A theoretical model for two new tellurite sulfates, namely ${\mathrm{Na}}_2{\mathrm{Cu}}_5\left({\mathrm{TeO}}_3\right){\left({\mathrm{SO}}_4\right)}_3{\left(\mathrm{OH}\right)}_4$ and ${\mathrm{K}}_2{\mathrm{Cu}}_5\left({\mathrm{TeO}}_3\right){\left({\mathrm{SO}}_4\right)}_3{\left(\mathrm{OH}\right)}_4$, is determined to be compatible with ab initio calculations. The results obtained in this paper show that some previous speculations in the literature about the couplings are correct, obtaining a model with a mixture of ferromagnetic and antiferromagnetic couplings. We use a combination of numerical techniques to study the magnetic properties of the model. Our numerical calculations based on the density-matrix renormalization group method reveal that the system presents Ising-like magnetization plateaus at rational values of the saturation magnetization.

In this paper we explore possible phases arising from doping neutral excitons into a Mott insulator in the context of moiré bilayers. We consider two moiré layers coupled together only through interlayer repulsion and there is a U(2) $\times$ U(2) symmetry. The densities of the two layers can be tuned to be $n_t=x,n_b=1-x$ with $n_t+n_b=1$. $x=0$ limit is a layer polarized Mott insulator and small $x$ regime can be reached by doping interlayer excitons at density $x$. Charge gap can remain finite at small $x$, as is demonstrated experimentally in the $\mathrm{W}{\mathrm{Se}}_2\text{−}\mathrm{h}\mathrm{B}\mathrm{N}\text{−}\mathrm{W}{\mathrm{Se}}_2/\mathrm{W}{\mathrm{S}}_2$ system. To capture the intertwinement of the spin and exciton degree of freedom, we propose a four-flavor spin model. In addition to the obvious possibility of single exciton condensation phase, we also identified more exotic physics with fractionalization: (i) We define spin gap ${\mathrm{\Delta }}_t,{\mathrm{\Delta }}_b$ for the two layers respectively. As long as the spin gap at either layer is finite, single exciton condensation is impossible and we can only have paired exciton condensation. If both spin gaps are finite, it can be a fractional exciton superfluid with paired exciton condensation coexisting with $Z_2$ spin liquid. Numerical evidences for such a phase will be provided. (ii) If the layer polarized Mott insulator at $x=0$ is in a U(1) spin liquid with spinon Fermi surface, the natural phase at $x>0$ hosts neutral Fermi surface formed by fermionic excitons. There are metallic counter-flow transport and also Friedel oscillations in layer polarization in this exotic phase. Our numerical simulation in one dimension observes an analog of this neutral Fermi surface phase. (iii) There could be a metal-insulator transition (MIT) by tuning $x$ in the universality class of a bandwidth tuned MIT. We provide one theory for such a continuous Mott transition and predicted a universal drag resistivity.

We design a novel formalism of determinant quantum Monte Carlo method for the half-filled Hubbard model with on-site Hubbard interaction $U$ and nearest-neighbor density-density interaction $V$ on the square lattice. The formalism is free of sign problem for $\left|U\right|\ge 4\left|V\right|$, and is achieved by introducing discrete auxiliary fields on the nearest-neighbor bonds alone. Based on this formalism, we study the ground-state phase diagram of the model systematically using the projector algorithm. Within the sign-problem free parameter space $\left|U\right|\ge 4\left|V\right|$, we obtain antiferromagnetism for $U\ge 4\left|V\right|>0$, charge density wave for $U<-4V$ and $V>0, s$-wave superconductivity for $U<0$ and small negative $V$, and phase separation for $U<0$ and a larger negative $V$. We also obtain the single particle gap and spin excitation spectra, and by comparison to mean field results, as well as available literatures, we discuss the possible phase boundary beyond the sign-problem free region. The unbiased numerically exact results can be taken as benchmarks in future studies. Finally, we discuss possible extension for longer-range interactions in the model.

Periodic driving has a longstanding reputation for generating exotic phases of matter with no static counterparts. This work explores the interplay between periodic driving, interaction effects, and ${\mathbb{Z}}_2$ symmetry that leads to the emergence of Floquet symmetry protected second-order topological phases in a simple but insightful two-dimensional spin-1/2 lattice. Through a combination of analytical and numerical treatments, we verify the formation of corner-localized 0 and $\pi$ modes, i.e., ${\mathbb{Z}}_2$ symmetry broken operators that commute and anticommute, respectively, with the one-period time evolution operator, as well as establish the topological nature of these modes by demonstrating their presence over a wide range of parameter values and explicitly deriving their associated topological invariants under special conditions. Finally, we propose a means to detect the signature of such modes in experiments, and we discuss the effect of imperfections.

Close to the semimetal-to-semiconductor topological phase transition, the band structure of HgCdTe is represented by massive Kane fermions that, in a semirelativistic approach, are characterized by two parameters: rest mass and velocity. Using terahertz magneto-optical spectroscopy, we explore the band structure evolution of HgCdTe films with hydrostatic pressure in the vicinity of the band-gap collapse. By analyzing the energies of interband optical transitions as a function of magnetic field, we have determined the rest mass of Kane fermions at different hydrostatic pressures. The pressure dependence of the rest mass allows us to obtain the hydrostatic deformation potential $a_c-a_v$ at low temperature, where $a_c$ and $a_v$ are the deformation potentials of the conduction and valence bands, respectively.

Elemental tellurium is a strongly spin-orbit coupled Peierls-distorted semiconductor whose band structure features topologically protected Weyl nodes. Using time-dependent density functional theory calculations, we show that impulsive optical excitation can be used to transiently control the amplitude of the Peierls distortion, realizing a mechanism to switch tellurium between three states: Weyl semiconductor, Weyl metal, and non-Weyl metal. Further, we present experimental evidence of this inverse Peierls distortion using time-resolved optical second harmonic generation measurements. These results provide a pathway to multifunctional ultrafast Weyl devices and introduce Peierls systems as viable hosts of light-induced topological transitions.

We study the effect of quasiperiodic Aubry-André disorder on the energy spectrum and eigenstates of a one-dimensional all-band-flat (ABF) diamond chain. The ABF diamond chain possesses three dispersionless flat bands with all the eigenstates compactly localized on two unit cells in the zero disorder limit. The fate of the compact localized states in the presence of the disorder depends on the symmetry of the applied potential. We consider two cases here: a symmetric one, where the same disorder is applied to the top and bottom sites of a unit cell and an antisymmetric one, where the disorder applied to the top and bottom sites are of equal magnitude but with opposite signs. Remarkably, the symmetrically perturbed lattice preserves compact localization, although the degeneracy is lifted. When the lattice is perturbed antisymmetrically, not only is the degeneracy is lifted but compact localization is also destroyed. Fascinatingly, all eigenstates exhibit a multifractal nature below a critical strength of the applied potential. A central band of eigenstates continue to display an extended yet nonergodic behavior for arbitrarily large strengths of the potential. All other eigenstates exhibit the familiar Anderson localization above the critical potential strength. We show how the antisymmetric disordered model can be mapped to a $\pi /4$ rotated square lattice with the nearest and selective next-nearest-neighbor hopping and a staggered magnetic field—such models have been shown to exhibit multifractality. Surprisingly, the antisymmetric disorder (with an even number of unit cells) preserves chiral symmetry—we show this by explicitly writing down the chiral operator.

We perform a detailed analysis of the magnetotransport and de Haas–van Alphen (dHvA) oscillations in crystal PdGa which is predicted to be a typical chiral fermion semimetal from the CoSi family holding a large Chern number. The unsaturated quadratic magnetoresistance and nonlinear Hall resistivity indicate that PdGa may be a multiband system with slight electron-hole compensation. Angular dependence of resistivity in PdGa shows weak anisotropy with twofold or threefold symmetry when the magnetic field rotates within the ($1\overline{1}0$) or (111) plane perpendicular to the current. Nine or three frequencies are extracted after the fast Fourier-transform analysis of the dHvA oscillations with $B\parallel \left[001\right]$ or $B\parallel \left[011\right]$, respectively, which is confirmed to be consistent with the Fermi surfaces obtained from first-principles calculations with spin-orbit coupling considered.

Magnon-mediated spin transport across a nonmagnetic metal (NM) and ferromagnetic insulator (FI) interface depends critically on electron-magnon coupling. We propose a route to enhance electron-magnon coupling from a transport viewpoint. A theoretical formalism for magnon-mediated spin current is developed based on the nonequilibrium Green's function. In the language of transport, the effective electron-magnon coupling at the NM/FI interface is determined by the self-energy of the FI lead, which is proportional to the density of states (DOS) at the interface due to the nonlinear process of electron-magnon conversion. By tuning the interfacial DOS, the spin conductance of two-dimensional (2D) and 3D NM/FI systems can be significantly enhanced by almost three orders of magnitude, setting up a new platform of manipulating electron-magnon coupling.

Spontaneous optical emission of graphene irradiated by intense single-cycle terahertz pulses was studied experimentally and explained theoretically. We found that the emitted photons are mainly polarized normally to the electric field of the terahertz pulse, which proves that the terahertz field not only heats the electrons, but also creates a strongly nonequilibrium momentum distribution. The comparison of the measured optical spectrum and polarization anisotropy with the results of numerical modeling allowed us to estimate the momentum isotropization time for electrons in graphene to be $\sim 25$ fs and roughly reconstruct the time evolution of the distribution function in $k$ space.

Recent experiments reported the puzzling zero-energy modes associated with ferromagnetic impurities in some iron-based superconductors with topological band structures. Here, we show that a sufficiently strong exchange coupling between a ferromagnetic impurity and a substrate can trigger a quantum phase transition, beyond which the phase of the topological surface superconducting order parameter around the impurity acquires a sign change. In such a case, we prove that besides a pair of impurity induced trivial Yu-Shiba-Rusinov states, a degenerate pair of Majorana modes can be induced at the boundary separating the two sign-change regimes and trapped around the impurity in the topological surface superconducting state. Furthermore, we show that our theory can explain the controversial observations and confusing features of the zero-energy modes from recent experiments in some iron-based superconductors.

First-principles calculations are performed to investigate Rashba-type and Zeeman effects in two-dimensional three-atom-layer structures $X\mathrm{Te}$/MnTe/$X\mathrm{Te}$ ($XMX, X=\text{Sn}$, Ge). It is found that the Rashba-type and Zeeman effects that typically compete against each other rather cooperatively work together and are improved in $XMX$, as compared to $X\mathrm{Te}/X\mathrm{Te}/X\mathrm{Te}$ ($XXX$). This cooperation and improvement are highly required in Majorana qubits for topological quantum computing, Josephson junction of topological superconductivity, and other phenomena. The Rashba-type effect is improved because replacing $X\mathrm{Te}$ by MnTe leads to a reduction of the thickness of $XMX$ and increases the interaction between the surface state of the lowest conduction bands and its neighboring states. The improved Zeeman effect in the $XMX$ structure arises from s-d and p-d exchange interactions between Te and Mn ions. The present strategy of simultaneously largely improving Rashba-type and Zeeman effects therefore provides a route to realize stable Majorana fermions, topological superconductivity, and other phenomena that require both large Rashba-type and Zeeman effects.

Ternary hydrides are ideal candidates for high-temperature superconductivity. However, their electronic properties correlate strongly with the charge transfer between their consistent elements. In this work we propose nine Na-B-H compounds, which combines charge modulation between Na and B/H atoms with pressure, with unusual structural motifs and interesting electronic properties. In the metallic $Pmma {\mathrm{NaBH}}_3$, with both ionic and covalent frameworks, formed by H atoms bonded to Na/B atoms, a low-frequency phonon dominated superconductivity is predicted with a $T_{\mathrm{c}}$ of 86.8 K, distinct from the high-frequency H-derived superconductivity associated with covalent H-based skeletons or H cages/sheets in superhydrides. Furthermore, in nonmetallic ${\mathrm{NaBH}}_n$ ($n=1,2$, and 6) and $R\text{−}3m {\mathrm{NaBH}}_3$, the B atom exhibits an ${sp}^3$-hybridized feature analogous to the C atom, resulting in the formation of several alkanelike B-H motifs, such as a graphanelike layer and an ethane-shaped molecule. Our work presents an important step toward the understanding of metal borohydrides.

The continuous spontaneous localization (CSL) model is an alternative formulation of quantum mechanics, which introduces a noise-coupled nonlinearly to the wave function to account for its collapse. We consider CSL effects on quantum computers made of superconducting transmon qubits. As a direct effect CSL reduces quantum superpositions of the computational basis states of the qubits: we show the reduction rate to be negligibly small. However, an indirect effect of CSL, dissipation induced by the noise, also leads transmon qubits to decohere, by generating additional quasiparticles. Since the decoherence rate of transmon qubits depends on the quasiparticle density, by computing their generation rate induced by CSL, we can estimate the corresponding quasiparticle density and thus the limit set by CSL on the performances of transmon quantum computers. We show that CSL could spoil the quantum computation of practical algorithms on large devices. We further explore the possibility of testing CSL effects on superconducting devices.

A model based on random matrix theory (RMT) is used to obtain the full conductance distribution $P\left(g\right)$ in three dimensions (3D) across the Anderson metal-insulator transition, and compared with the corresponding quasi-one-dimensional (Q1D) case where there is only a smooth crossover. We show that while the differences between the two are subtle, the details of the results from the RMT model agree very well with the existing numerical studies of the tight-binding Anderson model in 3D vs Q1D, including the critical region in 3D.

Twisting in bilayers introduces structural chirality with two enantiomers, i.e., left- and right-handed bilayers, depending on the oriented twist angle. The interplay between this global chirality and additional degrees of freedom, such as magnetic ordering and the local octahedral chirality arising from the geometry of the bonds, can yield striking phenomena. In this work, we focus on collinear antiferromagnetic ${\mathrm{CrI}}_3$ twisted homo-bilayers, which are characterized by a staggered octahedral chirality in each monolayer. Using symmetry analysis, density functional theory, and tight-binding model calculations we show that layer's twisting can lower the structural and magnetic point-group symmetries, thus activating pyroelectricity and the magneto-optical Kerr effect, which would otherwise be absent in untwisted antiferromagnetic homo-bilayers. Interestingly, both electric polarization and Kerr angle are controllable by the twist angle and their sign is reversed when switching between left- and right-twisted bilayers. We further unveil the occurrence of unconventional vortices with spin textures that alternate opposite chiralities in momentum space. These findings demonstrate that the interplay between twisting and octahedral chirality in magnetic bilayers and related van der Waals heterostructures represents an extraordinary resource for tailoring their physical properties for spintronic and optoelectronic applications.

Using ultrahigh magnetic field up to 120 T, a 1/2 magnetization plateau within $11.7–34.8$ T and a 3/4-like plateau within $55.6–87.0$ T have been observed in the $S$-1 skew chain antiferromagnet ${\mathrm{Ni}}_2{\mathrm{V}}_2{\mathrm{O}}_7$. By combining density functional theory calculations, exact diagonalization, and quantum Monte Carlo simulations, we find that the nearest-neighbor interchain interaction ($J_3/k_{\text{B}}=-78.5$ K) is much stronger than the intrachain interactions ($J_1/k_{\text{B}}=-1.0$ K and $J_2/k_{\text{B}}=6.3$ K), showing surprising “dimerization” of magnetic ions caused by the large $3d$-orbital overlap along the Ni1-Ni1 bond. Thus a “$\text{dimer}+\text{monomer}$” model is proposed to describe the magnetization process—the 1/2 plateau pertains to weakly coupled Ni2 monomers while the 3/4-like plateau to the strongly coupled Ni1 dimers. The possible supersolid phases from the Ni1 dimers are proposed.

Luttinger's theorem has long been taken as the key feature of Landau's Fermi liquid, which signals the presence of quasiparticles. Here, by the unbiased Monte Carlo method, violation of Luttinger's theorem is clearly revealed in the Falicov-Kimball (FK) model, indicating a robust correlation-driven non-Fermi-liquid characteristic under any electron density. Introducing hole carriers to the half-filled FK model leads to Mott insulator-metal transition, where the Mott quantum criticality manifests unconventional scaling behavior in transport properties. Further insight on the violation of Luttinger's theorem is examined by combining Hubbard-I approximation with a composite fermion picture, which emphasizes the importance of a mixed excitation of the itinerant electron and the composite fermion. Interestingly, comparing the FK model with a binary disorder system suggests that the two-peak band structure discovered by Monte Carlo and Hubbard-I approaches is underlying the violation of Luttinger's theorem.