In this paper, a sufficient reciprocity condition for general time-periodic modulated bianisotropic media is extracted from first principles. Reciprocity of various cases of significant importance, including stationary bianisotropic media, time-varying (TV) isotropic media, TV anisotropic media, and TV bianisotropic media, are investigated using this condition. We prove that synchronous time modulation of stationary bianisotropic yet reciprocal media (chiral, pseudochiral, and achiral) does not lead to nonreciprocity, unless the modulation function breaks time reversal symmetry. This is in contrast to recently published research. The theoretical results are validated using in-house finite difference time domain simulations.

Using bilayer films of $\beta$-Ta (5 nm)/${\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}$ (2 nm), we fabricate elliptical structures which exhibit uniaxial magnetic anisotropy, resulting in single magnetic domain behavior. We study induced spin-orbit torques (SOTs) in these devices with first- and second-order harmonic Hall measurements for current flowing along the long axis of the ellipses and external magnetic field applied in the film plane. We observe a giant response to the SOTs associated with magnetization reversal of the NiFe layer, and we correlate it quantitatively with a $d{\phi }_m/d{\phi }_H$ term, where ${\phi }_m$ and ${\phi }_H$ are the in-plane angles of the magnetization and the magnetic field, respectively. We discuss theoretical and applicative implications of this intriguing behavior.

Dispersion of acoustic plasmons and isotropic Landau parameters are calculated in three- and two-dimensional two-component electron-electron and electron-hole liquids at various concentration and mass ratios using the Landau-Silin kinetic equation and the random phase approximation for the self-energy. It is shown that the mode propagation and the strength of quasiparticle interaction are determined by the intercomponent screening and are asymmetric with respect to charge composition of two-component liquid. The well-defined acoustic plasmon-zero sound mode arises at strong difference in concentrations between components and its renormalization by the short-range exchange-correlation interaction is negligible. The acoustic plasmon mediated interparticle effective interaction in the fast component is weak in both the three- and two-dimensional electron liquids with parabolic dispersion, so the associated plasmonic superconductivity and the formation of acoustic plasmarons are unfavorable.

In this paper, the superconducting and normal state properties of ${\mathrm{TlBi}}_2$ with the ${\mathrm{AlB}}_2$-type structure were studied by resistivity, magnetization, and specific heat measurements. It was found that bulk superconductivity with $T_c=6.2$ K emerges in ${\mathrm{TlBi}}_2$, which is a phonon-mediated $s$-wave superconductor with a strong electron-phonon coupling (${\lambda }_{ep}=1.38$) and a large superconducting gap (${\mathrm{\Delta }}_0/k_B{T}_c=2.25$). We found that the $\rho (T$) exhibits an unusual $T$-linear dependence above 50 K, and can be well described by the Fermi-liquid theory below 20 K. Interestingly, its Kadowaki-Woods ratio $A/{\gamma }^2 [9.2\times {10}^{-5}\mathrm{\mu \Omega }\text{cm}{(\text{mol}{\mathrm{K}}^2/\mathrm{mJ})}^2]$ is unexpectedly one order of magnitude larger than that obtained in many heavy fermion compounds, although the electronic correlation is not so strong.

Quantum dots (QDs) acting as single-photon emitters in the blue-green range are fabricated and characterized at cryogenic temperature. They consist in CdSe dots inserted in (Zn,Mg)Se nanowires with a thick shell. Photoluminescence spectra, decay curves, and autocorrelation functions were measured under nonresonant continuous-wave and pulsed excitation. An analytical approach is applied simultaneously to the decay curves and correlation functions. It allows a quantitative description of how these two quantities are affected by the exciton rise due to biexciton feeding, the bright exciton decay, the effect of the dark exciton, and the reexcitation between two laser pulses. Linewidths at our limit of resolution (200 $\mu \mathrm{eV}$) are recorded. The reported correlation counts vary from a full control by reexcitation from traps, to a small contribution of reexcitation by mobile carriers or other QDs, as low as 5%.

We study a two-dimensional topological system driven out of equilibrium by the application of elliptically polarized light. In particular, we analyze the Bernevig-Hughes-Zhang model when it is perturbed using an elliptically polarized light of frequency $\mathrm{\Omega }$ described in general by a vector potential $\mathbf{A}\left(t\right)=(A_{0x}cos\left(\mathrm{\Omega }t\right),A_{0y}cos(\mathrm{\Omega }t+{\varphi }_0))$. Even for a fixed value of ${\varphi }_0$, we can change the topological character of the system by changing the $x$ and $y$ amplitudes of the drive. We therefore find a rich topological phase diagram as a function of $A_{0x}$, $A_{0y}$, and ${\varphi }_0$. In each of these phases, the topological invariant given by the Chern number is consistent with the number of spin-polarized states present at the edges of a nanoribbon.

Moiré lattices formed from semiconductor bilayers host tightly localized excitons that can simultaneously couple strongly to light and possess large electric dipole moments. This facilitates the realization of new forms of polaritons that are very strongly interacting and that have been predicted to lead to strong optical nonlinearities controlled by multiphoton resonances. Here we investigate the role of the nonlocal component of the exciton-exciton (dipolar) interactions on the optical response of these strongly interacting moiré exciton-polaritons under conditions of strong optical driving. We find that the nonlocal interactions can strongly influence the steady-state properties leading to multistabilities with spontaneously broken translational symmetry and pronounced distortions of the multiphoton resonances. We develop a self-consistent approach to describe the steady-state solution of moiré excitons coupled to a cavity field, treating the long-range interaction between the excitons and the photon field at the semiclassical level.

Advances in experimental tools allow to study quantum fidelity in unprecedented controlled settings. While the fidelity in metals, when adding a local impurity, is well known to show the Anderson orthogonality catastrophe (AOC), there remain outstanding questions in other quantum phases and settings. Here, we aim to tackle these by exploring systematically the ground-state fidelity of the Fermi liquid in the (extended) Aubry-André (AA) model, which allows to explore the AOC in both localized extended and critical phases. We discover that the AOC is typically exponential in the critical regime of the AA model and at the mobility edge of the extended AA model for an extended impurity, while it decays in the AA model with a power law for a weak single-site impurity. We explain this in terms of critical correlations and multipoint correlations. The OC is found to be exponential in the insulating regime, due to a fundamentally different, statistical mechanism, which is explained in detail. Furthermore, we consider a parametric perturbation to the AA model, and find an exponential OC numerically, in agreement with an analytical derivation which we provide here.

Recently it was demonstrated theoretically and experimentally that the presence of a moving conductor can break the reciprocity in the interactions between magnetic dipoles. In this article we investigate the influence of nonreciprocity on the dynamics of a pair of rigid $XY$ dipoles, which have been realized in experiments. In particular, we focus on the energy nonconservation, which is a consequence of the nonreciprocity. We find that the dynamics indeed has regimes, wherein the kinetic energy grows quadratically. However, whether energy absorption occurs depends strongly on the initial conditions on the dipoles. Simulations for various initial conditions reveal an intricate dependence, resulting in a rich structure of the energy absorbing regime in the initial condition space. Nevertheless, we provide a qualitative explanation of these observations, interpreting the absence of energy absorption as a confinement of the dynamics in phase space.

This study reports the observation of ferromagnetic (FM) order in the non–Au-based approximant crystals (ACs) using an approach whereby a total electron-per-atom ($e/a$) ratio of the spin-glass ${\mathrm{Ga}}_{50}{\mathrm{Pd}}_{36}{\mathrm{Gd}}_{14}$ 1/1 AC is lowered by simultaneously substituting certain ratios of a trivalent Ga and a zero-valent Pd by a monovalent Au. The emergence of FM order by this method was confirmed via magnetic susceptibility, magnetization, and specific-heat measurements. The findings of this study open up opportunities in developing long-range magnetic orders from stoichiometric ACs, quasicrystals, and even other (Ruderman–Kittel–Kasuya–Yosida) RKKY compounds with spin-glass ground state.

Multiple-$Q$ magnetic states, such as a skyrmion crystal, become a source of unusual transport phenomena and dynamics. Recent theoretical and experimental studies clarified that such multiple-$Q$ states ubiquitously appear under different crystal structures in metals and insulators. Toward a systematic understanding of the formation of the multiple-$Q$ states in various crystal systems, in this theoretical study we present a low-energy effective spin model with anisotropic exchange interactions in momentum space. We summarize symmetry rules for nonzero symmetric and antisymmetric anisotropic exchange interactions in momentum space, which are regarded as an extension of Moriya's rule. According to the rules, we construct the effective spin model for tetragonal, hexagonal, and trigonal magnets with primitive-lattice and multisublattice structures based on the symmetry of the crystal and wave vector. Furthermore, we describe the microscopic origin of the effective anisotropic exchange interactions in itinerant magnets by perturbatively analyzing a multiband periodic Anderson model with the spin-orbit coupling. We apply the effective spin model to an itinerant magnet in a $P6/mmm$ crystal, and we find various multiple-$Q$ states with a spin scalar chirality in the ground state. Our results provide a foundation of constructing effective phenomenological spin models for any crystal systems hosting the multiple-$Q$ states, which will stimulate further exploration of exotic multiple-$Q$ states in materials with the spin-orbit coupling.

We report our mechanical point-contact spectroscopy (MPCS) study on single-crystalline superconductor PdSb with $T_c\sim$ 1.32 K and ${\mu }_0{H}_{c2}\left(0\right)\sim$ 900 Oe, whose $T_c$ changes very slightly against hydrostatic pressure below 2.5 GPa. For MPCS with a soft Au tip, the Andreev reflection signals suggest the same superconducting temperature $T_c$ as in the pristine PdSb crystal and the conductance curves show a fully gapped behavior, yielding a superconducting gap value of ${\mathrm{\Delta }}_0=$ 0.23 meV with $2{\mathrm{\Delta }}_0/k_{\mathrm{B}}{T}_c\sim$ 4.25 in the strong-coupling regime. However, for MPCS with a hard W tip, an enhanced superconducting $T_c$ can be observed with a concomitant increase in the superconducting gap $\mathrm{\Delta }$ in a linear relation of 0.109 meV/kelvin, whereas $2{\mathrm{\Delta }}_0/k_{\mathrm{B}}{T}_c$ decreases from 4.25 to 3.5 in a negative correlation with $T_c$. From first-principles calculations, we observe a Lifshitz transition related to Pd-$4d_{z^2}$ orbitals under uniaxial pressure, which is absent under hydrostatic pressure. Such Lifshitz transition results in an enhanced density of states near the Fermi level $n\left(E_F\right)$, which is in line with the enhanced $T_c$. In addition, a soft phonon mode at $H$ is hardened under uniaxial pressure, suggesting a reduced coupling strength in this case. Such local strain effect provides a natural explanation for our MPCS results on PdSb.

Bragg coherent diffractive imaging (BCDI) is a noninvasive microscopy technique that can visualize the shape and internal lattice deviations of crystals with nanoscale spatial resolution and picometer deformation sensitivity. Its strain imaging capability relies on Fourier transform–based iterative phase retrieval algorithms, which are mostly developed under the kinematical approximation. Such approximation prohibits the application of BCDI on larger crystals, which are commonly seen in most emerging functional materials. Understanding the dynamical effect in BCDI, as well as developing a validated method for modeling BCDI at the dynamical diffraction limit, is crucial for applying BCDI to hierarchical systems that contain micron-sized crystals and grains. Here we report a comparative study on the impact of dynamical diffraction effects by comparing the reconstruction results from two measurements of the same crystal. Forward simulation is implemented to show subtle changes of interference fringes in the diffraction pattern due to the dynamical diffraction, and is compared directly with the experimental data.

We study the disorder-induced deterministic dispersion of particles uniformly driven in an array of narrow tracks. For different toy models with quenched disorder we obtain exact analytical expressions for the steady-state mean velocity $v$ and the dispersion constant $D$ for any driving force $f$ above the depinning threshold. For short-range correlated pinning forces we find that at large drives $D\sim 1/v$ for random-field type of disorder while $D\sim 1/v^3$ for the random-bond type. We show numerically that these results are robust: the same scaling holds for models of massive damped particles, soft particles, particles in quasi-one-dimensional or two-dimensional tracks, and for a model of a magnetic domain wall with two degrees of freedom driven either by electrical current or magnetic field. Crossover and finite-temperature effects are discussed. The universal features we identify may be relevant for describing the fluctuating dynamics of stable localized objects such as solitons, superconducting vortices, magnetic domain walls and skyrmions, and colloids driven in quasi-one-dimensional track arrays. In particular, the drive dependence of $D$ appears as a sensitive tool for characterizing and assessing the nature of disorder in the host materials.

Magnetization dynamics of bismuth-substituted yttrium iron-gallium garnet film $\left({\mathrm{Y}}_{3-z}{\mathrm{Bi}}_z\right)\left[{\mathrm{Fe}}_{2-x}{\mathrm{Ga}}_x\right]\left[{\mathrm{Fe}}_{3-y}{\mathrm{Ga}}_y\right]{\mathrm{O}}_{12}$ has been studied using a terahertz pump–optical probe spectroscopy. Two magnetic modes are observed whose frequencies intersect at a magnetization compensation point. The experimental dependence of the excited modes on terahertz pulse polarization, an external magnetic field and the temperature are analyzed. The theoretical description based on symmetry and the Lagrangian formalisms is proposed. Magnetic modes crossing is explained as interplay between exchange and anisotropy energies equally contributing near the compensation point. Simulations based on the Landau-Lifshitz-Gilbert equations show that a difference in magneto-optical susceptibilities of tetrahedral and octahedral iron sublattices can substantially enhance the sensitivity to the magnetic modes.

Cavity-mediated magnon-magnon coupling can lead to a transfer of spin-wave excitations between two spatially separated magnetic samples. In this paper, we present a scheme to substantially amplify this transfer efficiency through the application of a two-photon parametric drive on the cavity. Further, we enunciate the physical origin of the enhancement. The recurrent multiphoton absorption by the cavity opens up an infinite ladder of accessible energy levels, which can induce higher-order transitions within the magnon Fock space. This is reflected in a heightened spin-current response from one of the magnetic samples when the neighboring sample is coherently pumped. The enhancement induced by the parametric drive can be considerably high within the stable dynamical region. Specifically, near the periphery of the stability boundary, the spin current is amplified by several orders of magnitude. Such striking enhancement factors are attributed to the emergence of parametrically induced strong coherences precipitated by a long-lived mode, which is analogous to a bound state in continuum. While contextualized in magnonics, the generality of the principle would allow applications to energy transfer between systems contained in parametric cavities.

Hydrodynamic electrons flowing through a two-dimensional channel are predicted to undergo a plasma instability above a critical drift velocity. This Dyakonov-Shur (DS) instability terminates as a coherent nonlinear oscillator, which shows promise as a source of radiation that could fill the so-called terahertz gap. In this work, we study radial flow in a Corbino disk, and demonstrate how the DS instability is substantially enhanced in this geometry, both in terms of a lower critical drift velocity and a higher generated power. Interestingly, hydrodynamic electron flows were recently reported in a graphene sample of this geometry, and our results are therefore directly relevant to current efforts to detect this experimentally elusive phenomenon. The analysis is based on a hydrodynamic approach and features both linearized calculations as well as full numerical simulations of the Navier-Stokes equation.

We show that the quantum anomalous Hall effect almost always occurs in magnetic topological insulator thin films whenever the top and bottom surface layer magnetizations are parallel at some temperature, independent of the interior layer magnetization configuration. Using this criterion, we identify structures that have a quantum anomalous Hall effect even though they have collinear magnetic structures with no net magnetization, and discuss strategies for realizing these interesting magnetic states experimentally.

Plasmonic materials that exhibit excellent plasmonic behavior at high temperatures would greatly expand existing applications, but the search for such materials is ongoing. Transition metal nitrides with good conductivity and high-temperature stability are promising candidates for temperature-dependent plasmonic applications. Here, we systematically investigate the temperature-dependent surface plasmon and hot carrier properties of TiN and VN. Significantly, high temperatures significantly affect the phonon-assisted intraband transition processes. Meanwhile, TiN and VN exhibit efficient optical absorption and low surface plasmon loss at different temperatures, which are comparable to or even higher than Ag. In particular, the surface plasmon response of TiN extends to visible light, making it an excellent candidate for low-loss and broadband plasmonic applications at high temperatures. In contrast, VN exhibits a much narrower plasmonic window, which is mainly suitable for infrared low-loss plasmonic devices. For hot carriers, the energy and probability distributions of hot holes in TiN and hot electrons in VN exhibit robustness to temperature. Meanwhile, transport properties indicate that the temperature effect on hot holes is significantly higher than that on hot electrons in TiN and VN. Therefore, we expect that the reported results will provide theoretical guidance for the design of next-generation high-temperature plasmonic devices.