We study an XXZ spin chain, where each spin is coupled to an independent ohmic bath of harmonic oscillators at zero temperature. Using bosonization and numerical techniques, we show the existence of two phases separated by a Kosterlitz-Thouless transition. At low coupling with the bath, the chain remains in a Luttinger liquid (LL) phase with a reduced but finite spin stiffness, while above a critical coupling, the system is in a dissipative phase characterized by a vanishing spin stiffness. We argue that the transport properties are also inhibited: The LL is a perfect conductor, while the dissipative phase displays finite resistivity. Our results show that the effect of the bath can be interpreted as annealed disorder-inducing signatures of localization.

At a certain “magic” relative twist angle of two graphene sheets it remains a challenge to obtain a detailed description of the proliferation of correlated topological electronic phases and their filling dependence. We perform a self-consistent real-space Hartree-Fock study of an effective moiré lattice model to map out the preferred ordered phases as a function of Coulomb interaction strength and moiré flat-band filling factor. It is found that a quantum valley Hall phase, previously discovered at charge neutrality, is present at all integer fillings for sufficiently large interactions. However, except for charge neutrality, additional spontaneous spin/valley polarization is present in the ground state at nonzero integer fillings, leading to Chern insulator phases and anomalous quantum Hall effects at odd filling factors, thus constituting an example of interaction-driven nontrivial topology. At weaker interactions, all nonzero integer fillings feature metallic inhomogeneous spin/valley ordered phases which may also break additional point group symmetries of the system. We discuss these findings in the light of previous theoretical studies on and recent experimental developments related to magic-angle twisted bilayer graphene.

Low dimensionality in charge density wave (CDW) systems leads to anisotropic optical properties, in both equilibrium and nonequilibrium conditions. Here, we perform polarized two-color pump probe measurements on a quasi-one-dimensional material ${\mathrm{ZrTe}}_3$, in order to study the anisotropic transient optical response in the CDW state. Profound in-plane anisotropy is observed with respect to the polarization of probe photons. Below $T_{\mathrm{CDW}}$ both the quasiparticle relaxation signal and amplitude mode (AM) oscillation signal are much larger, with ${\mathbf{E}}_{\mathrm{pr}}$ nearly parallel to the $a$ axis (${\mathbf{E}}_{\mathrm{pr}}\parallel a$) than for ${\mathbf{E}}_{\mathrm{pr}}$ parallel to the $b$ axis (${\mathbf{E}}_{\mathrm{pr}}\parallel b$). This reveals that the ${\mathbf{E}}_{\mathrm{pr}}\parallel a$ signal provides much a larger response to the variation of the CDW gap. Interestingly, the lifetime of the AM oscillations observed with ${\mathbf{E}}_{\mathrm{pr}}\parallel b$ is longer than ${\mathbf{E}}_{\mathrm{pr}}\parallel a$. Moreover, at high pump fluence where the electronic order melts and the AM oscillations vanish for ${\mathbf{E}}_{\mathrm{pr}}\parallel a$, the AM oscillatory response still persists for ${\mathbf{E}}_{\mathrm{pr}}\parallel b$. We discuss the possible origins that lead to such an unusual discrepancy between the two polarizations.

Experiments on collisions of isolated electrons guided along the edges in quantum Hall setups can mimic mixing of photons with the important distinction that electrons are charged fermions. In the so-called electronic Hong-Ou-Mandel (HOM) setup uncorrelated pairs of electrons are injected toward a beam splitter. If the two electron wave packets were identical, then Fermi statistics would force the electrons to scatter to different detectors, yet this quantum antibunching may be confounded by Coulomb repulsion. Here we model an electronic HOM experiment using a quadratic two-dimensional saddle point potential for the beam splitter and unscreened Coulomb interaction between the two injected electrons subjected to a strong out-of-plane magnetic field. We show that classical equations of motion for the drift dynamics of electrons' guiding centers take on the form of Hamilton equations for canonically conjugated variables subject to the saddle point potential and the Coulomb potential where the dynamics of the center-of-mass coordinate and the relative coordinate separate. We use these equations to determine collision outcomes in terms of a few experimentally tuneable parameters: the initial energies of the uncorrelated electrons, relative time delay of injection, and the shape of the saddle point potential. A universal phase diagram of deterministic bunching and antibunching scattering outcomes is presented with a single energy scale characterizing the increase of the effective barrier height due to interaction of coincident electrons. We suggest clear-cut experimental strategies to detect the predicted effects and give analytical estimates of conditions when the classical dynamics is expected to dominate over quantum effects.

In this work, a doubly resonant photonic crystal (PhC) cavity using the merged bound state in the continuum (BIC) is proposed to obtain a higher second harmonic generation (SHG) efficiency. First, by scanning geometry parameters, the accidental BICs and a band-edge mode outside the light cone can be obtained. Then, as the lattice constant or the thickness of the slab is adjusted, the accidental BICs will merge. A supercell with large and small holes is constructed and the band-edge mode outside the light cone can be matched with the merged BIC mode. Finally, the heterostructure PhC cavity is designed. The merged BIC show a high quality factor of about 2000 for the PhC with finite size. The value is four times larger than the one in isolated BIC (about 500). Consequently, the SHG efficiency of the lattice constant near merged BIC of about 4800% ${\mathrm{W}}^{-1}$ is higher than the one of the isolated BIC (about 1000% ${\mathrm{W}}^{-1}$). This work is expected to provide guidance to the efficient nonlinear conversion in a small volume.

Ferroelectricity and superconductivity are prominent yet distinct quantum phenomena; materials exhibiting both of these phenomena are rare and of great fundamental and practical interest. Here, combining ab initio calculations and in situ measurements, we show that ferroelectric ${\mathrm{SnPS}}_3$ turns into a superconductor under pressure at 31.7 GPa, accompanied by a structural phase transition. Electronic band structure calculations reveal a partial flat band near the Fermi energy in compressed ${\mathrm{SnPS}}_3$, suggesting correlation effects as a possible origin of the observed superconducting state. The discovery of a pressure-induced ferroelectric-to-superconductor transition in ${\mathrm{SnPS}}_3$ raises the prospect of establishing this intriguing quantum phenomenon among a large class of metal phosphorous trichalcogenides, thereby broadening the material basis to elucidate the underlying physics.

The electronic structure of ${\mathrm{NdTe}}_3$ in the charge density wave phase (CDW) is investigated by angle-resolved photoemission spectroscopy. The combination of high-quality crystals and careful surface preparation reveals subtle and previously unobserved details in the Fermi surface topology, allowing an interpretation of the rich and unexplained quantum oscillations in the rare earth tritellurides ${\mathrm{RTe}}_3$. In particular, several closed Fermi surface elements can be observed that are related to CDW-induced replicas of the original bands, leading to the curious situation in which a CDW does not only remove Fermi surface elements but creates new ones that are observable in transport experiments. Moreover, a large residual Fermi surface is found in the CDW gap, very close to the position of the gapped normal-state Fermi surface. Its area agrees very well with high-frequency quantum oscillations in ${\mathrm{NdTe}}_3$ and its presence is explained by either a phase separation between normal state and CDW regions or by strong electron-phonon coupling combined with the quasi one-dimensional character of the CDW. Finally, we identify the origin of the low-frequency $\alpha$ quantum oscillations ubiquitous for the lighter R elements in the ${\mathrm{RTe}}_3$ family and responsible for the high mobility in these compounds.

Spectral winding of complex eigenenergies represents a topological aspect unique in non-Hermitian systems, which vanishes in one-dimensional (1D) systems under the open boundary conditions (OBC). In this Letter, we discover a boundary spectral winding in two-dimensional non-Hermitian systems under the OBC, originating from the interplay between Hermitian boundary localization and non-Hermitian nonreciprocal pumping. Such a nontrivial boundary topology is demonstrated in a non-Hermitian breathing Kagome model with a triangle geometry, whose 1D boundary mimics a 1D non-Hermitian system under the periodic boundary conditions with nontrivial spectral winding. In a trapezoidal geometry, this boundary spectral winding can even coexist with corner accumulation of edge states, instead of extended ones along the 1D boundary of a triangle geometry. An OBC type of hybrid skin-topological effect may also emerge in a trapezoidal geometry, provided the boundary spectral winding completely vanishes. By studying the Green's function, we unveil that the boundary spectral winding can be detected from a topological response of the system to a local driving field, offering a realistic method to extract the nontrivial boundary topology for experimental studies.

Second-order topological phases in artificial systems have been extensively studied, but studies in the phonons of atomic materials are limited. In this paper, we propose that phononic second-order topological phase exists in ${\mathrm{C}}_3\mathrm{N}$, a previously synthesized and intensively investigated two-dimensional material. Its nontrivial phase arises from the mismatch between the Wannier centers of the out-of-plane phonon modes and the atomic positions. Using a simplified force constant model, we find that gapped edge modes and in-gap corner modes only exist on the structures with broken pure-carbon-ring terminations, and this unexpected phenomenon can be explained by the electronlike filling anomaly for phonons. Further calculations reveal that these corner modes are robust to external disturbances. The nontrivial phononic phase in ${\mathrm{C}}_3\mathrm{N}$ provides an avenue in crystalline materials to explore higher-order topological phases in Bose systems.

The effect of finite-temperature magnetic excitations on the plasticity of body-centered cubic chromium is studied. In chromium, the magnetic order is disrupted by the $\frac{1}{2}\langle 111\rangle$ Burgers vectors of dislocations, creating magnetic frustrations partially resolved through the generation of magnetic faults. These faults may bear consequences on plasticity, the controlling parameter being their energy. Through the inclusion of finite-temperature magnetic excitations, we show in this work that these faults vanish below the Néel temperature, thus leaving $\frac{1}{2}\langle 111\rangle$ dislocations free to move. When the faults have disappeared, complex noncollinear magnetic structures are stabilized, surrounding the region sheared by these dislocations, with a negligible excess magnetic energy.

We investigate the nonlinear response of the Kitaev honeycomb lattice model in a weak magnetic field using the theory of two-dimensional (2D) coherent spectroscopy. We observe that at the isotropic point in the non-Abelian phase of this model, the nonlinear spectrum in the 2D frequency domain consists of sharp signals that originate from the flux excitations and Majorana bound states. Signatures of different flux excitations can be clearly observed in this spectrum, such that one can observe evidences of flux states with 4-adjacent, 2-nonadjacent, and 4-far-separated fluxes, which are not visible in linear response spectroscopy such as neutron scattering experiments. Moreover, in the Abelian phase we perceive that the spectrum in the frequency domain is composed of streak signals. These signals, as in the nonlinear response of the pure Kitaev model, represent a distinct signature of itinerant Majorana fermions. However, deep in the Abelian phase whenever a Kitaev exchange coupling is much stronger than the others, the streak signals are weakened and only single sharp spots are seen in the response, which resembles the dispersionless response of the conventional toric code.

We analyze, both experimentally and numerically, the nonlinear regime of the photoinduced coherent magnetization dynamics in cobalt-doped yttrium iron garnet films. Photomagnetic excitation with femtosecond laser pulses reveals a strongly nonlinear response of the spin subsystem with a significant increase of the effective Gilbert damping. By varying both laser fluence and the external magnetic field, we show that this nonlinearity originates in the anharmonicity of the magnetic energy landscape. We numerically map the parameter work space for the nonlinear photoinduced spin dynamics below the photomagnetic switching threshold. Corroborated by numerical simulations of the Landau-Lifshitz-Gilbert equation, our results highlight the key role of the cubic symmetry of the magnetic subsystem in reaching the nonlinear spin precession regime. These findings expand the fundamental understanding of laser-induced nonlinear spin dynamics as well as facilitate the development of applied photomagnetism.

The intercalation of magnetic elements in nonmagnetic van der Waals (vdW) materials is an effective way to design different (quasi) 2D magnets and produce exotic properties. More specifically, how exactly the intercalator is distributed within the synthetic crystal can also affect the physical properties substantially. In contrast to conventional $3d$ transition-metal intercalates of niobium and tantalum dichalcogenides, which commonly have $2\times 2$ or $\sqrt{3}\times \sqrt{3}$ type ordered intercalation, we report a disordered intercalation of Co atoms between the vdW gaps of $2H$-tantalum disulfide ($2H\text{-Ta}{\mathrm{S}}_2$). The obtained quasi-vdW ferromagnet ${\mathrm{Co}}_{0.27}\mathrm{Ta}{\mathrm{S}}_2$ shows both perpendicular magnetic anisotropy and thickness-tunable magnetic properties. More interestingly, the temperature dependence of electrical resistivity shows a semiconductorlike behavior, in contrast to the metallic feature of other analogs in this material family. This unexpected phenomenon can be understood through a variable-range hopping mechanism, which is due to highly disordered intercalation. Moreover, ${\mathrm{Co}}_{0.27}\mathrm{Ta}{\mathrm{S}}_2$ shows a side-jump scattering dominated anomalous Hall effect, which can also be related to the disordered distribution of Co intercalators.

We investigate dynamics of a single mobile impurity immersed in a bath of Anderson localized particles and focus on the regime of relatively strong disorder and interactions. In that regime, the dynamics of the system is particularly slow, suggesting, at short times, an occurrence of many-body localization. Considering longer timescales, we show that the latter is a transient effect and that, eventually, the impurity spreads subdiffusively and induces a gradual delocalization of the Anderson insulator. The phenomenology of the system in the considered regime of slow dynamics includes a subdiffusive growth of mean square displacement of the impurity, power-law decay of density correlation functions of the Anderson insulator, and a power-law growth of entanglement entropy in the system. We observe a similar regime of slow dynamics also when the disorder in the system is replaced by a sufficiently strong quasiperiodic potential.

Space-time reflection symmetry (PT symmetry) in non-Hermitian electronic models has drawn much attention over the past decade mainly because it guarantees that the band structures calculated under open boundary conditions be the same as those calculated under periodic boundary conditions. PT symmetry in electromagnetic (EM) models, which are usually borrowed from electronic models, has also been of immense interest, mainly because it leads to “exceptional” parameter values below which non-Hermitian operators have real eigenvalues, although PT symmetry is not the sole symmetry that allows such exceptional parameter values. In this work, we examine one-dimensional PT-symmetric non-Hermitian EM models to introduce novel concepts and phenomena. We introduce the band-structure concept of “fixed points,” which leads to “bidirectional” reflection zeros in the corresponding finite structures, contrary to a common belief about the EM structures with PT symmetry (and without parity inversion and time-reversal symmetries). Some of the fixed points manifest themselves as, what we name, “extended states in the bandgap” on the band structure while some other fixed points are the “turning points” of the band structure. The extended states in the bandgap are in fact the dual of the well-known “bound states in the continuum,” while the turning points allow us to observe “ideal” superluminal tunneling in the corresponding finite structures. By “ideal” superluminal tunneling, we mean the case where not only the transmission coefficient has an almost uniform phase over a broad bandwidth, but also the magnitudes of the transmission and reflection coefficients are almost equal to unity and zero, respectively, over the bandwidth.

Important aspects of exchange-coupled magnetic layered structures are related to the noncollinear arrangement of sublayer magnetizations which can arise from competition between bilinear (BL) and biquadratic (BQ) interlayer exchange coupling (IEC). In this work, the influence of coexisting BL and BQ IEC of different strengths on magnetization precession in layered systems is investigated both experimentally and theoretically. Laser-induced magnetization precession has been studied in the $\text{Fe/Si}(d_{\text{Si}}$) multilayers (MLS) as a function of the amplitude ($H$) and orientation angle (${\theta }_H$) of external magnetic field using time-resolved magneto-optical Kerr (TRMOKE) effect. Strongly changing characters of precession frequency dependencies $\omega (H,{\theta }_H)$ for Fe sublayer thickness $d_{\text{Fe}}=3$ nm and Si spacer-layer thicknesses ($d_{\text{Si}}$) varying in the range of 0.9–2.4 nm have been observed. Analytical formulas for acoustic and optic mode dispersion relations with coexisting BL and BQ IEC, scaled by $J_1$ and $J_2$ parameters, respectively, for the in-plane effective magnetic anisotropy and arbitrary magnetic field direction were derived, and very good agreement with the experimentally observed frequency dependencies has been obtained. It is shown that BQ coexisting with BL IEC significantly influences on the magnitude and form of dispersion relations. From analytical formula derived, it follows that zero-field optical mode frequency tends to zero as $|J_1|$ approaches $2J_2$. The acoustic and optic mode-crossing effect has been observed and it is found that values of crossing fields and frequency gaps strongly increase as ${\theta }_H$ angles decrease and depend on relative BL and BQ IEC strengths. The BL IEC is of ferromagnetic type with $J_1\approx 1.6$ mJ/${\mathrm{m}}^2$ for the MLS with $d_{\text{Si}}=0.9$ nm, and changes to antiferromagnetic one with $J_1\approx -0.9$ mJ/${\mathrm{m}}^2$ for the MLS with $d_{\text{Si}}=1.4$ nm, while the $J_2$ parameter of BQ IEC decreases from 1.8 to 1.0 mJ/${\mathrm{m}}^2$. The coupling strengths decrease by one to two orders of magnitude for the sample with $d_{\text{Si}}=2.4$ nm, but both mode frequencies are still observed and well reproduced by the theory. It is shown that $J_1$ and $J_2$ parameters obtained in the TRMOKE experiment coincide within the estimated error bars with the determined from independent measurements of magnetization processes in the static magneto-optical Kerr effect and interpreted with the use of analytical formulas derived. Numerical solutions of coupled Landau-Lifshitz-Gilbert (LLG) equations for acoustic and optic modes, with inclusion of BL IEC, intrinsic Gilbert damping, and spin-pumping damping terms, and extended to include BQ IEC, were performed and fitted to experimental data. It is shown that determined effective damping coefficients on $H$ and ${\theta }_H$ dependencies for acoustic and optic modes are very well simulated with the use of LLG equation solutions with Gilbert damping, spin-pumping-damping–related effective spin-mixing conductance, and spin-diffusion length parameters included. The dependencies of the parameters on $d_{\text{Si}}$ spacer-layer thickness are discussed and compared with available data for other systems.

We have studied the finite-temperature magnetic phase transition using a combination of first-principles calculations and the second-order Holstein-Primakoff approximation of the anisotropic Heisenberg model for hexagonal $\mathrm{Mn}X$ ($X=\mathrm{N}$, P, As, and Sb) monolayers. The $\mathrm{Mn}X$ monolayers are all half-metal with dynamically and thermally stable atomic structures at $T=300$ K. The hexagonal MnN is an out-of-plane easy-axis ferromagnetically ordered monolayer with the Curie temperature close to the room temperature. However, the other three $\mathrm{Mn}X$ ($X=\mathrm{P}$, As, and Sb) monolayers have an easy axis inside the plane with Curie temperature close to zero. The $\mathit{ab}$ initio energy difference between spin configurations is mapped into an anisotropic Heisenberg spin Hamiltonian which is solved using the second-order Holstein-Primakoff approximation. The anharmonic (magnon-magnon) interaction softens the magnetic excitation energy and reduces the magnon energy gap at the $\mathrm{\Gamma }$ point, which is crucial for the finite-temperature long-range magnetic order in two dimensions. As a result, the inclusion of magnon-magnon interaction dramatically reduces the Curie temperature of hexagonal $\mathrm{Mn}X$ monolayers. Although the magnon-magnon interaction is a perturbation in energy dispersion, it has an important effect on the finite-temperature long-range magnetic order in two-dimensional monolayers.

Recent studies on the kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ have revealed an intricate interplay between unconventional charge density waves (CDWs) and superconductivity (SC). Substitutions of Ti for V can perturb the CDW and produce two distinct SC phases with different pairing symmetries in the series of $\mathrm{Cs}{\mathrm{V}}_{3-x}{\mathrm{Ti}}_x{\mathrm{Sb}}_5$. Here, we performed a comprehensive high-pressure study on Ti-doped $\mathrm{Cs}{\mathrm{V}}_{3-x}{\mathrm{Ti}}_x{\mathrm{Sb}}_5$ ($x=0.04,0.15$) samples to elucidate the combined effects of physical pressure and chemical doping on the coexistence of CDWs and SC. Under high pressures, the M-shaped double superconducting dome observed in the parent ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ is retained but very weakened for $x=0.04$, and it is replaced by a single broad superconducting dome for $x=0.15$. Accordingly, the optimal superconducting $T_{\mathrm{c}}$ under high pressures decreases gradually with increasing Ti-doping level. On the other hand, the upper critical field first exhibits an obvious rise with enhanced flux pinning effects by disorders for $x=0.04$ and then is suppressed upon further increasing Ti doping. The constructed $T\text{−}P$ phase diagrams of $\mathrm{Cs}{\mathrm{V}}_{3-x}{\mathrm{Ti}}_x{\mathrm{Sb}}_5$ reveal the competition and coexistence of CDWs and SC. Our results provide more insight into the intertwined competing electronic orders in the Ti-doped kagome superconductors $\mathrm{Cs}{\mathrm{V}}_{3-x}{\mathrm{Ti}}_x{\mathrm{Sb}}_5$.

We theoretically investigate an intrinsic nonlinear anomalous Hall effect (INAHE) in space-time ($\mathcal{PT}$) symmetric antiferromagnetic metals. The INAHE is characterized by an asymmetric and nondissipative part of the second-order electric conductivity tensor in the clean limit in contrast to the Drude-type symmetric conductivity tensor with dissipation. By introducing a multipole description, we show that the emergence of the INAHE is due to active odd-parity magnetic quadrupoles or magnetic toroidal dipoles under magnetic orderings. In order to clarify the microscopic origin of the INAHE, we specifically consider a fundamental tight-binding model of a three-dimensional tetragonal system. We demonstrate that the INAHE arises from the effective coupling between magnetic ordering and antisymmetric spin-orbit interaction. We also discuss essential electron hopping paths driving the INAHE.

We derive recursive relations for the Schrieffer-Wolff (SW) transformation applied to the half-filled Hubbard dimer. While the standard SW transformation is set to block-diagonalize the transformed Hamiltonian solely at the first order of perturbation, we infer from recursive relations two types of modifications, variational or iterative, that approximate, or even enforce for the homogeneous case, the desired block-diagonalization at infinite order of perturbation. The modified SW unitary transformations are then used to design an test quantum algorithms adapted to the noisy and fault-tolerant era. This work paves the way toward the design of alternative quantum algorithms for the general Hubbard Hamiltonian.