Extensive research has explored the optical properties of topological insulating materials, driven by their inherent stability and potential applications. In this study, we unveil a functionality of three-dimensional integer quantum Hall (3D IQH) states as broadband filters for circularly polarized light, particularly effective in the terahertz (THz) frequency range under realistic system parameters. We also investigate the impact of practical imperfections, demonstrating the resilience of this filtering effect. Our findings reveal that this phenomenon is independent of the microscopic origin of the 3D IQH state, prompting discussions of its feasibility across diverse candidate materials. These results contribute to our understanding of fundamental optical properties and hold promise for practical applications in optical technologies.

Ultrashort laser pulses can be used to manipulate quantum materials and probe the ultrafast dynamics, where the pump photon energy can strongly affect the light-matter coupling mechanisms. Here, by pumping hole-doped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ using a low photon energy covering 135–330 meV, we reveal different excitations and relaxation dynamics upon above-gap and below-gap pumping by time- and angle-resolved photoemission spectroscopy. The above-gap pumping mainly leads to the injection of photoexcited carriers into the bulk conduction band and unoccupied surface states, while below-gap pumping leads to multiple sets of light-field dressed sidebands. In addition, the excited surface states above the Fermi energy also exhibit distinctive relaxation dynamics due to different relaxation channels involving intervalley, interband, and intraband scatterings. Our work reveals the essential role of pump photon energy on the light-matter coupling mechanisms, evolving from above-gap carrier injection to below-gap light-field dressing with distinct relaxation dynamics.

Exchange interactions govern the ordering between microscopic spins and the highest-frequency spin excitations—magnons—at the edge of the Brillouin zone. As well known from spontaneous Raman scattering (RS) experiments in antiferromagnets, such magnons couple to light in the form of two-magnon modes—pairs of magnon with opposite wavevectors. Experimental works on two-magnon modes driven by exchange perturbation in impulsive stimulated Raman scattering (ISRS) experiment posed a question about consistency between spin dynamics measured in the ISRS and RS experiments. Here, based on an extended spin correlation pseudovector formalism, we derive the analytical expression for observables in both types of experiments to determine possibly fundamental differences between the detected two-magnon spectra. We find that in both cases, the magnons from the edge of the Brillouin zone give the largest contribution to the measured spectra. However, the difference in the spectra stems from the fact that RS probes a population of a continuum of incoherent modes, while in the case of impulsively driven modes, they are coherent and their phase and amplitudes are detected. We show that for the continuum of modes, the sensitivity to the phase results in a relative shift of the main peaks in the two spectra, and the spectrum of the ISRS is significantly broadened and extends to the range above the maximum two-magnon mode frequency. Formally, this is manifested in the fact that the RS is described by an imaginary part of the Green function only, while the ISRS is described by the absolute value and hence additionally carries information about the real part of the Green function. We further derive two-magnon Raman tensor dispersion and the weighting factors, which define the features of the coupling to light of the modes from the different regions in the Brillouin zone. For more complex antiferromagnets, this potentially can be used for selective excitation of two-magnon modes with specific wavevectors.

We investigate the injection, transport, and detection of magnon spins in the van der Waals antiferromagnet chromium thiophosphate (${\mathrm{CrPS}}_4$). We electrically and thermally inject magnon spins by platinum contacts and examine the nonlocal resistance as a function of in-plane magnetic field up to 12 Tesla. We observe a large nonlocal resistance from both the electrically and thermally excited magnon modes above the spin-flip field where ${\mathrm{CrPS}}_4$ is in the collinear state. At 25 K for an in-plane field of range 5–12 T, we extract the magnon relaxation length ${\lambda }_m$ ranging $200–800\mathrm{nm}$ and a typical magnon conductivity of ${\sigma }_m\approx 1\times {10}^4{\mathrm{Sm}}^{-1}$, which is one order of magnitude smaller than in yttrium iron garnet (YIG) films at room temperature. Moreover, we find that ${\sigma }_m$ is almost zero for ${\mathrm{CrPS}}_4$ in the noncollinear state. In addition the nonlocal spin Seebeck effect shows a complex behavior as a function of field. Our results open up the way to understanding the role of the antiferromagnetic magnon modes on spin injection into antiferromagnets and implementation of two-dimensional magnets for scalable magnonic circuits.

Dipolar-octupolar (DO) pyrochlore systems ${\mathrm{Ce}}_2$(Zr,${\mathrm{Sn},\mathrm{Hf})}_2{\mathrm{O}}_7$ have garnered much attention as recent investigations suggest that they may stabilize a novel quantum spin ice (QSI), a quantum spin liquid (QSL) with an emergent $U\left(1\right)$ gauge field. In particular, the experimentally estimated microscopic exchange parameters place ${\mathrm{Ce}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ in the $\pi$-flux QSI regime, and recent neutron scattering experiments have corroborated some key theoretical predictions. On the other hand, to make a definitive conclusion, more multifaceted experimental signatures are desirable. In this regard, recent neutron scattering investigation of the magnetic field dependence of the spin correlations in ${\mathrm{Ce}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ may provide valuable information. However, there have not been any comprehensive theoretical studies for comparison. In this paper, we provide such information using gauge mean-field theory (GMFT), allowing for theoretical investigation beyond the perturbative regime. In particular, we construct the phase diagrams for the [110], [111], and [001] field directions. Furthermore, we demonstrate the distinctive evolution of the equal-time and dynamical spin structure factors as a function of the magnetic field for each field direction. These predictions will help future experiments confirm the true nature of the DO-QSI.

We investigate the localization properties of quasiperiodic tight-binding chains with hopping terms modulated by the interpolating Aubry-André-Fibonacci (IAAF) function. This off-diagonal IAAF model allows for a smooth and controllable interpolation between two paradigmatic quasiperiodic models: the Aubry-André and the Fibonacci model. Our analysis shows that the spectrum of this model can be divided into three principal bands, namely, two molecular bands at the edge of the spectrum and one atomic band in the middle, for all values of the interpolating parameter. We reveal that the states in the molecular bands undergo multiple reentrant localization transitions, a behavior previously reported in the diagonal IAAF model. We link the emergence of these reentrant phenomena to symmetry points of the quasiperiodic modulation and, with that, explain the main ground-state properties of the system. The atomic states in the middle band show no traces of localized phases and remain either extended or critical for any value of the interpolating parameter. Using a renormalization group approach, adapted from the Fibonacci model, we explain the extended nature of the middle band. These findings expand our knowledge of phase transitions within quasiperiodic systems and highlight the interplay between extended, critical, and localized states.

Using classical equilibrium and nonequilibrium molecular dynamics simulations together with an optimized, many-body embedded atom model EAM potential, the shear viscosity of liquid iron is calculated over the entire range of temperatures within which it is in its liquid phase and over a range of shear rates. The radial distribution function is also calculated for each of these molecular simulation systems. The validity of the results is assessed against computational, experimental, and theoretical observations. The shear viscosity obtained from equilibrium molecular dynamics simulations may not be a sufficient representation of actual values due to the combined effects of temperature and shear rate. Shear dilatant behavior was also observed at higher shear rates and could impact the local structure of liquid iron and the resulting flow dynamics.

We present high-pressure (2 GPa) ${}^7\mathrm{Li}$ nuclear magnetic resonance (NMR) measurements on single crystals of the hyperhoneycomb Kitaev magnet $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$. The spectra show evidence for a structural phase transition around 200 K and a coexistence of phases, consistent with the results of other measurement techniques. The NMR spectra and line shift measurements demonstrate a strong suppression of the local magnetic susceptibility at high pressure. However, the spin-lattice relaxation $(1/T_1)$ shows a clear power-law temperature dependence. This is inconsistent with a gapped singlet ground state of dimers and tetramers, as was previously proposed, and is instead evocative of a more exotic quantum spin-liquid-like ground state.

We discovered a family of symmetry-breaking magnetic soliton modes in isotropic ferromagnets, recognized as chiral magnetic solitons: left-handed, right-handed, and neutral magnetic solitons. In practical magnetic systems, the lifetime of these solitons is estimated to be in the nanosecond range due to damping effects, accounting for their absence in experimental observations thus far. Solving the Landau-Lifshitz-Gilbert(LLG) equation, the governing equation for magnetic dynamics, poses an exceptional challenge, rendering it a longstanding problem. Theoretically, we employed a sophisticated mathematical geometric mapping method to derive an exact solution to the LLG equation and uncovered these hidden chiral magnetic solitons. To facilitate the experimental realization and manipulation of unique magnetic soliton states, we proposed a scheme to regulate these chiral magnetic solitons using spin-transfer torque induced by spin currents. This approach holds promise for opening new avenues in spintronic applications based on chiral magnetic solitons.

The magnetic ground state of ${\mathrm{CaRuO}}_3$ is mystifying as it is poised on the verge of a quantum critical point. Minute perturbations via strain, chemical doping (or inhomogeneity or protonation), temperature, etc., alter the magnetic ground state of the system. The unit cell volume gets squeezed by 0.6% with 15% Cr doping in ${\mathrm{CaRuO}}_3$. Here we study temperature-dependent neutron diffraction, neutron depolarization, magnetization, and Raman spectroscopy to understand the magnetism and its correlation with crystal structure. The bulk magnetization data indicate a possible weak ferromagnetic order below 100 K, which is well supported by the spin-phonon coupling below $T_C$. Finally, the neutron depolarization analysis confirms about the ferromagnetic ground state below 100 K in 15% Cr-doped ${\mathrm{CaRuO}}_3$. The effect of Cr doping in combination with temperature-dependent structural modulations found to be the origin of ferromagnetism in ${\mathrm{CaRuO}}_3$.

Two-dimensional Euler insulators are novel kinds of systems that host multigap topological phases, quantified by a quantized first Euler number in their bulk. Recently, these phases have been experimentally realized in suitable two-dimensional synthetic matter setups. Here, we introduce the second Euler invariant, a familiar invariant in both differential topology (Chern-Gauss-Bonnet theorem) and in four-dimensional Euclidean gravity, whose existence still needs to be explored in condensed matter systems. Specifically, we first define two specific models in four dimensions that support a nonzero second Euler number in the bulk together with peculiar gapless boundary states. Second, we discuss its robustness in general spacetime-inversion invariant phases and its role in the classification of topological degenerate real bands through real Grassmannians. Our results naturally generalize the second Chern and spin Chern numbers to the case of four-dimensional phases that are characterized by real Hamiltonians and open doors for implementing such higher-dimensional phases in artificial engineered systems, ranging from ultracold atoms to photonics and electric circuits.

Response theories in condensed matter typically describe the response of an electron fluid to external electromagnetic fields, while perturbations on neutral particles are often designed to mimic such fields. Here, we study the response of fermions to a space-time-dependent velocity field, thereby sidestepping the issue of gauge charge. First, we use a semiclassical chiral kinematic theory to obtain the local density of current and extract the orbital magnetization. The theory immediately predicts a “converse vortical effect,” defined as an orbital magnetization driven by linear velocity. It receives contributions from magnetic moments on the Fermi surface and the Berry curvature of the occupied bands. Then, transcending semiclassics via a complementary Kubo formalism reveals that the uniform limit of a clean system receives only the Berry curvature contribution while other limits sense the Fermi surface magnetic moments too. We propose CoSi as a candidate material and suggest magnetometry of a sample under a thermal gradient to detect the effect. Overall, our study sheds light on the effects of a space-time-dependent velocity field on electron fluids and paves the way for exploring quantum materials using new probes and perturbations.

We obtain near-exact cusp behavior in a self-consistent orbital-free density functional theory treatment of atomic and molecular systems, with standard kinetic energy functionals, by the introduction of a physically motivated cost functional. The electron density, and its gradient and Laplacian, are also nearly exact in the vicinity of nuclei, and shell-structure emerges. This results in significantly reduced energy errors and provides a foundation for the development of next-generation kinetic-energy functionals.

We propose a method for far-field superresolution detection based on the time diffraction of evanescent waves, without any external driving or pumping. Time diffraction is induced by the turn-on effect of a laser beam, which creates an intrinsic temporal boundary, and relies on the nonlinear dispersion relation of evanescent waves in free space. By extending classical angular spectrum (AS) theory into the space-time domain, we theoretically investigate the time evolution of waves diffracted by subwavelength gratings. In addition to the stationary solution of traditional evanescent waves, we discover a high-frequency transient solution that decays over time but plays a crucial role in enabling superresolution. This transient wave creates a beat frequency in the temporal spectrum, allowing the retrieval of the grating period from far-field measurements. Our findings lay a theoretical foundation for the application of time-diffracted electromagnetic waves, potentially driving advancements in superresolution imaging and photonic applications beyond the diffraction limit.

We introduce a family of random matrices where correlations between matrix elements are induced via interaction-derived Boltzmann factors. Varying these yields access to different ensembles. We find a universal scaling behavior of the finite-size statistics characterized by a heavy-tailed eigenvalue distribution whose extremes are governed by the Fréchet extreme value distribution for the case corresponding to a ferromagnetic Ising transition. The introduction of a finite density of nonlocal interactions restores standard random-matrix behavior. Suitably rescaled average extremes, playing a physical role as an order parameter, can thus discriminate aspects of the interaction structure; they also yield further nonuniversal information. In particular, the link between maximum eigenvalues and order parameters offers a potential route to resolving long-standing problems in statistical physics, such as deriving the exact magnetization scaling function in the two-dimensional Ising model at criticality.

We compute the rotational anisotropy of the free energy of $\alpha \text{−}{\mathrm{RuCl}}_3$ in an external magnetic field. This quantity, known as the magnetotropic susceptibility, $k$, relates to the second derivative of the free energy with respect to the angle of rotation. We have used approximation-free, auxiliary-field quantum Monte Carlo simulations for a realistic model of $\alpha \text{−}{\mathrm{RuCl}}_3$ and optimized the path integral to alleviate the negative sign problem. This allows us to reach temperatures down to $30\mathrm{K}$—an energy scale below the dominant Kitaev coupling. We demonstrate that the magnetotropic spin susceptibility in this model of $\alpha \text{−}{\mathrm{RuCl}}_3$ displays scaling behavior $k=Tf(B/T)$ at high temperatures. Once the uniform susceptibility departs from the Curie law (i.e., at the energy scale of the exchange interactions), it appears to transition to an emergent scalinglike behavior, characterized by a different function $f$ at lower temperatures, stemming from the locality of torque fluctuations. We observe a remarkable numerical match between experiment and simulations and we also find qualitative agreement with the pure Kitaev model. In comparison, for the XXZ Heisenberg Hamiltonian, the scaling $k=Tf(B/T)$ breaks down at a temperature scale where the uniform spin susceptibility deviates from the Curie law and never reemerges at low temperatures.

We study the effect of quasiperiodic and periodic onsite potentials in a Hatano-Nelson model with next-nearest-neighbor hopping. By considering a nonreciprocal next-nearest-neighbor hopping and a quasiperiodic onsite potential under periodic boundary conditions, we show a breakdown of the typical correspondence between the delocalization-localization and complex-real transitions as a function of the potential strength. Moreover, we reveal that in the delocalized regime, when the potential strength increases, the eigenstates under open boundary conditions exhibit a bidirectional non-Hermitian skin effect, i.e., they tend to localize on both edges instead of localizing on either of the edges. However, when a periodic onsite potential is considered, the system not only exhibits a bidirectional skin effect but also shows a complete direction reversal of the skin effect as a function of the onsite periodic potential.

In two-dimensional systems, the influence of external alternating electric field on the quantum Hall effect is studied within the theory of open quantum systems. Charge carriers are treated as quantum particles interacting with the environment through a one-body (mean-field) mechanism. The dynamics of charge carriers is limited by the average collision time, which takes effectively into account two-body effects. The functional dependencies of average collision time on temperature and magnetic field are phenomenologically treated. Taking into consideration one-body, two-body, and non-Markovian effects, the electrical conductivity tensor is analytically derived in terms of cyclotron frequency and frequency of the external electric field. The theory is applied to describe the real and imaginary parts of the dynamic conductivity in GaAs/$\mathrm{Al}{}_x\mathrm{Ga}{}_{1-x}\mathrm{As}$ heterojunctions at various alternating electric fields. The dependence of magnetic moment of two-dimensional systems on the magnetic field is predicted at various temperatures and frequencies of the external electric field.