We investigate twisted bilayer graphene (TBG) at filling $\nu =-3$ in the presence of realistic heterostrain. Strain amplifies the band dispersion and drives the system beyond the strong-coupling regime of previous theoretical studies. We use DMRG to conduct an unbiased, large-scale numerical calculations that include all spin and valley degrees of freedom, up to bond dimension $\chi =24576$. We establish a global phase diagram that unifies a number of theoretical and experimental results. Near zero strain we find an intervalley-coherent quantized anomalous Hall (QAH-IVC) state, a competitive strong-coupling order that evaded past numerical studies. A tiny strain around $0.05\%$ drives a transition into an incommensurate Kekulé spiral (IKS) phase, supporting the mean-field prediction in [Kwan et al., Phys. Rev. X 11, 041063 (2021)]. Even higher strains above $0.2\%$ favor a flavor-symmetric metallic order, which may explain metals found at $\nu =-3$ in many experiments.

Recently, V-based kagome $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{K},\mathrm{Rb},\mathrm{Cs})$ attracted researcher interest for the existence of charge-density wave (CDW), superconductivity, and nontrivial ${\mathbb{Z}}_2$ topological invariant. ${\mathrm{CsV}}_8{\mathrm{Sb}}_{12}$ has the same V-based kagome net, considered as the substitution of Cs with orthorhombic ${\mathrm{V}}_2{\mathrm{Sb}}_2$ layer. Using angle-resolved photoemission spectroscopy (ARPES) and density-functional theory (DFT), we find out that the symmetry of the Fermi surface (FS) and the electron structure near $E_F$ change from $C_6$ to $C_2$ symmetry. With the appearance of the electronic states from ${\mathrm{V}}_2{\mathrm{Sb}}_2$ layer near $E_F$, the corresponding bands from V-based kagome move to high binding energy by the introduction of ${\mathrm{V}}_2{\mathrm{Sb}}_2$ layer. We observe two types of saddle points. One at $\overline{\mathrm{M}}$ originates from the kagome lattice and locates at high binding energy. Another one at $\overline{\mathrm{\Gamma }}$ arises from ${\mathrm{V}}_2{\mathrm{Sb}}_2$ layer and is situated near $E_F$. We study the influence of kagome distortion on the band structure using DFT, tight-binding model based on the ARPES experiment. We find that the ${\mathrm{V}}_2{\mathrm{Sb}}_2$-induced symmetry breaking and lattice distortion of the kagome net can lead to the shift of Dirac cones away from the high symmetry point. After considering spin-orbit coupling (SOC), the distorted kagome lattice shows the same topological properties as the pristine kagome lattice. Our findings will enhance comprehension of the microscopic mechanisms in V-based kagome metals and their topological properties.

Honeycomb ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is a prototype spin-orbit Mott insulator and Kitaev magnet. We report a combined structural and electrical resistivity study of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ single crystals. Laue back-scattering diffraction indicates twinning with $\pm {120}^{\circ }$ rotation around the $c^{*}$ axis while scanning electron microscopy displays nanothin lines parallel to all three $b$-axis orientations of twin domains. Energy dispersive x-ray analysis line scans across such domain walls indicate no change of the Ir signal intensity, i.e., intact honeycomb layers, while the Na intensity is reduced down to $\sim 2/3$ of its original value at the domain walls, implying significant hole doping. Utilizing focused-ion-beam microsectioning, the temperature dependence of the electrical resistance of individual domain walls is studied. It demonstrates the tuning through the metal-insulator transition into a correlated-metal ground state by increasing hole doping.

The Benalcazar-Bernevig-Hughes (BBH) quadrupole insulator model is a cornerstone model for higher-order topological phases. It requires $\pi$-flux threading through each plaquette of the two-dimensional Su-Schrieffer-Heeger model. Recent studies showed that particular $\pi$-flux patterns can modify the fundamental domain of momentum space from the shape of a torus to a Klein bottle with emerging topological phases. By designing different $\pi$-flux patterns, we propose two types of Klein-bottle BBH models. These models show rich topological phases, including Klein-bottle quadrupole insulators and Dirac semimetals. The phase with nontrivial Klein-bottle topology shows twined edge modes at open boundaries. These edge modes can further support second-order topology, yielding a quadrupole insulator. Remarkably, both models are robust against flux perturbations. Moreover, we show that different $\pi$-flux patterns dramatically affect the phase diagram of the Klein-bottle BBH models. Going beyond the original BBH model, Dirac semimetal phases emerge in Klein-bottle BBH models featured by the coexistence of twined edge modes and bulk Dirac points.

By delicately designing the subwavelength “meta-atoms,” the corresponding scattering properties can be flexibly engineered and one may provide more degrees of freedom for the implementation of custom-made light-matter interaction. In this paper, the magnetic medium loaded subwavelength hybrid meta-atoms (HMAs) are constructed, which exhibit nonreciprocal Fano resonance for the HMA immersed in air background due to the interference of Mie resonances associated with the angular momentum channels (AMCs) $m=\pm 1$ as well as the combining interaction for the Mie resonances with the AMCs $m=\pm 1$ and $-2$. The introduction of the epsilon-near-zero (ENZ) medium as the background medium results in the collapse of nonreciprocal Fano resonance due to the narrowed Mie resonances with much larger quality factors. As a consequence, the multipolar Mie resonances are superimposed solely with the uniform zeroth order mode, where two close intersections exist with one of them behaving as the nearly reciprocal Kerker effects showing the forward scattering and the other one behaving as the generalized nonreciprocal Kerker effects (GNKEs) showing the transverse unidirectional scattering. In addition, the operating frequency can be flexibly regulated by controlling the bias magnetic field, giving rise to the transformation between the reciprocal and nonreciprocal Kerker effects, as well as the frequency reversing of GNKEs with regard to the lower and higher order multipolar Mie resonances. In addition, the GNKE can even be modulated into the unidirectional scattering at nearly arbitrary direction with exceptional contrast by altering the permittivity of the ENZ background medium. By further considering magneto-optical materials the phenomenon can be expected at the optical regime and the research is also instructive for nonreciprocal photonics and potential applications.

Two of the primary sources of error in the Cluster dynamical mean field theory (CDMFT) technique arise from the use of finite size clusters and finite size baths, which makes the development of impurity solvers that can treat larger systems an essential goal. In this work we introduce an impurity solver based on the recently developed dynamical variational Monte Carlo (dVMC) method. Variational Monte Carlo possesses a favorable scaling as a function of system size, which enables the treatment of systems beyond the reach of current exact diagonalization solvers. To benchmark the technique, we perform a systematic set of CDMFT calculations on the single-band one-dimensional Hubbard model. We compare to results obtained with an exact diagonalization solver for small clusters, and against the exact solution in the thermodynamic limit obtained by Lieb and Wu for larger clusters. The development of improved impurity solvers will help extend the reach of quantum cluster methods, which can be applied to a wide range of strongly correlated electron systems, promising new insights into their emergent behavior.

Starting from Halperin multilayer systems we develop a hierarchical scheme, dubbed symmetrized multicluster construction, that generates bosonic and fermionic single-layer quantum Hall states (or vacua) of arbitrary filling factor. Our scheme allows for the insertion of quasiparticle excitations with either Abelian or non-Abelian statistics and quantum numbers that depend on the nature of the original vacuum. Most importantly, it reveals a fusion mechanism for quasielectrons and magnetoexcitons that generalizes ideas about particle fractionalization introduced in A. Bochniak, Z. Nussinov, A. Seidel, and G. Ortiz, Commun. Phys. 5, 171 (2022) for the case of Laughlin fluids. In addition, in the second quantization representation, we uncover the inherent topological quantum order (or the off-diagonal long-range order) characterizing these vacua. In particular, we illustrate the methodology by constructing generalized composite (generalized Read) operators for the non-Abelian Pfaffian and Hafnian quantum fluid states.

Chemical substitution is commonly used to explore new ground states in materials, yet the role of disorder is often overlooked. In Mn-substituted ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ (MnBFA), superconductivity (SC) is absent, despite being observed for nominal hole-doped phases. Instead, a glassy magnetic phase emerges, associated with the $S=5/2$ Mn local spins. In this work, we present a comprehensive investigation of the electronic structure of MnBFA using angle-resolved photoemission spectroscopy (ARPES). We find that Mn causes a small and orbital-specific reduction of the electron pockets, only partially disrupting nesting conditions. Based upon the analysis of the spectral properties, we observe, for all bands, an increase in the electronic scattering rate as a function of Mn content. This is interpreted as increasing band incoherence, which we propose as the primary contributor to the suppression of the magnetic order in MnBFA. This finding connects the MnBFA electronic band structure properties to the glassy magnetic behavior observed in these materials and suggests that SC is absent because of the collective magnetic impurity behavior that scatters the Fe-derived excitations. Additionally, our analysis shows that the binding energy ($E_B$) dependence of the imaginary part of the self-energy [$\text{Im}\mathrm{\Sigma }\left(E_B\right)$] is best described by a fractional scaling ($\text{Im}\mathrm{\Sigma }\left(E_B\right)\propto \sqrt{-E_B}$). These results indicate that Mn tunes MnBFA into an electronic disordered phase between the correlated Hund's metal in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and the Hund's insulator in ${\mathrm{BaMn}}_2{\mathrm{As}}_2$.

We have studied theoretically the effect of electrical magnetochiral anisotropy (eMChA) in $p$-type tellurium crystals. It is shown that the terms $k_i{B}_j$ in the hole Hamiltonian, linear in both the wave vector $\mathit{k}$ and the magnetic field $\mathit{B}$, do not lead to eMChA and one needs to include the higher-order terms like $k_i^3{B}_j$. Two microscopic mechanisms of the effect are considered. In the first one only elastic scattering of holes by impurities or imperfections is taken into consideration. In the second mechanism, besides the elastic scattering processes the hole gas heating and its energy relaxation are taken into account. It is demonstrated that both contributions to the magneto-induced rectification are comparable in magnitude. The calculation is performed by using two independent approaches, namely, in the relaxation-time approximation and in the limit of small chiral band parameter $\beta$. A bridge is thrown between the eMChA and magneto-induced photogalvanic effects.

Graphene nanoribbons (GNRs) are thin strips of graphene with unique properties due to their structure and nanometric dimensions. They stand out as basic components for the construction of different types of nanoelectromechanical systems (NEMS), including some very promising sensors and pumps. However, various phenomena, such as unintended mechanical vibrations, can induce undesired electrical currents in these devices. Here, we take a quantum mechanical approach to analyze how currents induced by fluctuations (either thermal or of some other kind) in suspended GNRs contribute to the electric current. In particular, we study the pumping current induced by the adiabatic variation of the Hamiltonian of the system when a transverse vibration (flexural mode) of a GNR suspended over a gate is excited. Our theoretical approach and results provide useful tools and rules of thumb to understand and control the charge current induced by fluctuations in GNR-based NEMS, which is important for their applications in nanoscale sensors, pumps, and energy harvesting devices.

We report low-temperature Raman signatures of the Heisenberg quantum magnet ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$, showing clear anomalies in phonon mode frequencies and linewidths below $\sim 100$ K. This crossover temperature lies in between the Jahn-Teller (JT) temperature scale ($>\text{room}$ temperature) and the temperature scale associated with the spin exchange interactions ($<12$ K). Our experimental observation is well captured by a secondary JT transition associated with a cooperative reorientation of the orbitals giving rise to anomalies in the temperature dependence of Raman frequencies and linewidths. Such orbital reorganization, in turn, affects the spin-spin exchange interactions that decide the fate of the magnet at lower temperatures and hence provide important clues to understand the energetics of the possible lower-temperature quantum paramagnetic phase.

Nodal-line semimetals are commonly believed to exist in $\mathcal{PT}$ symmetric or mirror-rotation symmetric systems. Here, we find a flux-induced parameter-dimensional second-order nodal-line semimetal (SONLS) in a two-dimensional system without $\mathcal{PT}$ and mirror-rotation symmetries. It has coexisting hinge Fermi arcs and drumhead surface states. Meanwhile, we discover a flux-induced second-order topological insulator (SOTI). We then propose a Floquet engineering scheme to create exotic parameter-dimensional hybrid-order nodal-line semimetals with abundant nodal-line structures and widely tunable numbers of corner states in a SONLS and SOTI, respectively. Our results break the perception of SONLSs and supply a convenient way to artificially synthesize exotic topological phases by periodic driving.

Weyl semimetal is a solid material with isolated touching points between conduction and valence bands in its Brillouin zone—Weyl points. Low-energy excitations near these points exhibit a linear dispersion and act as relativistic massless particles. Weyl points are stable topological objects robust with respect to most perturbations. We study effects of weak disorder on the spectral and transport properties of Weyl semimetals in the limit of low energies. We use a model of Gaussian white-noise potential and apply dimensional regularization scheme near three dimensions to treat divergent terms in the perturbation theory. In the framework of self-consistent Born approximation, we find closed expressions for the average density of states and conductivity. Both quantities are analytic functions in the limit of zero energy. We also include interference terms beyond the self-consistent Born approximation up to the third order in the disorder strength. These interference corrections are stronger than the mean-field result and nonanalytic as functions of energy. Our main result is the dependence of conductivity (in units $e^2/h$) on the electron concentration $\sigma ={\sigma }_0-0.891n^{1/3}+0.115(n^{2/3}/{\sigma }_0)ln\left|n\right|$.

The discovery of strong four-phonon (4ph) interactions in boron arsenide has spurred significant investigations into the influence of higher-order phonon anharmonic scattering on the thermal transport of materials. Considering the special relative regular residual feature and great potential applications in thermoelectrics, this study systematically investigates the anharmonic lattice dynamics and thermal transport behavior of monolayer group-IVB transition metal dichalcogenides $1T-M{X}_2 (M=\mathrm{Ti},\mathrm{Zr},\mathrm{Hf};X=\mathrm{S},\mathrm{Se})$ via ab initio calculations with the self-consistent phonon and Boltzmann transport theories. The calculation results indicate that $1T-M{X}_2$ has strong 4ph anharmonic scattering and that the phonon dispersion exhibits a significant temperature dependence, which stems from the obvious in-plane quartic potential well vibrations of the transition metal atoms. When 4ph scattering is considered, a remarkable reduction of thermal conductivity is observed in $1T-M{X}_2$, especially for $1T-\mathrm{Ti}{\mathrm{S}}_2$ and $1T-\mathrm{Ti}{\mathrm{Se}}_2$, with their conductivity reduction reaching 66 and 62% at room temperature, respectively. Moreover, the thermal conductivity of $1T-M{X}_2$ anomalously increases when the $M$ atoms transition from $\mathrm{Ti}\to \mathrm{Zr}\to \mathrm{Hf}$, which is in contrast to the traditional intuitive impression where heavy atomic systems always exhibit poor thermal transport performance. Such abnormal mass-dependent behavior of thermal conductivity mainly arises from the enhancement of the bond energy (reduction of phonon anharmonicity) in the Zr and Hf systems. The study results shed light on the physical mechanism of the thermal transport in monolayer $1T-M{X}_2$ and provides helpful insights into the necessity of considering the influence of 4ph effects on thermal transport.

Ultrathin ${L1}_0\mathrm{FePt}$ films with different $c$-axis orientations relative to the film plane are promising candidates for data storage materials. In this work, within the framework of density functional theory, we calculated the magnetic properties of ultrathin ${L1}_0\mathrm{FePt}$ (111) and (010) films with thicknesses ranging from 4 to 16 atomic monolayers (from about 0.8 to 3.5 nm). The highest average magnetic moments are observed for the thinnest films considered and, with increasing film thickness, the values converge towards the magnetic moment for bulk. The observed increase comes mainly from enhanced moments in the two atomic monolayers closest to the surface of the films. The easy axis of magnetization of (111) films prefers an alignment close to the tetragonal axis—an example of tilted magnetic anisotropy. The 6-monolayer (111) film (about 1.3 nm thick) inclines the easy axis of magnetization of about ${45}^{\circ }$ to the film plane, which can find use in applications. The (010) films show an in-plane easy magnetization axis in a unique ${L1}_0$ tetragonal direction. This is an unusual type of in-plane anisotropy, as the particular direction preference is very strong. The computational results encourage further experimental studies of ${L1}_0$ systems with tilted and in-plane fixed magnetic anisotropy.

We compute the momentum-resolved dynamical spin structure factor $S(k,\omega )$ of the SU(4) Heisenberg model on the honeycomb lattice assuming the $\pi$-flux Dirac spin liquid ground state by using two methods: (i) variationally using Gutzwiller projected particle-hole excitations of the $\pi$-flux Fermi sea, and (ii) in the noninteracting parton mean-field picture. The two approaches produce qualitatively similar results. Based on this analogy, we argue that the energy spectrum of the projected excitations is a gapless continuum of fractional excitations. Quantitatively, the Gutzwiller projection shifts the weight from higher to lower energies, thus emphasizing the lower edge of the continuum. In the mean-field approach, we obtained the $1/{\text{distance}}^4$ decay of the spin correlation function, and the local correlations show $S_{\text{MF}}^{33}\left(\omega \right)\propto {\omega }^3$ behavior.

$\left(1-x\right){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3\text{−}\left(x\right)\mathrm{BaTi}{\mathrm{O}}_3$ is one of the important Pb-free piezoelectric alloy systems. Despite extensive investigation over the years, the composition $x$–electric field (E) phase diagram of this alloy system has not received sufficient attention. Here, we performed an x-ray diffraction study in situ with electric fields at close composition intervals in a wide range: $0.0\le x\le 0.18$. We discovered that this system exhibits field-driven, reversible interferroelectric transformations in a large composition range: $0.02<x<0.14$. Based on this work, we present the first comprehensive $x$-E phase diagram of this system. Apart from addressing inconsistencies in an earlier study, our phase diagram can precisely explain the composition trend of both the weak-field (dielectric and piezoelectric) and high-field (electrostrain) properties of this Pb-free piezoelectric system.

The diamond nitrogen vacancy (NV) center remains an ever-increasing topic of interest. At present, it is considered an ideal example of a solid-state qubit applicable in quantum communication, computing, and sensing alike. With its success, the search for defects that share or improve upon its advantageous features is an ongoing endeavor. By performing large-scale high-throughput screening of 52 600 defects in $4H$ silicon carbide (SiC), we identify a collection of NV-like color centers of particular interest. From this list, the single most promising candidate consists of a silicon vacancy and chlorine substituted on the carbon site, and is given the name of chlorine vacancy (ClV) center. Through high-accuracy first-principle calculations, we confirm that the ClV center is similar to the NV center in diamond in its local structure and shares many qualitative and quantitative features in the electronic structure and spin properties. In contrast to the NV center, however, the ClV center in SiC exhibits emission in the telecom range near the C band.

We report high-resolution dilatometry studies on single crystals of the Shastry-Sutherland-lattice magnet ${\mathrm{NdB}}_4$ supported by specific heat and magnetometry data. Our dilatometric studies evidence pronounced anomalies at the phase boundaries, which imply strong magnetoelastic coupling. The evolution of the three zero-field phase transitions separating distinct antiferromagnetic phases at $T_{\mathrm{N}}=17.2$ K, $T_{\mathrm{IT}}=6.8$ K, and $T_{\mathrm{LT}}=4.8$ K can thus be traced in applied magnetic fields, which provides the magnetic phase diagrams for $B\parallel c$ up to 15 T and for $B\parallel \left[110\right]$ up to 35 T. In-field phases are discovered for both field directions and already known phases are confirmed. In particular, phase boundaries between different phases are unambiguously shown by sign changes of observed anomalies and corresponding changes in uniaxial pressure effects. For $B\left|\right|c$, we find a 1/4-magnetization plateau in addition to a previously reported plateau at 1/5 of the saturation magnetization. $T_{\mathrm{N}}$ increases for $B\parallel c$ in fields up to 15 T implying that magnetic moments of the all-in/all-out structure in the high-temperature AFM ordered phase are driven towards the $c$ axis in high magnetic fields. Uniaxial pressure dependencies $\partial T_{\mathrm{crit}}/\partial p_{\mathrm{c}}$ of the phase transition temperatures for magnetic fields and pressure applied along the $c$ axis are derived from the data.

Local distortions of the crystal structure and their influence on the electronic structure, electron-phonon interaction, and superconductivity are theoretically studied in the superconducting high-entropy alloy ${\left(\mathrm{TaNb}\right)}_{0.67}{\left(\mathrm{HfZrTi}\right)}_{0.33}$. Distortions of the crystal lattice are caused by the relaxation of atomic positions and are studied in the twelve models of supercells. The largest relative changes in the interatomic distances due to relaxation reach 8%. On average, local distortions tend to lower the density of states at the Fermi level and significantly reduce the electron-phonon coupling parameter $\lambda$. As a result, the calculated superconducting critical temperature is reduced to about 50% of the initial value, which shows the strong impact of structural disorder on superconductivity in this prototype high-entropy alloy. With the reduced value of $\lambda$, the theoretical $T_c$ is closer to the experiment for typical values of the Coulomb pseudopotential parameter ${\mu }^{*}$. The experimental $T_c$ can be reproduced by taking a slightly enhanced ${\mu }^{*}=0.176$, which leaves little room for the possibility of suppression of superconductivity by disorder.