Magnetization can be induced by an electric field in systems without inversion symmetry $\mathcal{P}$ and time-reversal symmetry $\mathcal{T}$. This phenomenon is called the magnetoelectric (ME) effect. The spin ME effect has been actively studied in multiferroics. The orbital ME effect also exists and has been mainly discussed in topological insulators at zero temperature. In this paper, we study the intrinsic orbital ME response in metals at finite temperature using the Kubo formula. The intrinsic response originates from the Fermi sea and does not depend on the dissipation. Especially in systems with $\mathcal{PT}$ symmetry, the extrinsic orbital ME effect becomes zero, and the intrinsic ME effect is dominant. We apply the response tensor obtained in this work to a $\mathcal{PT}$-symmetric model Hamiltonian with antiferromagnetic loop current order, demonstrating that the intrinsic ME effect is enhanced around the Dirac points.

Non-Hermiticity can alter the topological phases of Hermitian lattices and induce gapped edge states and in-gap corner states for the topologically trivial structures in Hermitian cases. To date, however, the realizations of non-Hermiticity-induced higher-order topological phases have often needed intricate designs, where the non-Hermitian systems on a square lattice are comprised of 16 atoms per unit cell with staggered on-site non-Hermitian components at least. It is still necessary to further investigate their topological characteristics in both Hermitian and non-Hermitian systems. In this paper, we suggest an extended quadrupole topological insulator that contains eight atoms in each unit cell and whose topological phases in Hermitian cases are governed by the couplings between adjacent atoms. By introducing external losses into such a higher-order topological insulator, we show that therein the trivial lattices can also give rise to nontrivial quadrupole topology with higher-order corner states. Calculating the biorthogonal nested Wilson loop characterizes the topology for such a non-Hermitian system. Interestingly, the non-Hermitian system has four Wannier bands, with two near zero and two deviating from zero and distributing symmetrically in the negative and positive sections, implying a topology similar to the nontrivial phase in the Hermitian case. Furthermore, both the positive and negative Wannier bands exhibit a nontrivial polarization of 0.5 whereas the remain Wannier bands near zero carry a trivial polarization, indicating that the non-Hermitian system hosts a nontrivial quadrupole topology. We also propose a complete design with acoustic resonators to implement the homologous non-Hermitian topology in acoustic systems to validate our theory. The numerical results match the analytical solution perfectly. Our work introduces an alternative method for fabricating non-Hermitian-induced quadrupole topological insulators and paves the way for future research into the non-Hermitian higher-order topology.

We calculate the lattice dielectric function of strongly anharmonic rutile ${\mathrm{TiO}}_2$ from ab initio anharmonic lattice dynamics methods. Since an accurate calculation of the $\mathrm{\Gamma }$ point phonons is essential for determining optical properties, we employ the modified self-consistent approach, including third-order anharmonicity as well as fourth-order anharmonicity. The resulting optical phonon frequencies and linewidths at the $\mathrm{\Gamma }$ point agree much better with experimental measurements than those from a perturbative approach. We show that the four-phonon scattering process contributes as much as the third-order anharmonic term to phonon linewidths of some phonon modes. Furthermore, incorporating the frequency dependence of phonon linewidth reveals that experimentally known but unidentified peaks of the dielectric function are due to the two-phonon process. This work emphasizes the importance of the self-consistent approach in predicting the optical properties of highly anharmonic materials.

A gap exists in the microscopic understanding the dynamic properties of the rotational organic cation (ROC) in the inorganic framework of the metal halide perovskites (MHP) to date. Herein, we develop a microscopic theory of Raman scattering for the ROC in MHP based on the angular momentum of a ROC exchanging with that of the photon and phonon. We systematically present the selection rules for the angular momentum transfer among the three lowest rotational levels. We find that the phonon angular momentum that arises from the inorganic framework and its specific values could be directly manifested by Stokes (or anti-Stokes) shift. Moreover, the initial orientation of the ROC and its preferentially rotational directions could be judged in Raman spectra. This study lays the theoretical foundation for the high-precision resolution and manipulation of molecular rotation immersed in the many-body environment by Raman technique.

Spin-related unidirectional magnetoresistance (UMR) exhibits nonreciprocal transport to either charge current or magnetization reversal, providing a potential application for direct electrical readout of in-plane magnetization using simple two-terminal geometry. To achieve such a UMR, it is usually believed that an adjacent heavy metal or other materials with strong spin-orbit coupling (SOC) as the spin polarizer is indispensable, leading to most efforts on multilayer structures rather than single-layer ferromagnetic films. Here, we report the observation of UMR in a single CoPt film by introducing a vertical composition gradient to break the structural inversion symmetry. Moreover, unlike conventional heavy-metal/ferromagnetic-metal bilayer films, the UMR in the CoPt film with a positive Co composition gradient is controllable as a function of current amplitude, which shows the decrease, sign reversal, and then enhancement with the increment of current density. These results reveal two competing UMR mechanisms with opposite signs in electrical current dependency, i.e., bulk Rashba effect induced by composition gradients and anomalous Nernst effect driven by a vertical temperature gradient. Our work provides a promising way to manipulate the UMR in a single ferromagnetic film with the combination of charge-spin conversion and magnetothermal effect.

Noncentrosymmetric $\mathrm{Cu}{\mathrm{B}}_2{\mathrm{O}}_4$ is a prototypical system with interacting spin sublattices of different dimensionality, exhibiting diverse magnetic structures and magnetic phase transitions. Here we report pressure-induced structural evolution of $\mathrm{Cu}{\mathrm{B}}_2{\mathrm{O}}_4$ and its effect on the magnetic phase diagram. Powder x-ray diffraction (XRD), Raman scattering, and infrared phonon measurements have been performed under high pressure at room temperature. Although XRD shows lattice stability up to $\sim 25\mathrm{GPa}$, Raman and IR phonon measurements reveal structural anomaly near 3 GPa, followed by a structural phase transition at $\sim 12\mathrm{GPa}$. Pressure evolution of the zero-phonon lines in the low-temperature optical absorption studies helps identifying the symmetry change of the Cu sites modifying the crystal-field energy levels at the structural transition. High-pressure magnetization measurements have been performed with magnetic field applied perpendicular to the tetragonal $c$ axis. With increasing pressure the saturation magnetization of the commensurate weak ferromagnetic phase increases with systematic reduction of its turnup critical field. Pressure-induced systematic suppression of the dc susceptibility in this phase is attributed to the reduced critical field, indicating pressure-driven enhanced stability of the weak ferromagnetic phase at high pressure. Whereas, below 10 K the critical field of the incommensurate-commensurate transition increases with pressure, indicating enhanced stability of the incommensurate helical order at high pressure.

In the current quest to synthesize hyperuniform materials with interesting applications, addressing the coupling of the objects composing the system to the physical properties of the host medium is crucial. With this aim we study a model system: vortices in the FeSe superconductor subject to a considerable magnetoelastic coupling with the host sample. We reveal that the low-field FeSe vortex structure is of the weakest hyperuniform type possibly due to the relevance of the anisotropic and long-ranged interaction term introduced by the magnetoelastic coupling. This work indicates that it is possible to tailor the hyperuniformity class of material systems by tuning the coupling of interacting objects with elastic properties of the host medium.

The behavior of natural monazite $\left(\mathrm{La},\mathrm{Ce},\mathrm{Nd}\right)\mathrm{P}{\mathrm{O}}_4$ at high pressure is investigated by Raman spectroscopy. Changes in the Raman spectra indicate a phase transition from monazite to a post-monazite phase. Single-crystal x-ray diffraction measurements of the decompressed sample reveal that the post-monazite phase has a distorted barite-type structure with space group $P{2}_1/n$. This phase transition is irreversible under quasihydrostatic conditions, and is reversible under nonhydrostatic conditions. Therefore, it is expected that the post-monazite phase cannot be found as a metastable phase at ambient conditions in impact craters due to nonhydrostatic conditions during shock metamorphism, supporting recent mineralogical observations in impact craters.

We present a physically motivated strategy for the construction of training sets for transferable machine learning interatomic potentials. It is based on a systematic exploration of all possible space groups in random crystal structures, together with deformations of cell shape, size, and atomic positions. The resulting potentials turn out to be unbiased and generically applicable to studies of bulk defects without including any defect structures in the training set or employing any additional active learning. Using this approach we construct transferable potentials for pure magnesium that reproduce the properties of hexagonal closed packed (hcp) and body centered cubic (bcc) polymorphs very well. In the process we investigate how different types of training structures impact the properties and the predictive power of the resulting potential.

The effective magnetic fields induced by spin-orbit torque (SOT), Dzyaloshinsky-Moriya exchange interaction, and magnetization damping in chiral domain wall systems are very important to manipulate magnetization switching, stimulate magnetization oscillation, and drive domain wall motion in the applications of various spintronic devices. However, magnetization damping as an effective magnetic field is usually ignored in magnetization switching measured by magnetic hysteresis loops due to its dynamical and instantaneous feature. Here, we demonstrate that in addition to the dampinglike SOT effective field, the perpendicular effective field measured by the shift of Hall magnetic hysteresis curves also contains the $z$-direction effective fields originating from magnetization damping under the total static magnetic field. These findings provide key insights for understanding the effective magnetic fields originated from the magnetic damping and dampinglike SOT effective field in chiral domain walls, and pave the path to practical application of the chiral domain wall systems.

We consider a two-dimensional generalization of the Su-Schrieffer-Heeger model which is known to possess a nontrivial topological band structure. For this model, which is characterized by a single parameter, the hopping ratio $0\le r\le 1$, the inhomogeneous superconducting phases induced by an attractive-$U$ Hubbard interaction are studied using mean-field theory. We show, analytically and by numerical diagonalization, that in lattices with open boundaries, phases with enhanced superconducting order on the corners or the edges can appear, depending on the filling. For finite samples at half filling, the corner site superconducting transition temperature can be much larger than that of the bulk. A novel proximity effect thus arises for $T_{c,\mathrm{bulk}}<T<T_{c,\mathrm{corner}}$, in which the corner site creates a nonzero tail of the superconducting order in the bulk. We show that such tails should be observable for a range of $r$ and $U$ values.

Coherent optical excitation of certain phonon modes in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ has been shown to induce superconducting-like interlayer coherence at temperatures higher than $T_c$. Recent work has associated these phenomena to a parametric excitation and amplification of Josephson plasma polaritons, which are overdamped above $T_c$ but are made coherent by the phonon drive. However, the dissipative response of uncondensed quasiparticles, which do not couple in the same way to the phonon drive, has not been addressed. Here, we investigate both the enhancement of the superfluid density, $\omega {\sigma }_2\left(\omega \right)$, and the dissipative response of quasiparticles, ${\sigma }_1\left(\omega \right)$, by systematically tuning the duration and energy of the mid-infrared pulse while keeping the peak field fixed. We find that the photoinduced superfluid density saturates to the zero-temperature equilibrium value for pulses made longer than the phonon dephasing time, while the dissipative component continues to grow with increasing pulse duration. We show that superfluid and dissipation remain uncoupled as long as the drive is on, and identify an optimal regime of pump pulse durations for which the superconducting response is maximum and dissipation is minimized.

Electromagnonics is an emerging field with a focus on entangling magnonic excitations to the microwave cavity photon modes with the prospect for use in quantum information science. Here, we discuss a class of Hamiltonians that embody a substantial steady-state photon-magnon entanglement enabled by a chiral coupling of the magnonic system to the cavity electric field. It is demonstrated how the entanglement can be controlled via external parameters. As a realization, we study a layered system that hosts an interfacial Dzyaloshinskii-Moriya interaction whose strength varies linearly with the cavity electric field rendering the low-energy spin excitations susceptible to an electric field and resulting in nonlinear magnon-photon dynamics. Accounting for interactions with the environment, we derive from the stochastic quantum Langevin equations explicit expressions evidencing the existence of a finite, steady-state entanglement and detailing its dependencies on external probes. The results point to particular types of electromagnonic systems that are potentially useful for quantum information applications.

We theoretically propose that hexagonal silicon-based crystals $P6/m\text{−}{\mathrm{Si}}_6$ and $P6/m\text{−}{\mathrm{NaSi}}_6$ are topological Dirac semimetals with superconducting critical temperatures of 12 K and 13 K, respectively, at ambient pressure. Band inversion occurs with the Fu-Kane topological invariant ${\mathbb{Z}}_2=1$, even in the absence of spin-orbit coupling. The Dirac nodes protected by $C_6$ crystal rotational symmetry remain gapless with spin-orbit coupling. Using first-principles calculations, we find pressure-induced topological phase transitions for $P6/m\text{−}{\mathrm{Si}}_6$ and $P6/m\text{−}{\mathrm{NaSi}}_6$ with critical external pressures of 11.5 GPa and 14.9 GPa, respectively. Above the critical pressures, the Dirac bands are gapped with ${\mathbb{Z}}_2=0$, while the superconducting states and the crystal symmetries are retained. Our results may shed light into a search for silicon-based topological materials with superconductivity.

Determining the (bare) electron mass $m_0$ in crystals is often hindered by many-body effects since Fermi-liquid physics renormalizes the band mass, making the observed effective mass $m^{*}$ depend on density. Here, we use a one-dimensional (1D) geometry to amplify the effect of interactions, forcing the electrons to form a nonlinear Luttinger liquid with separate holon and spinon bands, therefore separating the interaction effects from $m_0$. Measuring the spectral function of gated quantum wires formed in GaAs by means of magnetotunnelling spectroscopy and interpreting them using the 1D Fermi-Hubbard model, we obtain $m_0=(0.0525\pm 0.0015)m_{\text{e}}$ in this material, where $m_{\text{e}}$ is the free-electron mass. By varying the density in the wires, we change the interaction parameter $r_{\text{s}}$ in the range from $\sim 1$–4 and show that $m_0$ remains constant. The determined value of $m_0$ is $\sim 22\%$ lighter than observed in GaAs in geometries of higher dimensionality $D$ ($D>1$), consistent with the quasiparticle picture of a Fermi liquid that makes electrons heavier in the presence of interactions.

Recent research developments in the area of spacetime metamaterial structures and systems have raised new questions as to how the physics of fundamental phenomena is altered in the presence of spacetime modulation. In this context, we present a generalized and comparative description of the phenomenon of total internal reflection (TIR) at different dynamic interfaces. Such interfaces include, beyond the classical interfaces corresponding to the boundaries of moving bodies (moving interface–moving matter systems), interfaces formed by a traveling-wave step modulation of an electromagnetic parameter (e.g., refractive index) (moving interface–stationary matter systems) and fixed interfaces between moving-matter media (stationary interface–moving matter systems). We first resolve the problem using the evanescence of the transmitted wave as the criterion for TIR and applying the conventional technique of relative frame hopping (between the laboratory and rest frames), which results in closed-form formulas for the relevant critical (incidence, reflection, phase refraction, and power refraction) angles. We then introduce the concept of catch-up limit between the dynamic interface and the transmitted wave as an alternative criterion for the critical angle. We use this approach both to analytically verify the critical angle formulas, further validated by full-wave (FDTD) analysis, and to explain the related physics, using Fresnel-Fizeau drag and spacetime frequency transition considerations. These results might find various applications in ultrafast optics, gravity analogs, and quantum processing.

The Feynman path-integral variational approach to the polaron problem, along with the associated Feynman-Hellwarth-Iddings-Platzman (FHIP) linear-response mobility theory, provides a computationally amenable method to predict the frequency-resolved temperature-dependent charge-carrier mobility, and other experimental observables in polar semiconductors. We show that the FHIP mobility theory predicts non-Drude transport behavior, and shows remarkably good agreement with the recent diagrammatic Monte Carlo mobility simulations of Mishchenko et al. [Phys. Rev. Lett. 123, 076601 (2019)] for the abstract Fröhlich Hamiltonian. We extend this method to multiple phonon modes in the Fröhlich model action. This enables a slightly better variational solution, as inferred from the resulting energy. We carry forward this extra complexity into the mobility theory, which shows a richer structure in the frequency and temperature-dependent mobility, due to the different phonon modes activating at different energies. The method provides a computationally efficient and fully quantitative method of predicting polaron mobility and response in real materials.

Both quantum transport measurements in the Pauli blockade regime and microwave cavity transmission measurements are important tools for spin-qubit readout and characterization. Based on a generalized input-output theory we derive a theoretical framework to investigate how a double quantum dot (DQD) in a transport setup interacts with a coupled microwave resonator while the current through the DQD is rectified by Pauli blockade. We show that the output field of the resonator can be used to infer the leakage current and thus obtain insight into the blockade mechanisms. In the case of a silicon DQD, we show how the valley quasidegeneracy can impose limitations on this scheme. We also demonstrate that a large number of unknown DQD parameters including (but not limited to) the valley splitting can be estimated from the resonator response simultaneous to a transport experiment, providing more detailed knowledge about the microscopic environment of the DQD. Furthermore, we describe and quantify a back action of the resonator photons on the steady-state leakage current.

Although the non-Bloch band theory is a milestone in elaborating bulk energy bands of non-Hermitian systems under the open-boundary condition (OBC), vital issues related to multivalued functions of non-Hermitian energy bands remain unsolved. In this paper, we anatomize the bulk properties of one-dimensional multiband non-Hermitian systems under OBC. We put forward the energy-band branches (EBBs) to settle the multivalued functions of non-Hermitian energy bands, which become gapped or gapless corresponding to disconnected or connected EBBs in the complex energy plane, where the branch points and branch cuts play a crucial role. We clarify the precise significance of the non-Hermitian skin effect, which illustrates the asymptotic behavior of EBB eigenstates (bulk eigenstates) in the deep bulk and compensates previous non-Bloch band theory. We also obtain a general expression of open-boundary Green's functions based on such EBBs and generalized Brillouin zones, useful for studies on non-Hermitian dynamical evolution.