We employ the sign-problem-free projector determinant quantum Monte Carlo method to study a microscopic model of $\mathrm{SU}\left(N\right)$ fermions with singlet-bond and triplet-current interactions on the square lattice. We find the gapped singlet $p_x$ and gapless triplet $d_{x^2-y^2}$ density wave states in the half-filled $N=4$ model. Specifically, the triplet $d_{x^2-y^2}$ density wave order is observed in the weak triplet-current interaction regime. As the triplet-current interaction strength is further increased, our simulations demonstrate a transition to the singlet $p_x$ density wave state, accompanied by a gapped mixed-ordered area where the two orders coexist. With increasing the singlet-bond interaction strength, the triplet $d_{x^2-y^2}$-wave order persists up to a critical point after which the singlet $p_x$ density wave state is stabilized, while the ground state is disordered in between the two ordered phases. The analytical continuation is then performed to derive the single-particle spectrum. In the spectra of triplet $d_{x^2-y^2}$ and singlet $p_x$ density waves, the anisotropic Dirac cone and the parabolic shape around the Dirac point are observed, respectively. As for the mixed-ordered area, a single-particle gap opens and the velocities remain anisotropic at the Dirac point.

${\mathrm{Re}}_3{\mathrm{Ge}}_7$ is a unique electronic material showing low-temperature metal–insulator-like phase transition. Furthermore, bulk superconductivity can be induced in ${\mathrm{Re}}_3{\mathrm{Ge}}_7$ by hole doping. Here, we present transport properties, temperature-dependent high-resolution powder x-ray diffraction study, and electronic structure of ${\mathrm{Re}}_3{\mathrm{Ge}}_7$ in the vicinity of the electronic phase transition. Electrical resistivity and heat capacity measurements confirm the anomaly at 58.5 K in zero magnetic field. The Seebeck coefficient changes its sign and exhibits a maximum below the transition temperature indicating substantial changes in the electronic structure. Temperature-dependent structural studies confirm the second-order nature of phase transition, where unit cell parameters show abrupt change at the transition temperature. An analysis of interatomic distances indicates that the shrinkage of the unit cell volume is due to the interatomic changes taking place in the second coordination sphere of Re atoms. Electronic structure calculations reveal strong $5d-4p$ hybridization of valence orbitals near the Fermi level, which is situated between two deep pseudogaps of the density of states. The transition to the semiconducting state is accompanied by the opening of a band gap at the Fermi level right above the narrow pocket of hybridized states.

A trion magnetic polaron formed by the exchange interaction of a positively charged exciton (trion) with localized spins of ${\mathrm{Mn}}^{2+}$ ions is found experimentally in a 4-nm-wide ${\mathrm{Cd}}_{0.98}{\mathrm{Mn}}_{0.02}\mathrm{Te}/{\mathrm{Cd}}_{0.78}{\mathrm{Mn}}_{0.02}{\mathrm{Mg}}_{0.2}\mathrm{Te}$ quantum well containing resident holes. The experiment is performed at a temperature of 1.6 K using resonant excitation of the trion with circularly polarized light. The trion is formed from a resident hole, which is in a hole magnetic polaron state, and a photogenerated electron-hole pair. The dynamical evolution from the hole magnetic polaron to the trion magnetic polaron is accompanied by a spin flip of the electron, which results in negative circular polarization of the photoluminescence. The degree of circular polarization reaches $-8\%$ at zero magnetic field and strongly decreases in transverse magnetic fields exceeding 0.2 T. Our model considerations show that different localization sizes of the resident and photogenerated holes and the resulting difference in their exchange interaction with the ${\mathrm{Mn}}^{2+}$ spins maintains Mn spin polarization. The resulting exchange field of Mn acting on the electron provides a robust spin polarization of the trion magnetic polaron. We evaluate the electron exchange energy in the ${\mathrm{T}}^{+}\mathrm{MP}$ to be 0.19 meV, and the ${\mathrm{T}}^{+}\mathrm{MP}$ binding energy to be about 0.5–1 meV.

In this paper, based on the first-principles calculation combined with a $k·p$ model, we systematically study the ferroelectricity and bulk photovoltaic effect (BPVE) in elemental group-V monolayers. Our results indicate that the electric polarization in group-V monolayers can be effectively tuned by external uniaxial strain, showing a perfect linear relationship. In addition, the Berry curvature and BPVE are well correlated to spontaneous polarizations, which are dominated by structural distortion. Remarkably, a quadratic relation between the shift current and the spontaneous polarization is observed in group-V monolayers. Accordingly, by manipulating the structural distortion through strain engineering, the BPVE can be generated in a controllable manner. Our work not only establishes the polarization-dependent BPVE in ferroelectric materials, but also paves the way for the development of BPVE-based ferroelectric devices.

Combining the continuum elastic theory and the first-principles calculations, we systematically studied the internal strain of nanoparticles of face-centered-cubic (fcc) metals. By taking full account of the surface anisotropy, the theoretical results are found to be consistent with the previously reported experimental ones within a wide range of nanoparticle sizes. Furthermore, we demonstrate that among the fcc transition metals, the largest internal strain of the nanoparticles with size larger than 5 nm occurs in the last column of the Group VIII (Ni/Pd/Pt), which can be correlated with the filling status of $d$-band electrons of the metals. Most of the fcc metal nanoparticles tend to contract with decreasing size with the only exception of strontium, suggesting that most of the surfaces are dominated by the positive surface stress.

Determining the ground state of multiorbital Hubbard models is critical for understanding strongly correlated electron materials, yet, existing methods struggle to simultaneously reach zero temperature and infinite system size. The de facto standard is to approximate a finite dimension multiorbital Hubbard model with a $d=\infty$ version, which can then be formally solved via the dynamical mean-field theory (DMFT), although the DMFT solution is limited by the state of unbiased impurity solvers for zero temperature and multiple orbitals. The recently developed variational discrete action theory (VDAT) offers a new approach to solve the $d=\infty$ Hubbard model, with a variational ansatz that is controlled by an integer $\mathcal{N}$, and monotonically approaches the exact solution at an increasing computational cost. Here we propose a decoupled minimization algorithm to implement VDAT for the multiorbital Hubbard model in $d=\infty$ and study $\mathcal{N}=2-4$. At $\mathcal{N}=2$, VDAT rigorously recovers the multiorbital Gutzwiller approximation, reproducing known results. At $\mathcal{N}=3$, VDAT precisely captures the competition between the Hubbard $U$, Hund $J$, and crystal field $\mathrm{\Delta }$ in the two-orbital Hubbard model over all parameter space, with a negligible computational cost. For sufficiently large $U/t$ and $J/U$, we show that $\mathrm{\Delta }$ drives a first-order transition within the Mott insulating regime. In the large orbital polarization limit with finite $J/U$, we find that interactions have a nontrivial effect even for small $U/t$. VDAT will have far ranging implications for understanding multiorbital model Hamiltonians and strongly correlated electron materials.

We study the photoassisted shot noise (PASN) generated by time-dependent (TD) or random sources and transmission amplitudes. We show that it obeys a perturbative nonequilibrium (NEQ) fluctuation relation (FR) that fully extends the lateral-band transmission picture in terms of many-body correlated states. This FR holds in NEQ strongly correlated systems such as the integer or fractional quantum Hall regime as well as in quantum circuits formed by a normal or a Josephson junction (JJ) strongly coupled to an electromagnetic environment, with a possible temperature bias. We then show that the PASN is universally super-Poissonian, giving an alternative to a theorem by L. Levitov et al. which states that an ac voltage increases the noise. We show that this theorem is restricted to a linear dc current and that it does not apply to a nonlinear SIS (superconductor-insulator-superconductor) junction. Then we characterize minimal excitations in nonlinear conductors as those which ensure a Poissonian PASN, and show that these can carry a nontrivial charge value in the fractional quantum Hall regime. We also propose methods for shot noise spectroscopy and for a robust determination of the fractional charge which complement those we have proposed previously and that have been implemented experimentally [M. Kapfer et al., Science 363, 846 (2019) and R. Bisognin et al., Nat. Commun. 10, 2231 (2019)].

We investigate the electronic structure of the archetypical Mott insulator ${\mathrm{NiS}}_2$ by means of density functional theory calculations in which we explicitly account for the noncollinear antiferromagnetic order, as recently established in the isoelectronic analog $\mathrm{Ni}{(\mathrm{S},\mathrm{Se})}_2$. For metallic ${\mathrm{NiS}}_2$ under high pressures, our calculations predict a Fermi surface topology and volume which are in excellent agreement with recent quantum oscillation studies. However, we find that density functional theory wrongly predicts a metallic ground state even at ambient pressures, similar to previous nonmagnetic or collinear antiferromagnetic models. By including a Hubbard interaction $U$ and an on-site exchange interaction $J$, the metallic phase is suppressed, but even such an extended model fails to describe the nature of the metal-to-insulating phase transition and describes the insulating phase itself incorrectly. These results highlight the importance of more sophisticated computational approaches even deep in the insulating phase, far away from the Mott insulating phase transition.

Metasurfaces and metagratings possess vast wave-manipulation capabilities, including reflection and refraction of a plane wave into nonstandard directions. These require meticulously-designed subwavelength meta-atoms in each period of the metasurface which guarantee unitary coupling to the desired Floquet-Bloch mode or, equivalently, suppression of the coupling to other modes. Herein, we propose an entirely different scheme to achieve such modal control. Extending the concept of antireflective coatings to enable simultaneous manipulation of multiple modes, we show theoretically and experimentally that a simple superstrate consisting of only several uniform dielectric layers can be modularly applied to arbitrary periodic structures to yield perfect anomalous reflection. This multimodal antireflective coating (MMARC), designed based on an analytical model, presents a conceptually and practically simpler paradigm for wave control across a wide range of physical branches, from electromagnetics and acoustics to seismics and beyond.

Two-dimensional (2D) Stiefel-Whitney (SW) insulator (SWI) is a fragile topological state characterized by the second SW class in the presence of space-time inversion symmetry. So far, SWIs have been proposed in several electronic materials but seldom in phononic systems. Here we recognize that a large class of 2D buckled honeycomb crystals termed Xenes and their ligand-functionalized derivatives realize the nontrivial phononic SW topology. The phononic SWIs are identified by a nonzero second SW number $w_2=1$, associated with gaped edge states and robust topological corner modes. Despite the versatility of electronic topological properties in these materials, the nontrivial phononic SW topology is mainly attributed to the double band inversion between in-plane acoustic and out-of-plane optical bands with opposite parities due to the structural buckling of the honeycomb lattice. Our findings not only reveal an overlooked phononic topological property of 2D Xene-related materials, but also afford abundant readily synthesizable material candidates with simple phononic spectra for further experimental studies of phononic SW topology physics.

We explore the ground state and thermodynamic properties of the polaron coupled to quantum dispersive optical phonons in one spatial dimension. Calculations are performed using the finite–temperature Lanczos method augmented by a highly efficient construction of the variational Hilbert space. We focus on the electron removal spectral function as relevant for the angle-resolved photoemission experiments. We show that photoemission spectroscopy can be used to measure the phonon dispersion relation in a dilute system of polarons. The spectral weight of observed phonon bands is proportional to the phonon contribution to the wavefunction at finite phonon momentum. In addition, we demonstrate that when removing an electron from a polaron ground state, the polaron band does not appear in the spectral function. The latter becomes observable only at elevated temperatures.

Self-induced spin-orbit torques (SI-SOTs) in ferromagnetic (FM) layers have been overlooked when estimating the spin Hall angle (SHA) of adjacent nonmagnetic (NM) layers. In this work, we observe anomalous sign inversion of the total SOT in the spin-torque ferromagnetic resonance due to the enhanced SI-SOT, and successfully rationalize the sign inversion through a theoretical calculation considering the SHE in both the NM and FM layers. The findings show that using an FM layer whose SHA sign is the same as that of the NM achieves efficient SOT-magnetization switching with the assistance of the SI-SOT. The contribution of the SI-SOT becomes salient for a weakly conductive NM layer, and conventional analyses that do not consider the SI-SOT can overestimate the SHA of the NM layer by a factor of more than 150.

We theoretically study the propagation of a terahertz (THz) Gaussian beam through a thin sample of layered superconductor. We consider the beam axis and the superconducting layers to be perpendicular to the sample interface, while the electric field in the beam is perpendicular to the layers. We show that, in such a geometry, the Josephson current between the superconducting layers supports lensing of the beam instead of divergence on the Rayleigh range. Moreover, due to the nonlinearity, the focal length and waist of the transmitted beam depend on the incident beam intensity. These dependencies demonstrate nontrivial hysteresis behavior that can be observed in experiments with THz lasers.

We explore the effects of spin-orbit coupling on nuclear wave packet motion near an out-of-equilibrium molecular junction, where nonzero Berry curvature emerges as the antisymmetric part of the electronic friction tensor. The existence of nonzero Berry curvature mandates that different nuclear wave packets (associated with different electronic spin states) experience different nuclear Berry curvatures, i.e., different pseudomagnetic fields. Furthermore, for a generic, two-orbital two-lead model (representing the simplest molecular junction), we report significant spin polarization of the electronic current with decaying and oscillating signatures in the large voltage limit—all as a result of nuclear motion. These results are consistent with magnetic AFM chiral-induced spin selectivity experiments. Altogether, our results highlight an essential role for Berry curvature in condensed phase dynamics, where spin separation survives dissipation to electron-hole pair creation and emerges as one manifestation of nuclear Berry curvature.

We discuss the spin-$\frac{1}{2}{J}_1–J_2$ model on the triangular lattice using recently proposed bond-operator theory (BOT). In agreement with previous discussions of this system, we obtain four phases upon $J_2$ increasing: the phase with ${120}^{\circ }$ ordering of three sublattices, the spin-liquid phase, the state with the collinear stripe order, and the spiral phase. The ${120}^{\circ }$ and the stripe phases are discussed in detail. All calculated static characteristics of the model are in good agreement with previous numerical findings. In the ${120}^{\circ }$ phase, we observe the evolution of the quasiparticles' spectra and dynamical structure factors (DSFs) upon approaching the spin-liquid phase. Some of the considered elementary excitations were introduced first in our recent study of this system at $J_2=0$ using the BOT. In the stripe phase, we observe that the doubly degenerate magnon spectrum known from the spin-wave theory (SWT) is split by quantum fluctuations, which are taken into account more accurately in the BOT. As compared with other known findings of the SWT in the stripe state, we observe additional spin-1 and spin-0 quasiparticles which give visible anomalies in the transverse and longitudinal DSFs. We obtain also a special spin-0 quasiparticle named the singlon that produces a peak only in the four-spin correlator and which is invisible in the longitudinal DSF. We show that the singlon spectrum lies below energies of all spin-0 and spin-1 excitations in some parts of the Brillouin zone. The singlon spectrum at zero momentum can be probed by the Raman scattering.

Controlling the magnetization dynamics at the fastest speed is a major issue of fundamental condensed matter physics and its applications for data storage and processing technologies. It requires a deep understanding of the interactions between the degrees of freedom in solids, such as spin, electron, and lattice as well as their responses to external stimuli. In this paper, we systematically investigate the fluence dependence of ultrafast magnetization dynamics induced by below-bandgap ultrashort laser pulses in the ferrimagnetic insulators ${\mathrm{Bi}}_x{\mathrm{Y}}_{3-x}{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ with $1\le x_{\mathrm{Bi}}\le 3$. We demonstrate subpicosecond demagnetization dynamics in this material followed by a very slow remagnetization process. We prove that this demagnetization results from an ultrafast heating of iron garnets by two-photon absorption (TPA), suggesting a phonon-magnon thermalization time of $\le 0.6$ ps. We explain the slow remagnetization timescale by the low phonon heat conductivity in garnets. Additionally, we show that the amplitudes of the demagnetization, optical change, and lattice strain can be manipulated by changing the ellipticity of the pump pulses. We explain this phenomenon considering the TPA circular dichroism. These findings open exciting prospects for ultrafast manipulation of spin, charge, and lattice dynamics in magnetic insulators by ultrafast nonlinear optics.

The Verwey transition in magnetite $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$ is the prototypical metal-insulator transition and has eluded a comprehensive explanation for decades. A major element of the challenge is the complex interplay between charge order and lattice distortions. Here we use ultrafast electron diffraction (UED) to disentangle the roles of charge order and lattice distortions by tracking the transient structural evolution after charge order is melted via ultrafast photoexcitation. A dual-stage response is observed in which ${\mathrm{X}}_3, {\mathrm{X}}_1$, and ${\mathrm{\Delta }}_5\text{−}\mathrm{type}$ structural distortions occur on markedly different timescales of 0.7–3.2 ps and longer than 3.2 ps. We propose that these distinct timescales arise because ${\mathrm{X}}_3\text{−}\mathrm{type}$ distortions strongly couple to the trimeron charge order, whereas the ${\mathrm{\Delta }}_5\text{−}$ distortions are more strongly associated with monoclinic to cubic distortions of the overall lattice. Our work aids in clarifying the charge-lattice interplay using UED method and illustrates the disentanglement of the complex phases in magnetite.

Implementation of the hard-core-dimer Hilbert space in cold-Rydberg-atom simulators opens a route of investigating real-time dynamics of dimer liquids under Hamiltonian quench. Here, we consider an initial resonating valence bond state on the square lattice realizing a critical Coulomb phase with algebraic and dipolar correlations. Using its representation as a special point of a broad manifold of $\mathrm{SU}(2$)-symmetric, translationally invariant, projected entangled pair states (PEPS), we compute its nonequilibrium dynamics upon turning on intersite Heisenberg interactions. We show that projecting the time evolution onto the PEPS manifold remains accurate at small timescales. We also find that the state evolves within a PEPS submanifold characterized by a $\mathrm{U}(1$) gauge symmetry, suggesting that the Coulomb phase is stable under such unitary evolution.

We use matrix product techniques to investigate the performance of two algorithms for obtaining the ground state of a quantum many-body Hamiltonian $H=H_A+H_B$ in infinite systems. The first algorithm is a generalization of the quantum approximate optimization algorithm and uses a quantum computer to evolve an initial product state into an approximation of the ground state of $H$ by alternating between $H_A$ and $H_B$. We show for the one-dimensional (1D) quantum Ising model that the accuracy in representing a gapped ground state improves exponentially with the number of alternations. The second algorithm is the variational imaginary time ansatz, which uses a classical computer to simulate the ground state via alternating imaginary time steps with $H_A$ and $H_B$. We find for the 1D quantum Ising model that an accurate approximation to the ground state is obtained with a total imaginary time $\tau$ that grows only logarithmically with the inverse energy gap $1/\mathrm{\Delta }$ of $H$. This is much faster than imaginary time evolution by $H$, which would require $\tau \sim 1/\mathrm{\Delta }$.