We develop a consistent quantum description of surface plasmons interacting with quantum emitters and external electromagnetic field. Within the framework of macroscopic electrodynamics in dispersive and absorptive medium, we derive, in the Markov approximation, the canonical Hamiltonian, commutation relations, and coupling parameters for the plasmon modes in metal-dielectric structures of an arbitrary shape whose characteristic size is well below the diffraction limit. We then develop a new quantum approach bridging the macroscopic and canonical schemes which describes the interacting plasmons in terms of bosonic modes with linear dispersion whose coupling to the electromagnetic field and quantum emitters is mediated by the classical plasmons. By accurately accounting for medium optical dispersion and losses in the interactions of surface plasmons with light and localized electron excitations, this approach can serve as a framework for studying non-Markovian effects in plasmonics.

Understanding quantum many-body states of correlated electrons is one main theme in modern condensed-matter physics. Given that the Fermi-Hubbard model, the prototype of correlated electrons, was recently realized in ultracold optical lattices, it is highly desirable to have controlled numerical methodology to provide precise finite-temperature results upon doping to directly compare with experiments. Here, we demonstrate the exponential tensor renormalization group (XTRG) algorithm [Chen et al., Phys. Rev. X 8, 031082 (2018)], complemented by independent determinant quantum Monte Carlo, offers a powerful combination of tools for this purpose. XTRG provides full and accurate access to the density matrix and thus various spin and charge correlations, down to an unprecedented low temperature of a few percent of the tunneling energy. We observe excellent agreement with ultracold fermion measurements at both half filling and finite doping, including the sign-reversal behavior in spin correlations due to formation of magnetic polarons, and the attractive hole-doublon and repulsive hole-hole pairs that are responsible for the peculiar bunching and antibunching behaviors of the antimoments.

The frustrated XY model on the honeycomb lattice has drawn lots of attention because of the potential emergence of chiral spin liquid (CSL) with the increasing of frustrations or competing interactions. In this paper, we study the extended spin-$\frac{1}{2}$ XY model with nearest-neighbor $\left(J_1\right)$, and next-nearest-neighbor $\left(J_2\right)$ interactions in the presence of a three-spins chiral $\left(J_{\chi }\right)$ term using density matrix renormalization group methods. We obtain a quantum phase diagram with both conventionally ordered and topologically ordered phases. In particular, the long-sought Kalmeyer-Laughlin CSL is shown to emerge under a small $J_{\chi }$ perturbation due to the interplay of the magnetic frustration and chiral interactions. The CSL, which is a nonmagnetic phase, is identified by the scalar chiral order, the finite spin gap on a torus, and the chiral entanglement spectrum described by chiral $SU{\left(2\right)}_1$ conformal field theory.

We have investigated the charge dynamics of the correlated Dirac semimetallic perovskite ${\mathrm{CaIrO}}_3$ by infrared spectroscopy. The optical conductivity spectra show the $\omega$-linear-type interband transition and a tiny Drude response at low temperatures, suggesting that the Dirac node is pinned near the Fermi energy due to the Mott criticality. Moreover, a large polaron absorption appears at around 0.04 eV, when thermally excited carriers diminish and the plasma frequency decreases below the optical phonon energy. The electron correlation and resultant reduced plasma frequency are likely to enhance the electron-phonon interaction via the insufficient electronic screening, realizing the Dirac polaron.

Based on the Boltzmann transport theory, we study the origin of the anomalous temperature dependence of the Nernst coefficient ($\nu$) due to the phonon-drag mechanism. For narrow-gap semiconductors, we find that there are two characteristic temperatures at which a noticeable peak structure appears in $\nu$. Contrarily, the Seebeck coefficient ($S$) always has only one peak. While the breakdown of the Sondheimer cancellation due to the momentum dependence of the electron relaxation time is essential for the peak in $\nu$ at low $T$, the contribution of the valence band to the phonon-drag current is essential for the peak at higher $T$. By considering this mechanism, we successfully reproduce $\nu$ and $S$ of ${\mathrm{FeSb}}_2$, for which a gigantic phonon-drag effect is observed experimentally.

We develop the complete theory for the collective plasmon modes of an interacting electron system in the presence of explicit mass (or velocity) anisotropy in the corresponding noninteracting situation, with the effective Fermi velocity being different along different axes. Such effective mass anisotropy is common in solid state materials (e.g., silicon or germanium), where the Fermi surface is often not spherical. We find that the plasmon dispersion itself develops significant anisotropy in such systems, and the commonly used isotropic approximation of using a density of states or optical effective mass does not work for the anisotropic system. We predict a qualitatively new phenomenon in anisotropic systems with no corresponding isotropic analog, where the plasmon mode along one direction decays into electron-hole pairs through Landau damping while the mode remains undamped and stable along a different direction.

Topological phase, possessing inherent robustness against disorder, plays a central role in understanding matter, tailoring properties of artificial materials, and holding the promise of fault-tolerant quantum simulation and computing. Various topological phases and accessible controllability are being enthusiastically pursued in a wide spectrum ranging from condensed-matter to photonic systems. Recently, a topological photonic superlattice with arbitrary number of sites in each unit cell was proposed to provide richer topological properties. Here we successfully map such a multipartite structure into a photonic chip by using a femtosecond laser direct writing technique and demonstrate a direct observation of topological photonic superlattices in the quantum regime. Besides the tunable number of topologically protected edge states, we also observe stable interface states induced by the interference of topological linear modes, which exhibits Bloch-oscillation-like breathing dynamic behavior but free of nonlinearity.

Wave propagation and diffusion in linear materials preserve local reciprocity in terms of a symmetric Green's function. For wave propagations, the relation between the fields entering and leaving a system is more relevant than the detailed information about the fields inside it. In such cases, the global reciprocity of the scattering off a system through several ports is more important, which is defined as the symmetric transmission between the scattering channels. When a two-port system supports nonreciprocal (electromagnetic, acoustic) wave propagation, it is a (optical, phonon) diode directly following the definition. However, to date no concrete definition or discussion has been made on the global reciprocity of diffusive processes through a multiple-port system. It thus remains unclear what are the differences and relations between the three concepts, namely, local nonreciprocity, global nonreciprocity, and diode effect in diffusion. Here, we provide theoretical analysis on the frequency-domain Green's function and define the global reciprocity of heat diffusion through a two-port system, which has a different setup from that of a thermal diode. We further prove the equivalence between a heat transfer system with broken steady-state global reciprocity and a thermal diode, assuming no temperature-dependent heat generation. The validities of some typical mechanisms in breaking the diffusive reciprocity and making a thermal diode have been discussed. Our results set a general background for future studies on symmetric and asymmetric diffusive processes.

Our theoretical analysis shows that a femtosecond laser pulse can efficiently launch magnetization dynamics of ${\mathrm{Dy}}^{3+}$ ions in ${\mathrm{DyFeO}}_3$ and ${\mathrm{DyAlO}}_3$. Excitation of electrons from the ground state to the low-lying electronic level of ${\mathrm{Dy}}^{3+}$ ions by circularly or linearly polarized light can be seen as a result of an effective magnetic field acting on the magnetic moments of the rare-earth ions. It is shown that the launched magnetization dynamics can be expressed as a combination of coherent oscillations of mutually parallel and mutually antiparallel magnetic moments of ${\mathrm{Dy}}^{3+}$ ions, respectively. While the antiparallel magnetic moments lie in the plane perpendicular to the wave vector of light in the medium $\mathit{k}$, the parallel magnetic moments are aligned along $\mathit{k}$. The magnetization dynamics depend strongly on the duration and the shape of the pumping laser pulse, as well as on the anisotropy in properties of the rare-earth ion.

Antiferromagnetic spintronic devices have the potential to greatly outperform conventional ferromagnetic devices due to their ultrafast dynamics and high data density. A challenge in designing these devices is the control and detection of the orientation of the antiferromagnet. One of the most promising ways to achieve this is through the exchange bias effect. This is of particular importance in large-scale multigranular devices. Previously, due to the large system sizes, only micromagnetic simulations have been possible, with an assumed distribution of antiferromagnetic anisotropy directions and grain size. Here, we use an atomistic model where the distribution of antiferromagnetic anisotropy directions occurs naturally and where the exchange bias occurs due to the intrinsic disorder in the antiferromagnet. We perform large-scale simulations of exchange bias, generating realistic values of exchange bias. We find a strong temperature dependence of the exchange bias, in agreement with experimental observations, approaching zero at the blocking temperature of the antiferromagnet. We find that the experimentally observed increase in the coercivity at the blocking temperature occurs due to the superparamagnetic flipping of the antiferromagnet during the hysteresis loop cycle. We find a large discrepancy between the exchange bias predicted from a geometric model of the antiferromagnetic interface, indicating the importance of grain edge effects in multigranular exchange biased systems. The grain size dependence shows the expected peak due to a competition between the superparamagnetic nature of small grains and reduction in the statistical imbalance in the number of interfacial spins for larger grain sizes. Our simulations confirm the existence of single antiferromagnetic domains within each grain. The model gives insights into the physical origin of exchange bias and provides a route to developing optimized nanoscale antiferromagnetic spintronic devices.

We investigate systematically the spin-Nernst effect in Néel and zigzag ordered honeycomb antiferromagnets. Monolayers of transition-metal trichalcogenides, $\mathrm{Mn}\mathrm{P}{\mathrm{Se}}_3$, $\mathrm{Mn}{\mathrm{PS}}_3$, and ${\text{VPS}}_3$ show an antiferromagnetic Néel order while $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_3$, $\mathrm{Ni}{\mathrm{PS}}_3$, and $\mathrm{Ni}\mathrm{P}{\mathrm{Se}}_3$ show an antiferromagnetic zigzag order. We extract the exchange and Dzyaloshinskii-Moriya interaction parameters from ab initio calculations. Using these parameters, we predict that the spin-Nernst coefficient is at least two orders of magnitude larger in zigzag compared to the Néel ordered antiferromagnets. We find that this enhancement relies on the large band splitting due to the symmetry of magnetic configuration in the zigzag order. Our calculations indicate that the Dzyaloshinskii-Moriya interaction is the underlying factor for the spin-Nernst effect in both cases, although with different microscopic mechanisms. In the case of Néel antiferromagnets, magnon bands already possess a Berry curvature and introducing the Dzyaloshinskii-Moriya interaction splits the magnon bands with the opposite helicity throughout the Brillouin zone which results in an unbalanced population of magnons carrying opposite spins. In the case of zigzag antiferromagnets, magnon bands do not possess the Berry curvature but they are split for opposite helicity magnons due to symmetry of the system. In this case, introducing the Dzyaloshinskii-Moriya interaction induces the Berry curvature and results in the spin-Nernst effect. Due to large magnon band splitting, the spin-Nernst effect in zigzag antiferromagnets is stronger than Néel antiferromagnets.

We examine the role of strong nonlinearity on the topologically robust edge state in a one-dimensional system. We consider a chain inspired from the Su-Schrieffer-Heeger model but with a finite-frequency edge state and the dynamics governed by second-order differential equations. We introduce a cubic onsite nonlinearity and study this nonlinear effect on the edge state's frequency and linear stability. Nonlinear continuation reveals that the edge state loses its typical shape enforced by the chiral symmetry and becomes generally unstable due to various types of instabilities that we analyze using a combination of spectral stability and Krein signature analysis. This results in an initially excited nonlinear-edge state shedding its energy into the bulk over a long time. However, the stability trends differ both qualitatively and quantitatively when softening and stiffening types of nonlinearity are considered. In the latter, we find a frequency regime where nonlinear edge states can be linearly stable. This enables high-amplitude edge states to remain spatially localized without shedding their energy, a feature that we have confirmed via long-time dynamical simulations. Finally, we examine the robustness of frequency and stability of nonlinear edge states against disorder, and find that those are more robust under a chiral disorder compared to a nonchiral disorder. Moreover, the frequency-regime where high-amplitude edge states were found to be linearly stable remains intact in the presence of a small amount of disorder of both types.

Recent articles have suggested that the quasi-two-dimensional antiferromagnet ${\mathrm{MnPS}}_3$ may have nonreciprocal magnons, whereby magnons in a Brillouin zone corner at $+$q have different energies than those at $-$q. The magnons along the Brillouin zone boundaries were measured using neutron three-axis spectrometry, paying careful attention to the resolution function, to determine whether such nonreciprocity was present. The data show that, within the resolution, there are no significant differences between the magnons in opposite Brillouin zone corners. The findings are combined with a recently published study of ${\mathrm{MnPS}}_3$ by neutron resonant spin-echo spectroscopy and discussed within the context of the theories for nonreciprocal magnons.

First-order reversal curve (FORC) measurements are a powerful tool to study magnetization reversal processes and interactions in heterogeneous systems with broad coercivity distributions. In NdFeB hard magnets an additional soft magnetic component is often observed possibly originating from damaged surface grains. Here we use FORC to study the reversal processes and interactions in these permanent magnets at different temperatures between 50 and 350 K. The measured reversal curves reveal a strongly coupled switching of the soft and hard magnetic components above 250 K. Below this temperature the two components are decoupled and switch almost independently. This decrease in effective interaction at lower temperatures is also observed in the FORC diagrams by a relative reduction in intensity of the so called interaction peak. This result proves that FORC is a powerful method, contributing to a better understanding of magnetization reversal processes and interactions in permanent magnets.

Using density functional theory together with Monte Carlo simulations, we demonstrate that epitaxial strain, both compressive and tensile, attenuates the spin frustration of double perovskite ${\mathrm{Sr}}_2\mathrm{Fe}\mathrm{Os}{\mathrm{O}}_6$ to significantly enhance the critical temperature to 310 K, enabling room-temperature applications. We discover under tensile strain a tetragonal ($I4/m$)-to-monoclinic ($P{2}_1/n$) structural transition concomitant with an antiferromagnetic-to-ferrimagnetic transition. Furthermore, an indirect-to-direct band gap transition is observed with the valence and conduction states localized on different transition metal sublattices, opening a route to efficient electron-hole separation upon photoexcitation.

Spatiotemporal spin dynamics under spin-orbit interaction is investigated in a (001) GaAs two-dimensional electron gas using magneto-optical Kerr rotation microscopy. Spin-polarized electrons are diffused away from the excited position, resulting in spin precession because of the diffusion-induced spin-orbit field. Near the cancellation between the spin-orbit field and the external magnetic field, the induced spin precession frequency depends nonlinearly on the diffusion velocity, which is unexpected from the conventional linear relation between the spin-orbit field and the electron velocity. This behavior originates from an enhancement of the spin relaxation anisotropy by the electron velocity perpendicular to the diffused direction. We demonstrate that the spin relaxation anisotropy, which has been regarded as a material constant, can be controlled via diffusive electron motion.

We develop a theory of the coherent photogalvanic effect (CPGE) in low-dimensional superconductors in the fluctuating regime. It manifests itself in the appearance of a stationary electric current of Cooper pairs under the action of two coherent electromagnetic fields of light with frequencies lying in the sub-terahertz range. We derive the general formula for the electric current density, study the particular cases of linear and circular polarizations of the external light fields, and show that the current might have a nonmonotonous spectrum at certain polarization angles and turns out very sensitive to the proximity of the ambient temperature to the critical temperature of superconducting transition: Approaching the critical temperature, the peak in the spectrum becomes narrower, its frequency experiences a redshift, and the intensity of the peak drastically grows.

We employ momentum-resolved electron energy loss spectroscopy (M-EELS) on ${\mathrm{Bi}}_{2.1}{\mathrm{Sr}}_{1.9}{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ to resolve the issue of the kink feature in the electron dispersion widely observed in the cuprates. To this end, we utilize the GW approximation to relate the density response function measured in M-EELS to the self-energy, isolating contributions from phonons, electrons, and the momentum dependence of the effective interaction to the decay rates. The phononic contributions, present in the M-EELS spectra due to electron-phonon coupling, lead to kink features in the corresponding single-particle spectra at energies between 40 and 80 meV, independent of the doping level. We find that a repulsive interaction constant in momentum space is able to yield the kink attributed to phonons in ARPES. Hence, our analysis of the M-EELS spectra points to local repulsive interactions as a factor that enhances the spectroscopic signatures of electron-phonon coupling in cuprates. We conclude that the strength of the kink feature in cuprates is determined by the combined action of electron-phonon coupling and electron-electron interactions.

We investigate the magnetic excitations in view of the recent reports suggesting that the spin-wave energy may exhibit a significant dependence on the in-plane strain of a thin film of ${\mathrm{La}}_2{\mathrm{CuO}}_4$. The nature of dependence, as we find, can be explained naturally within a two-orbital model based on the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals. In particular, as the orbital-splitting energy between the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals increases with compressive strain, the zone-boundary spin-wave energy hardens. However, the hardening persists only until the orbital splitting reaches $\sim 2$ eV, beyond which there is no significant change. The behavior of zone-boundary spin-wave energy is explained in terms of the extent of hybridization between one of the exchange-split $d_{x^2-y^2}$ bands which is nearly half-filled and the $d_{3z^2-r^2}$ band. The role of second order antiferromagnetic superexchange process involving the interorbital hopping is also discussed.