The MoSH monolayer, one of the Janus transition metal sulfhydrates synthesized by stripping the top-layer S of ${\mathrm{MoS}}_2$ and replacing it with H atoms [Wan et al., ACS Nano 15, 20319 (2021)], has been predicted to host strong coupling two-gap superconductivity with a calculated critical temperature $T_c$ of about 28.58 K at atmospheric pressure. In this work, by using machine learning aided high-throughput calculations, we narrow down 180 possible configurations of two-dimensional Janus transition metal sulfhydrates ($MX\mathrm{H}$ monolayers, where $M=\text{transition}$ metal group elements and $X=\mathrm{S}$, Se, and Te) to 20 stable metals. Among them, we identify six low-energy monolayers that are potential high-$T_c$ superconductors. Notably, the $1T$-TiSH monolayer stands out with the highest $T_c$ of approximately 48 K, surpassing the superconducting properties of $1H$-MoSH ($T_c=28.58$ K) and the well-known ${\mathrm{MgB}}_2$ superconductor ($T_c=39$ K). By solving the anisotropic Migdal-Eliashberg equations, we find that $1T$-TiSH naturally exhibits a one-gap superconducting nature with strong electron-phonon coupling ($\lambda =2.79$) originating from the interactions of Ti $d_{xz,yz}$ orbitals and in-plane vibrations, which is different from and better than the $1H$-MoSH monolayer ($\lambda =1.60$). The presented results enrich families of Janus transition metal sulfhydrates and accelerate the design of novel two-dimensional superconductors.

Metal to insulator (MIT) phase transition accompanied by a structural phase transition (SPT) makes ${\mathrm{VO}}_2$ a potential material for investigations into a strongly correlated transition metal oxide system. It undergoes a high-temperature metallic to a low-temperature insulating phase. The MIT is associated with the SPT from the rutile tetragonal $\left(R\right)$ to the monoclinic ($M1$) phase. The structural transition occurs between $M1$ and $R$ via two other insulating metastable phases, namely, monoclinic ($M2$) and triclinic $\left(T\right)$. It has gained tremendous attention because of the half century old “chicken and egg” debate over the roles played by lattice distortion and electron-electron correlation. Despite several reports on the MIT and SPT between the $R$ and $M1$ phases, a combined and detailed investigation of the relation among the various stable and metastable structural phases is still missing. We have studied the temperature- and pressure-induced structural phase transitions in the ${\mathrm{V}}_{1-x}{\mathrm{Mg}}_x{\mathrm{O}}_2$ system by synchrotron x-ray diffraction and Raman spectroscopic measurements. We observe $M2\to T\to M1$ phase transition upon compression, which is completely reversible upon decompression. The transition pressures for $M2\to T$ and $T\to M1$ are observed to increase with the increase in doping concentration. The structural transitions from $M2$ to $T$ to $M1$ in ${\mathrm{VO}}_2$ are found to be second-order continuous phase transition. However, the temperature-driven $M2\to R$ phase transition is found to be first order. We argue that Mott-type first-order metal to insulator transition prompts the MIT from $R$ to $M2$, whereas a second-order structural phase transition/relaxation leads to the observation of $M2$ to $M1$ via the $T$ phase. We further investigated the isothermal and isobaric Grüneisen parameters for individual phonon modes and relaxations of the samples related to their thermal expansion.

Here we study the instabilities of a quadratic band crossing system to Chern insulating states and uncorrelated disorder. We determine the phase diagram in the plane of topological mass versus disorder strength, characterizing the system with respect to the spectral, localization, and topological properties. In the clean limit, the system has two gapped Chern insulating phases with Chern numbers $C=\pm 2$ and a trivial phase with $C=0$. For finite disorder, the quadratic band crossing points are unstable to emergent gapless Chern insulating phases with $C=\pm 1$ that are not present in the clean limit. These phases occupy a considerable region of the phase diagram for intermediate disorder and show features of topological Anderson insulators: it is possible to reach them through disorder-driven transitions from trivial phases.

Motivated by recent experiments on ${\mathrm{Ce}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ that reveal a dynamic, liquidlike ground state, we study the nearest-neighbor XYZ Hamiltonian of dipolar-octupolar pyrochlore magnets with the pseudofermion functional renormalization group (PFFRG), which is numerically implemented by the SpinParser software. Taking the interaction between the octupolar components to be dominant and antiferromagnetic, we map out the phase diagram demarcating the quantum disordered and magnetically ordered states. We identify four distinct phases, namely, the 0-flux and $\pi$-flux quantum spin ices and the all-in-all-out magnetic orders along the local $z$ and $x$ axes. We further use the static two-spin correlation output by the PFFRG algorithm to compute the polarized neutron scattering cross sections, which are able to capture several qualitative features observed experimentally, in the materially relevant parameter regime that stabilizes the $\pi$-flux quantum spin ice. Our results provide support for a quantum spin-liquid ground state in ${\mathrm{Ce}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$.

We microscopically analyze the nearest-neighbor Heisenberg model on the maple leaf lattice through neural quantum state (NQS) and infinite density matrix renormalization group (iDMRG) methods. Embarking to parameter regimes beyond the exact dimer singlet ground state with a dimer bond spin exchange coupling $J_d$ varied against the exchange strength $J$ of all other bonds, the iDMRG (NQS) method finds a dimer state paramagnetic phase for $J_d/J>1.464$ ($J_d/J>1.39$) and a canted ${120}^{\circ }$ magnetic order for $J_d/J<1.419$ ($J_d/J<1.23$). Assessing training convergence inaccuracies of the NQS method and the influence of finite cylindric circumference in the iDMRG method, we discuss the possible existence of an intermediate phase between the magnet and the dimer paramagnet.

Ruthenium-based pyrochlore oxides exemplify a unique material system in which the competition of Mottness ($U$), Hundness ($J_{\mathrm{H}}$), and effective $p-d$ hybridization brings exotic physical characteristics. To understand and manipulate these interactions, we have investigated the structural, magnetic, and electrical properties of the pyrochlore ruthenate ${\mathrm{Y}}_2\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ and its doped variants ${\left({\mathrm{Y}}_{1-x}\mathrm{C}{\mathrm{a}}_x\right)}_2\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$. The results of our magnetic measurements reveal a systematic suppression of antiferromagnetic (AFM) ordering and increased frustration with the progressive substitution of Ca at the Y site leading to a nonequilibrium glassy magnetic state at low temperature above the doping level of 10%. Moreover, the screening of the net effective moment of Ru in the doped samples, as derived from the experimental results, is attributed to the induced itinerant character of $\mathrm{R}{\mathrm{u}}^{5+}$. Such an effect is also reflected in the substantially reduced electrical resistivity, possibly due to enhanced $d-p$ hybridization. Consistent with the experimental results, density functional theory calculations confirm the bulk AFM ordering and a drop in the band gap of the doped samples. We further claim the low-temperature magnetic state of ${\mathrm{Y}}_2\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ to possess a noncollinear all-out-type spin configuration, which transforms into a randomized glassy phase upon hole doping.

We study the high Curie temperature ferromagnet (${\mathrm{Cr}}_{0.35}{\mathrm{Sb}}_{0.65}{)}_2\mathrm{Te}{}_3$ ($T_C=192\mathrm{K}$), using $T$-dependent x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and angle-resolved photoemission spectroscopy (ARPES). The $T$-dependent (25–220 K) XAS-XMCD evolution of $\mathrm{Cr}3d$ and $\mathrm{Te}5p$ unoccupied site- and orbital-projected states shows a systematic modification, which we interpret as due to spin-splitting below $T{}_C$. The $T$-dependent XMCD intensity and leading-edge spin-sensitive shifts ${\gamma }_{\text{expt}}\left(T\right)$ follow bulk magnetization. ARPES measurements with $h\nu =78\mathrm{eV}$ show a metallic state with Sb $5p$ band dispersions at and near Fermi level ($E_F$), consistent with bulk band-structure calculations for ${\left({\mathrm{Cr}}_{0.33}{\mathrm{Sb}}_{0.67}\right)}_2\mathrm{Te}{}_3$. However, surface-sensitive ARPES with $h\nu =8.4\mathrm{eV}$ above and below $T{}_C$ show linear band dispersions just below $E_F$, suggesting a remnant of Dirac-type dispersions. Assuming the linear dispersion survives above $E_F$, it implies a topologically trivial ferromagnet as the estimated Dirac point energy lies above the largest ${\gamma }_{\text{expt}}(T=25\mathrm{K})$. The $\mathrm{Cr}3d$ XAS-XMCD spectra can be simulated by charge transfer multiplet cluster model calculations with an exchange field $H_{\text{ex}}$ which quantitatively reproduces the experimental XMCD. At $T=25\mathrm{K}$, the required exchange field $H_{\text{ex}}$ of $\sim 48\mathrm{T}$ corresponds to a Zeeman energy $\zeta =2.8\mathrm{meV}<T_C=192\mathrm{K}$ (= 16.5 meV) $\ll {\gamma }_{\text{expt}}\sim 140\mathrm{meV}$. The results indicate the role of $\mathrm{Cr}3d$ exchange interactions in causing spin-sensitive shifts in $\mathrm{Cr}3d$ states, and inducing comparable spin-sensitive shifts via hybridization in $\mathrm{Te}5p$ states of ${\left({\mathrm{Cr}}_{0.35}{\mathrm{Sb}}_{0.65}\right)}_2\mathrm{Te}{}_3$.

Graphene nanoribbons are a promising candidate for fault-tolerant quantum electronics. In this scenario, qubits are realized by localized states that can emerge on junctions in hybrid ribbons formed by two armchair nanoribbons of different widths. We derive an effective theory based on a tight-binding ansatz for the description of hybrid nanoribbons and use it to make accurate predictions of the energy gap and nature of the localization in various hybrid nanoribbon geometries. We use quantum Monte Carlo simulations to demonstrate that the effective theory remains applicable in the presence of Hubbard interactions. We discover, in addition to the well-known localizations on junctions, which we call “Fuji”, a new type of “Kilimanjaro” localization smeared out over a segment of the hybrid ribbon. We show that Fuji localizations in hybrids of width $N$ and $N+2$ armchair nanoribbons occur around symmetric junctions if and only if $N(mod3)=1$, while edge-aligned junctions never support strong localization. This behavior cannot be explained relying purely on the topological $Z_2$ invariant, which has been believed to be the origin of the localizations to date.

Artificial media provide a unique playground to test fundamental theories, allowing one to probe the laws of electromagnetism in the presence of hypothetical axions. While some materials are known to realize this physics, here we propose the nonlocal extension of axion electrodynamics. Compared to the usual axion case, the suggested metamaterial features similar optical properties including Kerr and Faraday rotation. However, the external sources in this structure do not induce dyon charges, eliminating the well-celebrated Witten effect. We put forward the design of such a nonreciprocal nonlocal metamaterial and discuss its potential applications.

We present a nonstandard Hubbard model applicable to arbitrary single-particle potential profiles and interparticle interactions. Our approach involves a treatment of Wannier functions, free from the ambiguities of conventional methods and applicable to finite systems without periodicity constraints. To ensure the consistent evaluation of Wannier functions, we develop a perturbative approach, utilizing the barrier penetration coefficient as a perturbation parameter. With the defined Wannier functions as a basis, we derive the Hubbard Hamiltonian, revealing the emergence of density-induced and pair tunneling terms alongside standard contributions. Our investigation demonstrates that long-range interparticle interactions can induce a mechanism for repulsive particle pairing. This mechanism relies on the effective suppression of single-particle tunneling due to density-induced tunneling. Contrary to expectations based on the standard Hubbard model, an increase in interparticle interaction does not lead to an insulating state. Instead, our proposed mechanism implies the coherent motion of correlated electron pairs, similar to bound states within a multiwell system, resistant to decay from single-electron tunneling transitions. These findings carry significant implications for various phenomena, including the formation of flat bands, the emergence of superconductivity in twisted bilayer graphene, and the possibility of a metal-insulator transition.

In this study, we employ a generalized tight-binding (TB) framework to explore the electronic characteristics of $\mathrm{BiTe}X$ ($X$ = Cl, Br, and I) compounds. Our methodology relies on Slater-Koster integrals (SK) and the linear combination of atomic orbitals (LCAO) approach as the basis for our investigation. By refining generalized TB models through first-principles calculations, we establish an efficient and accurate quantum mechanical technique for predicting the electronic properties of halogenated bismuth telluride. Our research delves into various models that provide unique insights into the electronic behavior of $\mathrm{BiTe}X$. Within an orthogonal TB model, the contributions from $s$ and $p$ orbitals define the band gap, making it suitable for analyzing low-energy phenomena near the $\mathrm{\Gamma }$ point. Furthermore, this model extends to incorporate the interplay between $d$ and $p$ orbitals, resulting in a more comprehensive depiction of the band structure. Conversely, a nonorthogonal TB model emphasizes the contributions of $s$ and $p$ orbitals, revealing intricate details of the electronic structure, albeit with potential computational complexities. Remarkably, this model demonstrates excellent agreement with first-principles calculations. Collectively, these models unveil the nuanced electronic nature of $\mathrm{BiTe}X$, furnishing adaptable computational tools for materials science and quantum device applications.

Having previously been the subject of decades of semiconductor research, cadmium arsenide (${\mathrm{Cd}}_3{\mathrm{As}}_2$) has now reemerged as a topological material, realizing ideal three-dimensional Dirac points at the Fermi level. These topological Dirac points lead to a number of extraordinary transport phenomena, including strong quantum oscillations, large magnetoresistance, ultrahigh mobilities, and Fermi velocities exceeding graphene. The large mobilities persist even in thin films and nanowires of ${\mathrm{Cd}}_3{\mathrm{As}}_2,$ suggesting the involvement of topological surface states. However, computational studies of the surface states in this material are lacking, in part due to the large 80-atom unit cell. Here we present the computed Fermi-arc surface states of a ${\mathrm{Cd}}_3{\mathrm{As}}_2$ thin film, based on a tight-binding model derived directly from the electronic structure. We show that despite the close proximity of the Dirac points, the Fermi arcs are very long and straight, extending through nearly the entire Brillouin zone. The shape and spin properties of the Fermi arcs suppress both back- and side scattering at the surface, which we show by explicit integrals over the phase space. The introduction of a small symmetry-breaking term, expected in a strong electric field, gaps the electronic structure, creating a weak topological insulator phase that exhibits similar transport properties. Crucially, the mechanisms suppressing scattering in this material differ from those in other topological materials such as Weyl semimetals and topological insulators, suggesting a new route for engineering high-mobility devices based on Dirac semimetal surface states.

We investigate the influence of local decoherence on a symmetry-protected topological (SPT) phase of the two-dimensional (2D) cluster state. Mapping the 2D cluster state under decoherence to a classical spin model, we show a topological phase transition of a ${\mathbb{Z}}_2^{\left(0\right)}\times {\mathbb{Z}}_2^{\left(1\right)}$ SPT phase into a trivial phase occurring at a finite decoherence strength. To characterize the phase transition, we employ three distinct diagnostic methods, namely, the relative entropy between two decohered SPT states with different topological edge states, the strange correlation function of ${\mathbb{Z}}_2^{\left(1\right)}$ charge, and the multipartite negativity of the mixed state on a disk. All the diagnostics can be obtained as certain thermodynamic quantities in the corresponding classical model, and the results of the three diagnostic tests are consistent with each other. Given that the 2D cluster state possesses universal computational capabilities in the context of measurement-based quantum computation, the topological phase transition found here can also be interpreted as a transition in the computational power.

Electronic materials with a flat-band provide a fertile foundation for exploiting emerging quantum phenomena. Progress has been made in various two-dimensional (2D) systems, especially in a geometric frustrated kagome lattice system and a moire superlattice system for the study of unconventional superconductivity. However, intrinsic superconductivity has not yet been reported in 3D flat-band systems. The orbital degree of freedom might bring new vitality into the field of flat bands and superconductivity. Here, we propose an orbital-designed 3D flat-band model and its realization in various materials containing degenerate $p$-electrons, $d$-electrons, or molecular orbitals. More importantly, the possibility of superconductivity in some of these 3D flat-band materials, e.g., ${\mathrm{K}}_2\mathrm{Pb}$ and ${\mathrm{K}}_2\mathrm{Bi}$, is revealed via first-principles calculations. Interestingly, these are intrinsically multiband superconductors. Our findings would expand the scope of flat-band studies, shedding light on the exploration of interacting effects and emerging quantum phases in 3D materials.

Vortices are pointlike topological defects in superconductors whose motion dictates superconducting properties and controls device performance. In superconductor-ferromagnet heterostructures, vortices interact with topological defects in the ferromagnet such as linelike domain walls. While in previous heterostructure generations, vortex-domain wall interactions were mediated by stray fields; in new heterostructure families, more important become exchange fields and spin-orbit coupling. However, spin-orbit coupling's role in vortex-domain wall interactions remains unexplored. Here we uncover, via numerical simulations and Ginzburg-Landau theory, that Rashba spin-orbit coupling induces magnetoelectric interactions between vortices and domain walls that crucially depend on the wall's winding direction—its helicity. The wall's helicity controls whether vortices are pushed or dragged by Néel walls, and their gliding direction along Bloch walls. Our work capitalizes on interactions between topological defects from different order parameters and of different dimensionality to engineer enhanced functionality.

Harmonic generation provides an efficient tool for the study of ultrafast nonlinear dynamics. We report on time-resolved optical-pump terahertz harmonic generation spectroscopic investigation of ultrafast nonlinearity in a prototypical three-dimensional Dirac semimetal, ${\mathrm{Cd}}_3{\mathrm{As}}_2$. A transient population inversion characterized by excessive nonthermal Dirac electrons and holes is found to be very sensitive and responsive to a periodic terahertz drive, leading to very efficient terahertz third-harmonic generation. Based on the Boltzmann transport theory, we analyze the terahertz field-driven kinetics of the transient Dirac fermions that is responsible for the observed strong terahertz nonlinearity.

We investigate temperature-dependent dynamical and structural properties of Cu-Ti melts within a compositional range of 24–69 at.% Ti. Accurate data of viscosity, density, and structure have been obtained by employing the electrostatic levitation technique, which enables containerlessly processing of the samples. Within the Cu-Ti system, the viscosity features a nonmonotonous trend, with the melt of the highest viscosity located at the compositions with intermediate Ti contents. This compositional trend of the liquid dynamics is not reflected in the average packing fraction derived by the macroscopic density, which is almost independent of the Ti concentration. By studying the melt structure using x-ray and neutron diffraction, particularly the direct access of the concentration-concentration structure factor $S_{\text{CC}}$, we show that the slowdown of the melt dynamics upon mixing is rather due to chemical effects associated with the preferred formation of Cu-Ti pairs. This leads to a contraction of the interatomic distances, whereas the almost ideal mixing behavior in the molar volume of the melt is a result of the simultaneous decrease of the average coordination number.

We elucidate the origin of the ultrashort laser-driven lattice disorder in germanium through nonadiabatic quantum molecular dynamics simulations. The total disorder is dissected into disorder components arising from electron-phonon coupling, covalent bond softening, and ionic thermal activation caused by potential energy surface modification, using which thermal and nonthermal effects are quantified. We find that, although the bond softening effect initially dominates irrespective of the excitation density, the eventual ultrashort laser-driven phase transition involves both the thermal and nonthermal elements in it, with the level of their effects regulated by the electronic excitation density.

Giant spin-induced ferroelectric polarization has been observed in the multiferroic $\mathrm{CaM}{\mathrm{n}}_7{\mathrm{O}}_{12}$, the origin of which remains controversial. Various theoretical models have been proposed in this regard, some of which put the preformed orbital order as a prerequisite factor, while others attribute it to the combination of Dzyaloshinskii-Moriya interactions and exchange striction. In this paper, we resolve this issue by replacing Ca with disordered $\mathrm{B}{\mathrm{i}}^{3+}/\mathrm{A}{\mathrm{g}}^{+}$ ions through high-pressure and high-temperature methods, leading to the successful synthesis of single-phase isostructural $\left(\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{A}{\mathrm{g}}_{0.5}\right)\mathrm{M}{\mathrm{n}}_7{\mathrm{O}}_{12}$. No orbital order is observed down to 30 K, while a giant electric polarization of $\sim 1900\mu \mathrm{C}{\mathrm{m}}^{-2}$ is realized below $T_{\mathrm{N}}\approx 80$ K when the system enters an antiferromagnetically ordered state. The low-temperature spin structure adopts the magnetic point group of ${31}^{\prime }$, the same as $\mathrm{CaM}{\mathrm{n}}_7{\mathrm{O}}_{12}$. Our results strongly indicate that the giant ferroelectric polarization is primarily caused by exchange striction instead of orbital order.