temperature. Finally, at a high shear rate, a third regime corresponding to a strongly anisotropic stringlike flowing phase appears.

Phase diagrams exhibiting extended solid-solution and lenslike melting are often reproduced using ideal solutions, where ideal mixing considers a fully random configurational entropy of mixing. In the field of irreversible thermodynamics, experimental measurements of the composition variation of high-temperature electronic transport and molten-state properties suggest, however, a strong role for short-range atomic ordering in these systems. Herein, measurements of the thermopower and resistivity are reported for Cu-Ni solid solutions as a function of temperature and composition. The electronic transport properties were interpreted with an irreversible thermodynamic framework, revealing a large electronic contribution to the entropy of mixing. By considering a cluster model for the configurational entropy that uses the electronic contribution to inform the existence of ordered associates, we rationalize such a contribution of the electronic entropy with the ideal entropy of mixing commonly used to model such systems. These results suggest that the short-range order of the atoms plays a significant role in both solid and liquid states, even when there are no dominant intermetallic compounds in these alloys.

Just as electronic shot noise in driven conductors results from the granularity of charge and the statistical variation in the arrival times of charge carriers, there are predictions for fundamental noise in magnon currents due to angular momentum being carried by discrete excitations. The inverse spin Hall effect as a transduction mechanism to convert spin current into charge current raises the prospect of experimental investigations of such magnon shot noise. Spin Seebeck effect measurements have demonstrated the electrical detection of thermally driven magnon currents and have been suggested as an avenue for accessing spin current fluctuations. Using spin Seebeck structures made from yttrium iron garnet on gadolinium gallium garnet, we demonstrate the technical challenges inherent in such noise measurements. While there is a small increase in voltage noise in the inverse spin Hall detector at low temperatures associated with adding a magnetic field, the dependence on field orientation implies that this is not due to magnon shot noise. We describe theoretical predictions for the expected magnitude of magnon shot noise, highlighting ambiguities that exist. Further, we show that magnon shot noise detection through the standard inverse spin Hall approach is likely impossible due to geometric factors. Implications for future attempts to measure magnon shot noise are discussed.

The successful preparation of freestanding oxide perovskites down to the few-layer limit inspires the development of two-dimensional multifunctional materials exhibiting both ferroelectric and ferromagnetic properties. In this study, we performed a comprehensive first-principles investigation on the few-layer perovskite ${\mathrm{YMnO}}_3$. Our findings reveal that its monolayer is an antiferromagnetic semiconductor, while other few layers are ferromagnetic half metals. All of them have an in-plane polarization with moderate energy barrier, although the “polarization” of metallic few layers may be not switchable. The distortion modes of few-layer ${\mathrm{YMnO}}_3$ are different from its orthorhombic bulk counterpart which is paraelectric. The symmetry mode analysis reveals that the monolayer ${\mathrm{YMnO}}_3$ displays proper ferroelectric behavior. We have investigated the polarization and magnetism of few-layer polar oxide perovskites and found that they strongly depend on the number of layers. These results will contribute to our current understanding of the properties of few-layer polar oxide perovskites.

The antiskyrmion, as a topological structure, can be stabilized by anisotropic Dzyaloshinskii-Moriya interaction. We investigated the dynamic properties of antiskyrmions through micromagnetic simulations. The field-dependent evolution of the antiskyrmion expresses a square shape or an elongated shape, due to its spin configuration with the combination of Néel-type and Bloch-type walls. This spin configuration with the broken rotational symmetry leads to a notable mode coupling between the breathing mode and the azimuthal spin wave mode under microwave excitation. The same mode coupling can also exist in the skyrmion system with a small in-plane magnetic anisotropy. Our findings emphasize that the rotational symmetry of the spin configuration may play a significant role in the dynamic properties of antiskyrmions and their relevance to magnonics applications.

The quantum Hall effect, which was observed in two-dimensional (2D) electron gases under an external magnetic field, stands out as one of the most remarkable transport phenomena in condensed matter. However, a long-standing puzzle remains regarding the observation of the relativistic quantum Hall effect (RQHE). This effect, predicted for a single 2D Dirac cone immersed in a magnetic field, is distinguished by the intriguing feature of half-integer Hall conductivity (HIHC). In this work, we demonstrate that the condensed-matter realization of the RQHE and direct measurement of the HIHC are feasible by investigating the underlying quantum transport mechanism. We reveal that the manifestation of HIHC is tied to the presence of dissipative half-integer quantized chiral channels circulating along the interface between the RQHE system and a Dirac metal. Importantly, we find that the Ohmic scaling of the longitudinal conductance of the system plays a key role in directly measuring the HIHC in experiments. Furthermore, we propose a feasible experimental scheme based on three-dimensional topological insulators to directly measure the HIHC. Our findings not only uncover the distinct transport mechanism of the HIHC for the RQHE but also pave the way for the measurement of the HIHC in future experiments.

On-demand emission of individual electrons for the implementation of flying qubits and quantum electron-optics experiments requires precise knowledge and tunability of emission times and energies. Crucially, for confined electron sources such as driven quantum dots, the effect of local Coulomb interaction on these emission properties needs to be understood, in particular if multiple particles are emitted close in time or near simultaneously. This paper theoretically analyzes electron pair emission from an ac driven quantum dot, detailing the competing effects of the electron-electron interaction, the time-dependent potential forming the quantum dot, and of the quantum-state properties, such as degeneracy, on the emission times and energies. We complement a numerical analysis of the coherent Schrödinger evolution of two particles in a driven potential with a master-equation description for strongly interacting electrons tunneling stochastically into a weakly coupled conductor. This captures a broad range of different influences on the emitted particles and thereby guides the development of single-electron sources with higher control over two-particle emission properties.

We analyze the influence of disorder and strong correlations on the topology of two-dimensional Chern insulators. A mean-field calculation in the half-filled Haldane model with extended Hubbard interactions and Anderson disorder shows that the disorder favors topology in the interacting case and extends the topological phase to a larger region of the Hubbard parameters. In the absence of a staggered potential, we find a novel disorder-driven topological phase with Chern number $C=1$, with the coexistence of topology with long-range spin and charge orders. More conventional topological Anderson insulating phases are also found in the presence of a finite staggered potential.