The discovery of two-dimensional nontrivial topological insulators has stimulated researchers’ enthusiasm to exploit exotic topological quantum states in low dimensions. Here, we study the intriguing topological flat bands in arsenene monolayers decorated with H and transition metal magnetic atoms by using first-principles simulations and tight-binding models. Remarkably, the functionalized As monolayer can exhibit a time-reversal symmetry breaking Chern insulator (CI) with a unique topologically nontrivial flat band formed by an antibonding state of As $p_z$ and Cr $d_{z^2}$ orbitals. Under applying tensile strain, the As monolayer undergoes a quantum phase transition and behaves as a ferrovalley insulator (FVI). The topological quantum phase transition between CI and FVI is rationalized well by a five-orbital tight-binding model. Our work sheds light on manipulating the rare topological flat bands in layered two-dimensional quantum materials, showing opportunities for promising applications in strongly correlated topological electronics.

We investigate the physics of one-dimensional symmetry-protected topological (SPT) phases protected by symmetries whose symmetry generators exhibit spatial modulation. We focus in particular on phases protected by symmetries with linear (i.e., dipolar), quadratic, and exponential modulations. We present a simple recipe for constructing modulated SPT models by generalizing the concept of decorated domain walls to spatially modulated symmetry defects, and develop several tools for characterizing and classifying modulated SPT phases. A salient feature of modulated symmetries is that they are generically only present for open chains, and are broken upon the imposition of periodic boundary conditions. Nevertheless, we show that SPT order is present even with periodic boundary conditions, a phenomenon we understand within the context of an object we dub a “bundle symmetry.” In addition, we show that modulated SPT phases can avoid a certain no-go theorem, leading to an unusual algebraic structure in their matrix product state descriptions.

We numerically and experimentally demonstrated a higher-order topological state in a three-dimensional (3D) photonic crystal (PhC) with a complete photonic band gap. Two types of cubic lattices were designed with different topological invariants, which were theoretically and numerically confirmed by the finite difference of their Zak phases. Topological boundary states in the two-dimensional interfaces and hinge states in the one-dimensional corners were formed according to the higher order of bulk-boundary correspondence. Microwave measurements of the fabricated 3D PhC containing two boundaries and one corner showed a localized intensity, which confirmed the boundary and hinge states.

The ordinary and quantum Hall effects with fixed carrier types, electron or hole, exhibit well-defined chirality under external magnetic fields, but the analogous chirality of the quantum anomalous Hall effect (QAHE) remains little explored. Here, for the electronic structures of intrinsic QAHE systems, we show that the local Berry curvature is linked with the angular-momentum difference $l_{\delta z}$ of the inverted bands. In the presence of rotation symmetries, the value of $l_{\delta z}$ will be quantized and set as the Chern number. The quantized $l_{\delta z}$ will also give rise to a unique quantized magnetic circular dichroism effect in the case of resonance absorption. By $l_{\delta z}$, we find that the chiral flow-direction of an electron in the QAHE is not explicitly related to the intrinsic spin axis, thereby enabling the spin-momentum-unlocking edge states in an antiferromagnetic QAHE system. The validity of our theory is confirmed through rigorous examinations using perturbation theory, $\mathbf{k}·\mathbf{p}$ modeling, tight-binding modeling, and first-principle calculations, which predicts exotic behavior in experimentally accessible quantum materials.

We study heat radiation and radiative heat transfer for nanoparticles in the presence of an infinitely long cylinder in different geometrical configurations, based on its electromagnetic Green's tensor. The heat radiation of a single particle can be enhanced by placing it close to a nanowire, and this enhancement can be much larger as compared to placing it close to a plate of the same material. The heat transfer along a cylinder decays much slower than through empty vacuum, being especially long-ranged in the case of a perfectly conducting nanowire, and showing nonmonotonic behavior in the case of a SiC cylinder. Exploring the dependence on the relative azimuthal angle of the nanoparticles, we find that the results are insensitive to small angles, but they can be drastically different when the angle is large, depending on the material. Finally, we demonstrate that a cylinder can either enhance or block the heat flux when placed perpendicular to the interparticle distance line, where the blocking in particular is strongly enhanced compared to the geometry of a sphere of same radius.

Ferrovalley, spin, and piezoelectric polarizations are novel characteristics for electronic materials, and so far, there are few reports that these properties coexist in a single system. By first-principles calculations, we predict a series of highly stable intrinsic ferrovalley materials, i.e., single layer $\mathrm{Cr}\mathit{XY}$ ($X=\mathrm{S}$, Se, Te, $Y=\mathrm{F}$, Cl, Br, I) with large valley polarization up 76.1 meV and appropriate Curie temperature higher than room temperature. For CrSF, the large bandgap of spin-up (spin-down) is as high as 1.96 eV (3.11 eV), which is beneficial for generating 100% spin polarized carriers by optical excitation or electrical gating. The quasianomalous valley Hall effect with valley contrast properties can be induced by compressive strain in CrSI and CrTeX ($X=\mathrm{Cl}$, Br, I). Meanwhile, the large in-plane $(-5.78-3.54\mathrm{pm}/\mathrm{V})$ and outside plane piezoelectric polarization ($-5.03-2.53\mathrm{pm}/\mathrm{V}$) of $\mathrm{Cr}\mathit{XY}$ are higher than most reported two-dimensional materials. Our work provides a pathway for a wide variety of applications in nanoelectronics, spintronics, valleytronics, piezoelectrics, and other demanding areas.

We present a generalized dynamical cluster theory for addressing the cluster effects of off-diagonal disorder. In this work, we introduce an extended local degree of freedom, namely the coupling space, to encapsulate the off-diagonal disorder in an auxiliary medium, and to form a cluster coupling space so that each site of the cluster can be homogenously handled. Within the cluster coupling space, the self-consistent auxiliary dynamical cluster approximation (ADCA) with full symmetry of the lattice is developed to treat the diagonal and off-diagonal disorders on an equal footing. ADCA recovers the coherent-potential approximation at the single-site limit and the dynamical cluster approximation for diagonal-only disorder. Especially for the disordered vibrational system, ADCA conserves the force-constant sum rule. As important tests, we apply ADCA to the one- and three-dimensional disordered lattices, and we find that the ADCA density of state results with appropriate cluster size are in very good agreement with the exact and supercell calculations, especially for the localized defect modes. Moreover, the important interplay of mass and force-constant disorders is revealed by comparing the geometrically and arithmetically averaged density of states calculated by ADCA, presenting both effects of significantly enhanced localization and delocalization. The development of ADCA provides an effective approach for simulating both diagonal and off-diagonal disorders in materials.

Quantum spin liquid states have garnered significant attention as potential precursors for high-temperature superconductors. Researchers are aiming to achieve high-temperature superconductivity through regulation. However, previous studies have indicated that candidate materials with honeycomb structures, such as $\mathrm{N}{\mathrm{a}}_2\mathrm{Ir}{\mathrm{O}}_3$ and $\alpha -\mathrm{L}{\mathrm{i}}_2\mathrm{Ir}{\mathrm{O}}_3$, remain in a magnetically ordered and insulating state. Pressure serves as an effective regulatory tool by adjusting atomic interactions through interatomic spacing manipulation, thereby influencing the band structure near the Fermi surface and consequently tuning quantum-state evolution. In this study, interlayer Li were substituted by Ag atoms in $\alpha -\mathrm{L}{\mathrm{i}}_2\mathrm{Ir}{\mathrm{O}}_3$ to obtain the $\mathrm{A}{\mathrm{g}}_3\mathrm{LiI}{\mathrm{r}}_2{\mathrm{O}}_6$, and its transitions in structure and physical properties as functions of temperature and pressure were investigated. It has been observed that $\mathrm{A}{\mathrm{g}}_3\mathrm{LiI}{\mathrm{r}}_2{\mathrm{O}}_6$ remains stable between −190 and ${300}^{\circ }\mathrm{C}$ without undergoing any structural phase transitions. High-pressure phase transitions occur at 3.0–7.5 and 12.0–16.1 GPa. The first structural phase transition, as deduced from high-pressure x-ray diffraction and Raman spectroscopy, is associated with Ir-Ir dimerization and $\mathrm{Ir}{\mathrm{O}}_6$ octahedral distortion. Corresponding resistance measurements indicate a decreasing rate reduction in resistance near 5.2 GPa due to dimerization. Further compression leads to the existence of a minimum room-temperature resistance at $\sim 19.5$ GPa. A transition from negative to positive magnetoresistance occurs at 12.4 GPa under 2 K. Further analysis suggests that the transition from negative to positive magnetoresistance may be connected to the valence change of Ag from +1 to 0. Although the desired insulator-to-metal transition was not achieved, we have explored the correlation between structural and physical property transitions under high pressure, laying the groundwork for future investigations.

Single-shot all-optical helicity-independent switching (AO-HIS) induced by a femtosecond laser pulse has been mainly reported in gadolinium-based rare-earth (RE)–transition-metal alloys such as GdFeCo or GdCo, but the mechanism leading to magnetization switching is a hotly debated topic. Here, we elaborate on a large number of $\mathrm{G}{\mathrm{d}}_y\mathrm{R}{\mathrm{E}}_{1-x-y}\mathrm{C}{\mathrm{o}}_x$ (RE = Dy, Tb, Ho) alloys to tune various magnetic parameters in order to define what the criteria are for observing AO-HIS in such systems. The state diagrams show that two laser-fluence thresholds must be considered: the fluence which induces the single laser-pulse switching ($F_{\mathrm{switch}}$) and the fluence at which the material breaks into a multidomain state ($F_{\mathrm{multi}}$). Those two fluences are shown to behave very differently as a function of the material properties and the laser-pulse duration. The criterion to ensure the observation of AO-HIS is $F_{\mathrm{multi}}>F_{\mathrm{switch}}$. Taking into account the parameters defining the conditions for which multidomain states are created and considering only the angular momentum transfer from the Gd sublattice to the rest of the system explains in large our experimental results. The importance of the compensation in the ferrimagnetic alloys is also discussed. We believe the defined criteria will be an important tool for designing ultrafast spintronic devices based on all-optical switching.

We present a numerical simulation of the x-ray magnetic circular dichroism (XMCD) of the $L_{2,3}$ and $M_{2,3}$ edges of Ru in antiferromagnetic ${\mathrm{RuO}}_2$ using a combination of density functional plus dynamical mean-field theory and configuration interaction treatment of the Anderson impurity model. We study the dependence of the dichroic spectra on the orientation of the Néel vector and discuss it in the context of altermagnetism. An approximate equivalence between the XMCD spectra for geometries with x rays propagating parallel and perpendicular to the Néel vector is found and shown to be exact in the absence of valence spin-orbit coupling and a core-valence multipolar interaction.

A combination of density functional theory in its local density approximation (LDA) with $\mathbf{k}$- and $\omega$-dependent self-energy found from fluctuational–exchange-type random-phase approximation (FLEX–RPA) is utilized here to study superconducting pairing interaction in a prototype cuprate superconductor ${\mathrm{HgBa}}_2{\mathrm{CuO}}_4$. Although the FLEX–RPA methodology has been widely applied in the past to unconventional superconductors, previous studies were mostly based on tight–binding-derived minimal Hamiltonians, while the approach presented here deals directly with the first-principle electronic structure calculation of the studied material where spin and charge susceptibilities are evaluated for a correlated subset of the electronic Hilbert space as it is done in popular $\text{LDA}+U$ and $\text{LDA}+$dynamical mean-field theory methods. Based on our numerically extracted pairing interaction among the Fermi-surface electrons we exactly diagonalize a linearized BCS gap equation, whose highest eigenstate is expectantly found corresponding to $d_{x^2-y^2}$ symmetry for a wide range of on-site Coulomb repulsions $U$ and dopings that we treat using virtual crystal approximation. Calculated normal-state self-energies show a weak $\mathbf{k}$ and strong frequency dependence with particularly large electronic mass enhancement in the vicinity of a spin-density wave instability. Although the results presented here do not bring any surprisingly new physics to this very old problem, our approach is an attempt to establish the numerical procedure to evaluate material specific coupling constant $\lambda$ for high-$T_c$ superconductors without reliance on tight–binding approximations of their electronic structures.

The McMillan-Ginzburg-Landau (MGL) model for charge density waves (CDW) is employed in a systematic phenomenological study of the different phases that have been probed in recent experiments involving transition metal dichalcogenides. We implemented an efficient imaginary time evolution method to solve the MGL equations, which enabled us to investigate the role of different coupling parameters on the CDW patterns and to perform calculations with different energy functionals that lead to several experimentally observed singularities in the CDW phase profiles. In particular, by choosing the appropriate energy functionals, we were able to obtain phases that go beyond the well-known periodic phase slips (discommensurations), exhibiting also topological defects (i.e., vortex-antivortex pairs), domain walls where the CDW order parameter is suppressed, and even CDW with broken rotational symmetry. Finally, we briefly discuss the effect of these different CDW phases on the profile and critical temperature of the competing superconducting state.

Polymorphism in two-dimensional (2D) materials presents a fertile ground for introducing new functionalities and designing novel architectures. Here, using first-principles calculations, we investigate the polymorphs of monolayer $1T\text{−}{\mathrm{TaTe}}_2$, including the high-symmetry phase and various charge density wave (CDW) phases ($3\times 1, 4\times 1, 3\times 3, 2\sqrt{3}\times 2\sqrt{3}, \sqrt{13}\times \sqrt{13}, 4\times 4$, and $\sqrt{19}\times \sqrt{19}$) with diverse physical properties. The high-symmetry $1T$ phase is predicted to be a quantum anomalous Hall metal with ferromagnetism. However, after undergoing the CDW phase transitions, the ferromagnetism vanishes and the nontrivial topological properties are also altered. Particularly, the $4\times 4$ CDW phase with the second lowest total energy exhibits a novel topological insulating state, while the $3\times 3$ CDW phase, possessing the lowest total energy, behaves as a normal metal. We further propose that charge doping can effectively modulate the relative stability of the CDW phases. Upon introducing slight hole doping, the $4\times 4$ CDW becomes the most energetically stable state followed by the $3\times 3$ CDW phase. These findings show the rich landscape of structures and properties of $1T\text{−}{\mathrm{TaTe}}_2$, which will strongly stimulate further investigations and lay the foundation for the development of new electronic devices.

We develop a theoretical framework for describing light-matter interactions in cavity quantum electrodynamics (QED), optimized for efficient convergence at arbitrarily strong coupling strengths and is naturally applicable to low-dimensional materials. This Hamiltonian is obtained by applying a unitary gauge transformation on the $\mathrm{p}·\mathrm{A}$ Hamiltonian, with a shift on both the matter coordinate and the photonic coordinate, then performing a phase rotation and transforming in the reciprocal space of the matter. By formulating the light-matter interaction in terms of an upper-bounded effective coupling parameter, this method allows one to easily converge eigenspectra calculations for any coupling strength, even far into the ultra-strong and deep-strong coupling regimes. We refer to this approach as the reciprocal asymptotically decoupled (RAD) Hamiltonian. The RAD Hamiltonian allows for a fast convergence of the polariton eigenspectrum with a much smaller matter and photon basis, compared to the commonly used $\mathrm{p}·\mathrm{A}$ or dipole gauge Hamiltonians. The RAD Hamiltonian also allows one to go beyond the commonly used long-wavelength approximation and accurately describes the spatial variations of the field inside the cavity, which ensures the conservation of momentum between light and matter.

A spin-1/2 chain system coupled to the elastic subsystem of the crystal is considered. Due to that coupling, the intersite spin nematic ordering in the spin subsystem takes place. The phase transition to the ordered phase is studied using the exact quantum-mechanical approach. It is shown how the nonzero temperature and the external magnetic field affect the transition to the intersite spin nematic phase and the phase itself.

In ferromagnetic metals where the anomalous Hall effect (AHE) occurs, one expects a corresponding thermal Hall transport due to electrons that may be dubbed the anomalous thermal Hall effect (ATHE). The same mechanisms that are responsible for AHE, such as the intrinsic Berry curvature effect (usually the most important) or the side-jump mechanism, should play significant roles in ATHE as well. Despite the obvious correlations between AHE and ATHE expected on generalized Wiedemann-Franz law, however, the actual thermal Hall conductivities from each of these contributions have not been evaluated quantitatively. Here, we investigate the intrinsic and the impurity-scattering-induced side-jump contributions to the AHE and the ATHE in $3d$ ferromagnetic metals, bcc Fe, hcp Co, and fcc Ni. We find that the sum of the intrinsic and the side-jump contributions to anomalous thermal Hall conductivity (ATHC) agrees with the experimental values in Fe and Co rather well. In Ni, the calculated ATHC is much larger than the experimental value. We attribute this difference to the importance of electron-phonon scattering in Ni.

By means of density functional theory plus dynamical mean-field theory, we investigate the correlated electronic structures of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under high pressure. Our calculations show that ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ is a multiorbital Hund metal. Both the $3d_{z^2}$ and $3d_{x^2-y^2}$ orbitals of Ni are close to being half filled and contribute the bands crossing the Fermi level. Band renormalization and orbital selective electronic correlation are observed. Through imaginary-time correlation functions, the discovery of high-spin configuration, spin-frozen phase, and spin-orbital separation shows that the system is in a frozen moment phase at high temperatures above 290 K and is a Fermi liquid at low temperatures, which is further confirmed by the calculated spin, orbital, and charge susceptibilities under high temperatures. Our paper uncovers Hundness in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under high pressure.

Waves in a variety of fields in physics, such as mechanics, optics, spintronics, and nonlinear systems, obey generalized eigenvalue equations. To study non-Hermitian physics of those systems, in this paper, we construct a non-Bloch band theory of generalized eigenvalue problems. Specifically, we show that eigenvalues of a transfer matrix lead to a certain condition imposed on the generalized Brillouin zone, which allows us to develop a theory to calculate the continuum bands. As a concrete example, we examine the non-Hermitian skin effect of photonic crystals composed of chiral metamaterials by invoking our theoretical framework. When the medium has circularly polarized eigenmodes, we find that each eigenmode localizes at either of the edges, depending on whether it is left- or right-circularly polarized. In contrast, when the medium only has linearly polarized eigenmodes, every eigenmode localizes to the edge of the same side independent of its polarization. We demonstrate that the localization lengths of those eigenmodes can be determined from the chiral parameters and eigenfrequencies of the photonic crystal.

Motivated by the recent experimental breakthrough on the observation of the fractional quantum anomalous Hall (FQAH) effects in semiconductor and graphene moiré materials, we explore the rich physics associated with the coexistence of FQAH effect and the charge density wave (CDW) order that spontaneously breaks the translation symmetry. We refer to a state with both properties as “FQAH crystal.” We show that the interplay between FQAH effect and CDW can lead to a rich phase diagram including multiple topological phases and topological quantum phase transitions at the same moiré filling. In particular, we demonstrate the possibility of direct quantum phase transitions from a FQAH crystal with Hall conductivity ${\sigma }_H=-2/3$ to a trivial CDW insulator with ${\sigma }_H=0$ and, more interestingly, to a QAH crystal with ${\sigma }_H=-1$.