We present a microscopic calculation of the inverse Faraday effect in metals. We derive a static local magnetic moment induced on the application of high-frequency light, using the Eilenberger formulation of quasiclassical theory. We include the effect of disorder and formulate a theory applicable across the entire temperature range, in the absence of external applied fields. For light-induced electric fields of amplitude $\sim 100\text{kV/cm}$, the induced fields are large $\sim 0.1\text{T}$ for metallic Nb. The predictions of our theory agree with recent experimental and theoretical results [O. H.-C. Cheng et al., Nat. Photon. 14, 365 (2020) and J. Hurst et al., Phys. Rev. B 98, 134439 (2018)]. An extension of this approach to superconductors would open a new route of inducing orbital magnetic field and potentially vortices in superconductors.

Materials with linear electronic dispersion often feature high carrier mobilities and unusually strong nonlinear optical interactions. In this work, we investigate the (THz) nonlinear dynamics of one such material, HgCdTe, with an electronic band dispersion heavily dependent on both temperature and stoichiometry. We show how the band gap, carrier concentration and band shape together determine the nonlinear response of the system. At low temperatures, carrier generation from Zener tunneling dominates the nonlinear response with a reduction in the overall transmission. At room temperature, quasiballistic electronic dynamics drive the largest observed nonlinear optical interactions, leading to a transmission increase. Our results demonstrate the sensitivity of these nonlinear optical properties of narrow-gap materials to small changes in the electronic dispersion and carrier concentration.

We present the spin-wave theory of the excitation spectrum and quench dynamics of the non-Hermitian transverse-field Ising model. The complex excitation spectrum is obtained for a generic hypercubic lattice using the linear approximation of the Holstein-Primakoff transformation together with the complex bosonic Bogolyubov transformation. In the one-dimensional case, our result compares very well with the exact quasiparticle dispersion relation obtained via a fermionic representation of the problem, at least in the regime of large dissipation and transverse field. When applied to the quench dynamics, however, we show that the linear spin-wave approximation breaks down and the bosonic theory is plagued by a divergence at finite times. We understand the origin of this instability using a single-mode approximation. While limited to short times, we show that this approach allows us to characterize the dynamics arising from the quench of the dissipative term and the light-cone structure of the propagation quantum correlations. Furthermore, for the one-dimensional case, the linear spin-wave dynamics shows good agreement with the exact fermionic solution, both for the local magnetization and the spin-spin correlations.

We study fluctuation properties in the energy spectra of finite-size honeycomb lattices—graphene billiards—subject to the Haldane-model on-site potential and next-nearest-neighbor tunneling at critical points, referred to as Haldane graphene billiards in the following. The billiards have the shapes of a rectangular billiard with integrable dynamics, one with chaotic dynamics, and one whose shape has, in addition, threefold rotational symmetry. It had been shown that the spectral properties of the graphene billiards coincide with those of the nonrelativistic quantum billiard with the corresponding shape, both at the band edges and in the region of low-energy excitations around the Dirac points at zero energy. There, the dispersion relation is linear and, accordingly, the spectrum is described by the same relativistic Dirac equation for massless spin-1/2 particles as relativistic neutrino billiards, whose spectral properties agree with those of nonrelativistic quantum billiards with violated time-reversal invariance. Deviations from the expected behavior are attributed to differing boundary conditions and backscattering at the boundary, which leads to a mixing of valley states corresponding to the two Dirac points, that are mapped into each other through time reversal. We employ a Haldane model to introduce a gap at one of the two Dirac points so that backscattering is suppressed in the energy region of the gap and demonstrate that there the correlations in the spectra comply with those of the neutrino billiard of the corresponding shape. Here, the phase transition from nonrelativistic to relativistic quantum behavior is achieved by adjusting the Haldane parameters.

Within the context of first-principles techniques, we present a theoretical and computational framework to quickly determine, at finite momentum, the static self-consistent (longitudinal) charge response to an external perturbation that enters the determination of the scattering cross section of inelastic scattering processes such as EELS. We also determine the (transverse) charge response computed in short-circuit conditions. The all-order quasimomentum expansion of the transverse charge response to an atomic displacement are identified with dynamical Born effective charges, quadrupoles, octupoles, etc. Theoretically, we demonstrate that the transverse charge response can be related to the longitudinal one via a well-defined static long-range dielectric function, going beyond the random phase approximation. Our theoretical advancements allow for an efficient use of perturbation theory in the computational implementation. Due to its more favorable scaling, our method provides an interesting alternative to the use of the $2n+1$ theorem, especially for the study of semiconductors and metals with large unit cells. For semiconductors, we apply our developments to the computation of the piezoelectric properties of a large cell solid solution of semiconducting hafnium oxide containing 96 atoms. We here show that the clamped ion piezoelectric response, which is determined solely by dynamical quadrupoles, can be decomposed into real-space localized contributions that mostly depend on the chemical environment, paving the way for the use of machine-learning techniques in the material search for optimized piezoelectrics. We further apply our methodology to determine the density response of metals. We here find that the leading terms of the charge expansion are related to the Fermi energy shift of the potential if admitted by symmetry, and by Born effective charges which do not sum to zero over the atoms. These terms are then linked to the leading order expansion of the macroscopic electron-phonon coupling in metals. We apply our developments to the TEM-EELS spectroscopy of lithium intercalated graphites, where we find that approximating the density response via the use of the atomic form-factor in the long-wavelength limit does not take into account the anisotropy of the atomic chemical bonding in the crystal.

We reexamine the internal structure of skyrmioniums stabilized in quasi-two-dimensional chiral magnets with easy-axis uniaxial anisotropy. Skyrmioniums are particlelike states of two nested skyrmions with opposite polarities contributing to zero topological charge. The physical principles of skyrmionium stability are drawn from both the analytical analysis with a trial function and from numerical simulations within the framework of micromagnetism. We deduce that the radii of the internal skyrmion with the positive polarity and the ring-shaped external skyrmion with the negative polarity are mutually dependent, which constitutes the paradigm of communicating skyrmions. For large central skyrmions, the skyrmionium transforms into a narrow circular domain wall, whereas for small internal radii, the ring expands, which occurs at the verge of collapsing into an ordinary isolated skyrmion. We show that skyrmioniums may form lattices of two varieties depending on the polarity of the internal skyrmion. At the phase diagram (magnetic field)-(uniaxial anisotropy), both skyrmionium lattices share the same area with one-dimensional spiral states and remain metastable solutions for the whole range of control parameters. By expanding at the critical line, skyrmionium lattices do not release isolated skyrmioniums. Isolated skyrmioniums of just one type exist apart from the corresponding lattice in a narrow field region restricted by the critical line of expansion from below and by the line of collapse above.

Entanglement in quantum many-body systems can exhibit universal phenomena governed by long-distance properties. We study universality and phase transitions of the entanglement inherent to open many-body systems, namely, the entanglement between a system of interest and its environment. Specifically, we consider the Tomonaga-Luttinger liquid (TLL) under a local measurement and analyze its unconditioned nonunitary evolution, where the measurement outcomes are averaged over. We quantify the system-environment entanglement by the Rényi entropy of the post-measurement density matrix, whose size-independent term encodes the universal low-energy physics. We develop a field-theoretical description to relate the universal term to the effective ground-state degeneracy known as the $g$ function in a boundary conformal field theory, and use the renormalization group method to determine its value. We show that the universal contribution is determined by the TLL parameter $K$ and can exhibit singularity signifying an entanglement phase transition. Surprisingly, in certain cases the size-independent contribution can increase as a function of the measurement strength in contrast to what is naïvely expected from the $g$-theorem. We argue that this unconventional behavior could be attributed to the dangerously irrelevant term which has been found in studies of the resistively shunted Josephson junction. We also check these results by numerical calculations in the spin-$\frac{1}{2}$ XXZ chain subject to a site-resolved measurement. Possible experimental realization in ultracold gases, which requires no postselections, is discussed.

The controlled manipulation of spins on ultrashort timescales is among the most promising solutions for novel high-speed and low-power-consumption spintronic and magnetic recording applications. To do so, terahertz (THz) light pulses can be used to drive coherent magnetization dynamics in ferromagnetic thin films. We were able to resolve these dynamics on the nanoscale employing THz-pump x-ray resonant magnetic scattering from the labyrinth-type domain network of a Co/Pt multilayer with perpendicular magnetic anisotropy. Our results reveal THz-driven ultrafast demagnetization as well as coherent local magnetization oscillations at the THz fundamental frequency of $2.5\mathrm{THz}$. We observe a temporal lag between femtosecond demagnetization and the start of the coherent magnetization oscillations that can be understood by a time-dependent damping. The dynamics of the domain and domain-wall contributions are found to be highly correlated, suggesting the applicability of THz spin control in magnetic nanostructures.

The Heisenberg quantum antiferromagnet on the maple leaf lattice has been shown to feature highly exotic phases, and therefore material realizations are intensely sought after. We determine the magnetic Hamiltonian of the copper mineral bluebellite using density-functional theory based energy mapping. Due to significant distortion of the spin-1/2 maple leaf lattice, we find two of the five distinct nearest-neighbor couplings to be ferromagnetic. The solution of this Hamiltonian with density matrix renormalization group calculations points us to the surprising insight that this particular imperfect maple leaf lattice, due to the strongly ferromagnetic ${\mathrm{Cu}}^{2+}$ dimer, realizes an effective $S=1$ breathing kagome Hamiltonian. In fact, this is another highly interesting Hamiltonian that has rarely been realized in materials. Analysis of the effective model within a bond-operator formalism then allows us to identify a valence bond solid ground state and extract thermodynamic quantities using a low-energy bosonic mean-field theory. We resolve the puzzle of the apparent one-dimensional character of bluebellite as our calculated specific heat has a Bonner-Fisher-like shape, in good agreement with the experiment.

We report numerical simulations of coupled sine-Gordon and heat diffusion equations describing dynamic states stimulated by a trapped vortex driven by dc current in a stack of up to $N=321$ Josephson junctions. It is shown that the Cherenkov wake behind the vortex shuttle trapped in the stack can trigger proliferation of counterpropagating vortices and antivortices which get synchronized and form large-amplitude standing electromagnetic waves. This happens if the dc current density $J$ exceeds a threshold value $J_s$ which can be well below the Josephson interlayer critical current density $J_c$ for underdamped junctions. The cavity modes stimulated by the vortex-antivortex pair production cause peaks in the radiated power $P_N\left(J\right)$ with a nearly monochromatic spectrum at discrete values of $J$ corresponding to the zero-field Fiske resonances. The power $P_N\left(J\right)$ was evaluated for small rectangular stacks in the magnetodipole approximation and for large stacks in a single mode state. For small stacks, the highest peak in $P_N\left(J\right)$ increases rapidly, $P_N\propto N^6$, with the number of junctions at $N\le 81$ and gradually slows down to $P_N\propto N^2$ at $161\le N\le 321$. For stacks larger than the radiation wavelength, we obtained $P_N\propto N^5$ at $N\lesssim 200–300$ and $P_N\propto N^2$ at larger $N$. At $N\le 321$ and representative parameters of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$, we observed moderate overheating and no hotspots. The vortex-antivortex pair production can amplify THz radiation from ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ mesas for which trapping Josephson vortices could be used to stimulate THz emission at subcritical currents and optimize the radiation output.

We propose and demonstrate that the behaviors of long-range, two-site quantum resources can effectively diagnose quantum phases. In an XX spin chain with symmetry-breaking quantum phase transitions, we reveal that the gradually decaying and damped oscillating modes of quantum coherence or quantum discord, along with two-site distance, can identify two spin-liquid phases, respectively. Moreover, based on our analytical results of spin correlation functions, we confirm the existence of long-range entanglement in the system and establish a connection between two-site entanglement and quantum phases. Furthermore, for the extended Ising model with topological phase transitions, we find that coherence and quantum discord behaviors can also signify topological quantum phases. In particular, we discover the quantum resource freezing phenomenon, where topologically protected long-range quantum resources may have potential applications in quantum information processing.

Disordered hyperuniform many-body systems are exotic states of matter with novel optical, transport, and mechanical properties. These systems are characterized by an anomalous suppression of large-scale density fluctuations compared to ordinary liquids. The structure factor of disordered hyperuniform systems often obeys the scaling relation $S\left(k\right)\sim \mathcal{B}{k}^{\alpha }$ with $\mathcal{B},\alpha >0$ in the limit $k\to 0$. Ground states of $d$-dimensional free fermionic gases, which are fundamental models for many metals and semiconductors, are key examples of quantum disordered hyperuniform states with important connections to random matrix theory. However, the effects of electron-electron interactions as well as the polarization of the electron liquid on hyperuniformity have not been explored thus far. In this paper, we systematically address these questions by deriving the analytical small-$k$ behaviors (and, associated, $\alpha$ and $\mathcal{B}$) of the total and spin-resolved structure factors of quasi-one-dimensional, two-dimensional, and three-dimensional electron liquids for varying polarizations and interaction parameters. We validate that these equilibrium disordered ground states are hyperuniform, as dictated by the fluctuation-compressibility relation. Interestingly, free fermions, partially polarized interacting fermions, and fully polarized interacting fermions are characterized by different values of the small-$k$ scaling exponent $\alpha$ and coefficient $\mathcal{B}$. In particular, partially polarized fermionic liquids exhibit a unique form of multihyperuniformity, in which the net configuration exhibits a stronger form of hyperuniformity (i.e., larger $\alpha$) than each individual spin component. The detailed theoretical analysis of such small-$k$ behaviors enables the construction of corresponding equilibrium classical systems under effective one- and two-body interactions that mimic the pair statistics of quantum electron liquids. Our paper thus reveals that highly unusual hyperuniform and multihyperuniform states can be achieved in simple fermionic systems and paves the way for harnessing unique hyperuniform scaling relations for applications, such as the construction of accurate density functionals.

Eigenstate thermalization has been numerically shown to occur for few-body observables in a wide range of nonintegrable models. For intensive sums of few-body observables, a weaker version of eigenstate thermalization known as weak eigenstate thermalization has been proved to occur in general. Here, we unveil a stricter weak eigenstate thermalization phenomenon that occurs in quadratic models exhibiting quantum chaos in the single-particle sector (quantum-chaotic quadratic models) and in integrable interacting models. In such models, we argue that few-body observables that have a properly defined system-size independent norm are guaranteed to exhibit at least a polynomially vanishing variance (over the entire many-body energy spectrum) of the diagonal matrix elements, a phenomenon we dub normal weak eigenstate thermalization. We prove that normal weak eigenstate thermalization is a consequence of single-particle eigenstate thermalization, i.e., it can be viewed as a manifestation of quantum chaos at the single-particle level. We report numerical evidence of normal weak eigenstate thermalization for quantum-chaotic quadratic models such as the three-dimensional Anderson model in the delocalized regime and the power-law random banded matrix model, as well as for the integrable interacting spin-$\frac{1}{2}$ XYZ and XXZ models.

The ponderomotive force is an effective static force that a particle feels in an oscillating field, whose static potential may be called the ponderomotive potential. We generalize this notion to periodically driven quantum many-body systems, and propose it as a convenient tool to engineer their nonequilibrium steady states beyond the single-particle level. Applied to materials driven by light, the ponderomotive potential is intimately related to the equilibrium optical conductivity, which is enhanced close to resonances. We show that the ponderomotive potential from the incident light may be used to induce exciton condensates in semiconductors, to generate attractive interactions leading to superconductivity in certain electron-phonon systems, and to create additional free-energy minima in systems with charge/spin/excitonic orders. These effects are presented with experimentally relevant parameters.

Using a broadband dielectric spectroscopy approach (1 to ${10}^{14}$ Hz) we prove that the tungsten bronze $\mathrm{C}{\mathrm{a}}_{0.3}\mathrm{B}{\mathrm{a}}_{0.7}\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_6$ (CBN-30) displays a ferroelectric phase transition of mixed displacive and order-disorder character, and its paraelectric phase does not show traces of relaxor behavior but precursor effects as polar fluctuations below about 550 K. The analysis of the submegahertz dielectric response, together with infrared and Raman spectroscopy, reveals that simultaneous polarization mechanisms are responsible for the phase transition. The comparison of the excitations found in CBN-30 with those of (Sr,Ba)$\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_6$ reveals that these mechanisms are congruous, although in CBN-30 the main relaxation process behaves differently due to the different domain structure. The excitations are phenomenologically assigned to phonons, to an anharmonic vibration of cationic origin which plays the role of a soft central mode (${\nu }_{\mathrm{THz}}$), and to a relaxation in the gigahertz range (${\nu }_{01}$), probably due to polarization fluctuations of nanometric size which carry the main part of the permittivity and split below $T_{\mathrm{C}}$ into several weaker excitations with different polarization correlation lengths. The overall dielectric response is therefore explained by the coexistence of several excitations with different thermal behaviors, corroborating the complexity of the tetragonal tungsten bronze structures.

In this paper, we present a comprehensive investigation of the distinctive magnetic properties involving the remarkable occurrences of negative magnetization (NM), exchange bias (EB), and spin reorientation (SR) in the $\mathrm{ErF}{\mathrm{e}}_{0.5}{\mathrm{Co}}_{0.5}{\mathrm{O}}_3$ compound. The dc magnetization data, recorded in field-cooled-cooling mode, reveal a net zero magnetization at the compensation temperature $\left(T_{\mathrm{COMP}}\right)$ of 24 K, leading to the NM phenomenon in the compound. Rietveld refinement of the neutron diffraction (ND) patterns over 1.5–300 K elucidates the SR of Fe/Co spins at 100 K $\left(T_{\mathrm{SR}}\right)$ and Er magnetic ordering <4 K $\left(T_{\mathrm{N}}^{\mathrm{Er}}\right)$ resulting in ${\mathrm{\Gamma }}_4\left(G_x\right)$, ${\mathrm{\Gamma }}_{24}(G_z,G_x)$, ${\mathrm{\Gamma }}_2\left(G_z\right)$, and ${\mathrm{\Gamma }}_{257}(G_z;A_y^{\mathrm{Er}}{G}_z^{\mathrm{Er}})$ magnetic structures at T > $T_{\mathrm{SR}}$, $T_{\mathrm{SR}}\ge T>T_{\mathrm{COMP}}$, $T_{\mathrm{N}}^{\mathrm{Er}}<T\le T_{\mathrm{COMP}}$, and $T\le T_{\mathrm{N}}^{\mathrm{Er}}$, respectively. It is, therefore, evident that the SR of Fe/Co moments from ${\mathrm{\Gamma }}_4\left(G_x\right)$ to ${\mathrm{\Gamma }}_2\left(G_z\right)$ gets completed at the $T_{\mathrm{COMP}}$ of 24 K, and the Er magnetic ordering into an unusual ${\mathrm{\Gamma }}_{57}(A_y^{\mathrm{Er}}{G}_z^{\mathrm{Er}})$ spin configuration takes place at $T\le 4$ K. Anomalies in dc magnetization data (coercivity and remanent magnetization) at $T_{\mathrm{SR}}$, $T_{\mathrm{COMP}}$, and $T_{\mathrm{N}}^{\mathrm{Er}}$ are also reflected in the ac susceptibility data. Intriguingly, EB field $\left(H_{\mathrm{EB}}\right)$ in the compound does not change its polarity across $T_{\mathrm{COMP}}$ and remains positive even above $T_{\mathrm{COMP}}$. The observed positive $H_{\mathrm{EB}}$ at T > $T_{\mathrm{COMP}}$ can be attributed to a complex spin arrangement as evident from the ND, whereas for T ≤ $T_{\mathrm{COMP}}$, positive $H_{\mathrm{EB}}$ has its usual explanation within the framework of the Meiklejohn-Beam model. The maximum positive $H_{\mathrm{EB}}$ and a broad hump in $H_{\mathrm{EB}}$ at $T_{\mathrm{COMP}}$ and $T_{\mathrm{SR}}$, respectively, indicate a correlation between the EB and SR in the compound. Additionally, cooling-field dependence of the EB shows a peak value ∼5 kOe; thereafter, an unusual suppression of $H_{\mathrm{EB}}$ up to 70 kOe cooling field is found. The observed NM below $T_{\mathrm{COMP}}$ is elucidated using the Cooke's model, where the polarized Er moment, under the internal magnetic field of the ordered canted antiferromagnetic Fe/Co sublattice, competes with the ferromagnetic Fe/Co moment. This results in a complete cancellation of magnetization at $T_{\mathrm{COMP}}$ and the emergence of NM below $T_{\mathrm{COMP}}$ in the compound. The specific heat data reveal a Schottky anomaly, inferring the dominant polarized nature of the Er moment below $T_{\mathrm{COMP}}$. In this paper, we underscore the pivotal role of Er and Fe/Co exchange coupling in shaping the intriguing and complex magnetic properties—NM and EB—of the compound. These findings highlight the potential utility of the compound in spintronic applications.

Finding an exact solution for a realistic interacting quantum many-body problem is often challenging. There are only a few problems where an exact solution can be found, usually in a narrow parameter space. Here, we propose a spin-$\frac{1}{2}$ Heisenberg model on a square lattice with spatial anisotropy and bond depletion for the nearest-neighbor antiferromagnetic interactions but not for the next-nearest-neighbor interactions. This model has an exact and unique dimer ground state at $J_2/J_1=\frac{1}{2}$; a dimer state is a product state of spin singlets on dimers (here, staggered nearest-neighbor bonds). We examine this model by employing the bond-operator mean-field theory and exact diagonalization. These analytical and numerical methods precisely affirm the correctness of the dimer ground state at the exact point $(J_2/J_1=\frac{1}{2})$. As one moves away from the exact point, the dimer order melts and vanishes when the spin gap becomes zero. The mean-field theory with harmonic approximation indicates that the dimer order persists for $-0.35\lesssim J_2/J_1\lesssim 1.35$. However, in nonharmonic approximation, the upper critical point lowers by 0.28 to 1.07, but the lower critical point remains intact. The exact diagonalization results suggest that the latter approximation fares better. The model reveals Néel order below the lower critical point and stripe magnetic order above the upper critical point. It has a topologically equivalent model on a honeycomb lattice where the nearest-neighbor interactions are still spatial anisotropic, but the bond depletion shifts into the isotropic next-neighbor interactions. Moreover, these models can also be generalized in the three dimensions.

We employ a classical limit grounded in SU(4) coherent states to investigate the temperature-dependent dynamical spin structure factor of the $S=1/2$ ladder consisting of weakly coupled dimers. By comparing the outcomes of this classical approximation with density matrix renormalization group and exact diagonalization calculations in finite-size ladders, we demonstrate that the classical dynamics offers an accurate approximation across the entire temperature range when the interdimer coupling is weak and a good approximation in the high-temperature regime even when the interdimer coupling is strong. This agreement is achieved after appropriately rescaling the temperature axis and renormalizing expectation values to satisfy a quantum sum rule, following D. Dahlbom et al. [Phys. Rev. B 109, 014427 (2024)]. We anticipate the method will be particularly effective when applied to 2D and 3D lattices composed of weakly coupled dimers, situations that remain challenging for alternative numerical methods.

Current-induced spin-orbit torque (SOT) provides an efficient strategy for electrical manipulation of magnetism in spintronic devices. However, despite more than one decade of research for current-induced torque optimization, the charge-to-spin conversion efficiency and the resistivity in the spin-current source materials do not satisfy the requirement for next-generation low-consumption devices. Authors of recent theoretical works have predicted that an oxide or nitride interface can induce orbital angular momentum (OAM) accumulation through the orbital Rashba-Edelstein effect (OREE) and exert orbital torques to the adjacent magnetic layer. Here, we report on the remarkable dampinglike torque contribution from the OREE at the Pt/Co-O interface. In the Co/Co-O/Pt structures, the insertion of a thin natural-oxidized Co-O layer results in the sign reversal of SOT efficiency, suggesting the existence of a competing SOT source. In addition, the SOT efficiency and spin Hall magnetoresistance ratio in the Pt/Co structure were elevated significantly by introducing a Co-O underlayer. Anomalous SOT has also been observed in the Pt/Co/Co-O/Pt structures with perpendicular magnetic anisotropy by using the loop-shift technique and current-induced magnetization switching. After excluding the bulk contribution from the Co-O layer and its possible manipulation to the spin current from the Pt layer, we attribute this anomalous SOT origin to the OREE at the Co-O/Pt interface. In this paper, we not only provide a strategy for generating additional spin current for manipulating the SOT but also demonstrate an important clue toward microscopic understanding of how the out-of-equilibrium OAM interacts with the local magnetization.