A recent experiment of ${\mathrm{MgB}}_2$-type structure $\alpha \text{−}{\mathrm{MoB}}_2$ has realized $\sim 32$ K superconductivity (SC) at 90 GPa, exhibiting the highest superconducting transition temperature ($T_{\mathrm{c}}$) among transition-metal diborides. Although the SC was characterized by the electron-phonon coupling (EPC), the microscopic mechanism of how the large EPC constant and high $T_{\mathrm{c}}$ are attained is unclear. Here, based on first-principles calculations, we found that in contrast to ${\mathrm{MgB}}_2$, B atoms contribute most to electronic states near Fermi level ($E{}_{\mathrm{F}}$), Mo $d_{z^2}$ orbital is more dominant component in ${\mathrm{MoB}}_2$ and provides two impressive peaks in density of states near $E_{\mathrm{F}}$ associated with emergent double Van Hove singularities (VHS). The EPC analysis reveals that the electronic sates around double VHS could strongly interact with the softened acoustic modes of Mo out-of-plane vibration, giving rise to a large single gap with the $T_{\mathrm{c}}$ up to $\sim 37$ K, which distinctly differs from the superconducting feature of ${\mathrm{MgB}}_2$. Furthermore, by electron doping into ${\mathrm{MoB}}_2$, the VHS is tuned to be aligned with the $E_{\mathrm{F}}$ and $T_{\mathrm{c}}$ can be increased to $\sim 43$ K. Our findings not only elucidate the microscopic mechanism of observed high $T_{\mathrm{c}}$ in ${\mathrm{MoB}}_2$, but also demonstrate that ${\mathrm{MoB}}_2$ provides an ideal platform to explore the role of the VHS in emergent strong EPC SC.

We combine structural and magnetic measurements to compare the different magnetic phase diagrams between the pressure and substitution studies in ${\mathrm{CeTiGe}}_3$. We report on the structural, magnetic, and electrical transport properties of single crystals of ${\mathrm{CeTi}}_{1-x}{\mathrm{V}}_x{\mathrm{Ge}}_3$ ($x=0$, 0.1, 0.2, 0.3, 0.4, 0.9, and 1), and of polycrystalline samples ($x=0.5$, 0.6, 0.7, 0.8), as well as structural properties of ${\mathrm{CeTiGe}}_3$ under pressure up to $9\mathrm{GPa}$. The ferromagnetic ordering in ${\mathrm{CeTiGe}}_3$ is suppressed with V doping in ${\mathrm{CeTi}}_{1-x}{\mathrm{V}}_x{\mathrm{Ge}}_3$, and suggests a possible ferromagnetic quantum critical point near $x=0.45$. We perform a detailed crystalline electric field (CEF) analysis, and the magnetic susceptibility data in pure ${\mathrm{CeTiGe}}_3$ and ${\mathrm{CeVGe}}_3$ can be well explained by the CEF model. The proposed CEF energy levels suggest that there is a gradual change of the ground state from $|\pm 5/2\rangle$ in ${\mathrm{CeTiGe}}_3$ to $|\pm 1/2\rangle$ in ${\mathrm{CeVGe}}_3$, and a suppression of CEF splitting energies near the quantum critical region. When hydrostatic pressure is used instead of chemical substitution, the quantum critical point is avoided by the appearance of magnetic phases above around $4.1\mathrm{GPa}$. In the substitution study, the ferromagnetic and antiferromagnetic regions are well separated, whereas they touch in the pressure study. We observe a different trend in the temperature dependence of the resistivity maximum in both studies, suggesting that the CEF splitting energy is suppressed by V substitution but enhanced by pressure. We also observe different responses in lattice constants between the two studies, highlighting the fact that substitution effects cannot be reduced to chemical pressure effects only. Nevertheless, when the magnetic phase diagrams of both hydrostatic pressure and substitution are compared, we find a common critical lattice constant $c=5.78\AA$ where the ferromagnetic ordering is suppressed in both studies.

We have synthesized YbPdAs with the hexagonal ZrNiAl-type structure in which the Yb atoms form a distorted kagome sublattice in the hexagonal-basal plane. Magnetic, transport, and thermodynamic measurements indicate that YbPdAs is a low-carrier Kondo lattice compound with an antiferromagnetic transition at $T_{\mathrm{N}}$ = 6.6 K, which is slightly suppressed in applied magnetic fields up to 9 T. The magnetic entropy at $T_{\mathrm{N}}$ recovers only 33% of $Rln2$, the full entropy of the ground-state doublet of the Yb-ions. The resistivity displays a $-lnT$ dependence between 30 and 15 K, followed by a broad maximum at $T_{\mathrm{coh}}$ = 12 K upon cooling. Below $T_{\mathrm{coh}}$, the magnetoresistance changes from negative to positive, suggesting a crossover from single-ion Kondo scattering processes at intermediate temperatures to coherent Kondo lattice behaviors at low temperatures. Both the Hall resistivity measurements and band-structure calculations indicate a relatively low carrier concentration in YbPdAs. Our results suggest that YbPdAs could provide an opportunity for examining the interplay of Kondo physics and magnetic frustration in low carrier systems.

Accurate simulation of electronic excitations and deexcitations are critical for complementing complex spectroscopic experiments and can provide validation to theoretical approaches. Using a generalized framework, we contrast the accuracy and validity of orbital-constrained and linear-response approaches that build upon Kohn-Sham density functional theory (DFT) to simulate emission spectra of electronic origin and propose an efficient approximation, named many-body x-ray emission spectroscopy or MBXES, for simulating such processes. We show analytically as well as with computed examples that for electronic (de)excitation leading to an appreciable change in polarization (i.e., density rearrangement), the adiabatic approximation in a response-based formalism will be inadequate for the calculation of oscillator strength. Thus, such a change (e.g., in the net electrostatic dipole moment of a finite system) can be used as a metric for evaluating the applicability of the adiabatic response-based approach and can be particularly valuable in x-ray emission spectroscopy. On the other hand, MBXES, the flexible method introduced in this paper, can compute oscillator strengths accurately at a much lower computational expense on the basis of two DFT-based self-consistent field calculations. Using illustrative examples of emission spectra, the efficacy of the MBXES method is demonstrated by comparison with its parent theory, orbital-optimized DFT, and with experiments.

For several decades now, ultrahigh-mobility GaAs two-dimensional electron systems (2DESs) have served as the hallmark platform for various branches of research in condensed-matter physics. Fundamental to this long-standing history of success for GaAs 2DESs was continuous sample quality improvement, which enabled scattering-free transport over macroscopic lengthscales as well as the emergence of a diverse range of exotic many-body phenomena. While the recent breakthrough in the quality of GaAs 2DESs grown by molecular beam epitaxy is highly commendable in this context, it is also important and timely to establish an up-to-date understanding of what obstructs us from pushing the mobility limit even further. Here, we present mobility data taken at a temperature of 0.3 K for a wide variety of state-of-the-art GaAs 2DESs, exhibiting a maximum, world-record mobility of $\mu \simeq 57\times {10}^6{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ at a 2DES density of $n=1.55\times {10}^{11}/{\mathrm{cm}}^2$. We also provide comprehensive analyses of the collective scattering mechanisms that can explain the results. Furthermore, based on our study, we discuss potential scenarios in which GaAs 2DES mobility values exceeding $100\times {10}^6{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ could be achieved.

We propose a quantum-mechanical model to calculate the nonlinear differential conductance of a single molecular junction immersed in a solvent, either in pure form or as a binary mixture with varying volume fraction. The solvent mixture is captured by a dielectric continuum model for which the resulting spectral density is determined within the Gladstone-Dale approach. The conductance of the molecular junction is calculated by a real-time diagrammatic technique. We find a strong variation of the conductance maximum for varying volume fraction of the solvent mixture. Importantly, the calculated molecular nonlinear conductance shows a very good agreement with experimentally measured data for common molecular junctions in various polar solvent mixtures.

Weyl semimetals exhibit a myriad of exotic transport responses, among which, the Goos-Hänchen (GH) and Imbert-Fedorov (IF) effects have recently garnered substantial attention. Besides the usual parametric dependence inherited from the underlying Hamiltonian to describe a Weyl system, the IF shift particularly carries a topological identity—it depends on the chirality of the Weyl cones. Observing such signatures following the trail of theoretical predictions applied to clean systems can be severely obfuscated by surface potentials induced by localized impurities that are naturally present in real materials hosting the semimetallic phase. Classifying these potentials, we study their effects on GH and IF shifts to provide useful guidance to experiments that are tuned to the objective of characterizing Weyl semimetals and leveraging them to provide the basis for future technological advances. A transfer matrix-based approach is proposed to study the profile of Weyl wave functions scattering from the impurity potentials. As we unfold, the presence of such potentials can lead to several remarkable effects such as the complete nullification of the IF shift and valley inversion.

The high-order topological phase exhibits nontrivial gapless states at the boundaries whose dimension is lower than bulk by two. However, this phase has not been observed experimentally in two-dimensional (2D) materials up to now. In this paper, using first-principles calculations and the tight-binding model, we propose that the experimentally synthesized ${\mathrm{C}}_2\mathrm{N}$ is a 2D second-order topological insulator (SOTI) with one-dimensional gapped edge states and zero-dimensional gapless corner states. ${\mathrm{C}}_2\mathrm{N}$ exhibits a large bulk gap of 2.45 eV and an edge gap of 0.32 eV, making it an excellent candidate to evidently present the nontrivial corner states in experiments. The robustness of the corner states against the edge disorders has been explicitly identified. Moreover, another three ${\mathrm{C}}_2\mathrm{N}$-like materials are also found to host the nontrivial SOTI phase including an experimentally synthesized material aza-fused $\pi$-conjugated microporous polymers. The four 2D SOTIs proposed in our present paper provide excellent candidates for studying the novel high-order topological properties in future experiments.

Spin currents proportional to the first and second order of the electric field are calculated in a specific tight-binding model with time-reversal symmetry. Specifically, a tight-binding model with time-reversal symmetry is constructed with chiral hopping and spin-orbit coupling. The spin conductivity of the model is calculated using the Boltzmann equation. As a result, it is clarified that the first-order spin current of the electric field vanishes, while the second-order spin current can be finite. Furthermore, the spin current changes its sign by reversing the chirality of the model. The present results reveal the existence of spin currents in systems with time-reversal symmetry depending on the chirality of the system. They may provide useful information for understanding the chirality-dependent spin polarization phenomena in systems with time-reversal symmetry.

Motivated by recent extensive studies on the $M{A}_2{Z}_4$ family materials, we have predicted three ${\mathrm{MoP}}_2{X}_2{Y}_2$-type monolayers, including ${\mathrm{MoP}}_2{\mathrm{B}}_2{\mathrm{Te}}_2, {\mathrm{MoP}}_2{\mathrm{Al}}_2{\mathrm{S}}_2$, and ${\mathrm{MoP}}_2{\mathrm{Ga}}_2{\mathrm{S}}_2$, which possess excellent dynamic and thermal stabilities and peculiar electronic properties. According to first-principle calculations, the band structures show that the three monolayers are direct band-gap semiconductors with a direct band gap of $\sim 1\mathrm{eV}$. The band energy extrema occurs at $K/K^{\prime }$ points, giving rise to a pair of degenerate valleys. A large valley spin splitting is found at the valence band under the spin-orbit coupling effect, resulting in valley-contrasting Berry curvature. Moreover, these materials have strong optical absorption intensity in the visible light regions, especially ${\mathrm{MoP}}_2{\mathrm{Ga}}_2{\mathrm{S}}_2$. Our research shows that these three ${\mathrm{MoP}}_2{X}_2{Y}_2$ monolayers, as close relatives of $M{A}_2{Z}_4$ family materials, have potential valleytronics and photovoltaics applications.

Magnetic resonances driven by current-induced torques are crucial tools to study magnetic materials but are very limited in frequency and mechanical efficiency. We propose an alternative mechanism, voltage-induced torque, to realize high efficiency in generating high-frequency magnetization dynamics. When a ferromagnet–topological insulator–ferromagnet trilayer heterostructure is operated as an adiabatic quantum motor, voltage-induced torque arises from the adiabatic motion of gapped topological electrons on the two interfaces and act oppositely on the two ferromagnetic layers, which can excite the exchange mode where the two ferromagnetic layers precess with a $\pi$-phase difference. The exchange mode resonance, bearing a much higher frequency than the ferromagnetic resonance, is accompanied by topological charge pumping, leading to a sharp peak in electrical admittance at the resonance point. Because the output current is purely adiabatic while dissipative current vanishes identically, the proposed voltage-driven exchange resonance entails a remarkably high mechanical efficiency close to unity, which is impossible in any current-driven systems.

We propose a theoretical model to study the quantum phase transition in a double-cavity magnonic system. We find that the system exhibits a second-order phase transition from a parity-symmetric phase to a parity-symmetry-broken phase or a first-order phase transition from a parity-symmetric phase to a bistable phase when the driving strength of one cavity is above a critical value. We obtain the phase diagram, the critical point, and the corresponding critical exponent to characterize the phase transition. We can identify different phase transitions by the different behaviors of the mean magnon number and correlation fluctuation in the vicinity of the critical point. In particular, we show that the phase transition in one cavity can be precisely and efficiently controlled by adjusting the parameters of the other cavity, which suggests that we can easily observe the phase transition at low driving strength in experiment. The effects of additional microwave pulses on the dynamical behavior of the phase-transition observable and the experimental feasibility of our theoretical scheme are discussed as well.

We study the emergence of topological superconductivity in a two-dimensional Weyl system, composed of stacked Su-Schrieffer-Heeger (SSH) chains. A previous analysis of the model showed that the addition of an attractive Hubbard interaction between spinful electrons leads to a superconducting state that has an intricate pairing structure, but is topologically trivial. Here we consider a pairing interaction that couples spinless fermions on opposite sublattices within the same unit cell. We observe that this physically motivated, momentum-independent pairing interaction induces a topological superconducting state, characterized by a gap function with a nontrivial phase, as well as Majorana and Fermi arc edge states along the edge perpendicular to the direction of the SSH dimerization. In addition, we observe a transition as a function of pairing interaction strength and chemical potential, indicated by a change in the sign of the topological charge carried by each of the four Bogoliubov-Weyl nodes.

In this paper, we revisit the high pressure behavior of ${\mathrm{BaZrO}}_3$ by a combination of first-principle calculations, Raman spectroscopy and x-ray diffraction under high pressure. We confirm experimentally the cubic-to-tetragonal transition at $10\mathrm{GPa}$ and find no evidence for any other phase transition up to $45\mathrm{GPa}$, the highest pressures investigated, at variance with past reports. We reinvestigate phase stability with density functional theory considering not only the known tetragonal ($I4/mcm$) phase but also other potential antiferrodistortive candidates. This shows that the tetragonal phase becomes progressively more stable upon increasing pressure as compared to phases with more complex tilt systems. The possibility for a second transition to another tilted phase at higher pressures, and in particular to the very common orthorhombic $Pnma$ structure, is therefore ruled out.

The wave transport properties of two-dimensional stealthy hyperuniform distributions of rigid scatterers embedded in a waveguide are experimentally characterized for scalar waves in airborne audible acoustics. The nonresonant nature of the scatterers allows us to directly link these properties to the geometric distribution of points through the structure factor. The transport properties are analyzed as a function of the stealthiness $\chi$ of their hyperuniform point pattern and compared to those of a disordered material in the diffusive regime, which are characterized by the Ohm's law through both the classical mean free path and the corrected mean free path by the collective approximation considering the effects of correlation. Different scattering regimes are theoretically and numerically identified showing transparent regions, isotropic band gaps, and anisotropic scattering depending on $\chi$. The robustness of these scattering regimes to losses, which are unavoidable in audible acoustics is experimentally unveiled.

We numerically show that a vortex pinning occurs in a superconducting quasicrystal without impurities and defects. This vortex pinning is intrinsic since the superconducting order parameter in quasicrystals is always inhomogeneous due to the lack of the translational symmetry. We propose that experiments influenced by vortex pinning effects can detect the atomic-scale inhomogeneous superconducting order parameter in quasicrystals. We develop a numerical method to solve the Bogoliubov-de Gennes equations and gap equations in large systems, which is based on the localized-Krylov subspace and a sparse modeling technique. Two two-dimensional quasicrystals, the Penrose and Amman-Beenker tiling, are considered.

A real-space formulation of the numerical renormalization group (NRG) procedure is introduced. The real-space construction, dubbed eNRG, is more straightforward than the NRG discretization, and allows a faithful description of the coupling between quantum dots and conduction states even if the design of the couplings is intricate. General features of the two procedures are discussed comparatively. A more specific comparison is then developed, based on computations of the zero-bias transport properties for an Anderson-model description of a quantum wire side-coupled to a single quantum dot. An eNRG computation is shown to reproduce accurately the temperature-dependent electrical conductance for the uncorrelated model in the continuum limit, while significant deviations mark the corresponding NRG computation for thermal energies comparable to the conduction bandwidth. A combination of analytical and numerical results for the transport properties of the correlated model then provides a more exacting check on the accuracy of the eNRG procedure. A recent NRG analysis mapping the transport properties of a single-electron transistor onto universal functions of the temperature scaled by the Kondo temperature is extended to the side-coupled device, on the basis of eNRG reasoning. Numerical results for the electrical conductance, thermopower, and thermal conductance in side-coupled geometry are then shown to agree very well with the mappings. The numerical results are also checked against the thermal dependence of the thermopower measured by Köhler et al. [Phys. Rev. B 77, 104412 (2008)] in ${\mathrm{Lu}}_{0.9}^{\phantom{†}}{\mathrm{Yb}}_{0.1}^{\phantom{†}}{\mathrm{Rh}}_2^{\phantom{†}}{\mathrm{Si}}_2^{\phantom{†}}$, and the remarkably accurate, recent conductance measurements by Xu et al. [Chin. Phys. Lett. 38, 087101 (2021)].

Employing soft x-ray magnetic circular dichroism (XMCD) and soft x-ray absorption spectroscopy (XAS), we have investigated electronic structures and spin configurations of Co-substituted ${\mathrm{Mn}}_3\mathrm{Ga}$ (${\mathrm{Mn}}_{3-x}{\mathrm{Co}}_x\mathrm{Ga}$) Heusler films, which are promising spintronic materials exhibiting perpendicular magnetic anisotropy (PMA). Co $2p$ XAS and XMCD measurements, combined with the charge-transfer multiplet analysis, show that the substituted Co ions in ${\mathrm{Mn}}_{3-x}{\mathrm{Co}}_x\mathrm{Ga}$ consist of mainly ferromagnetic metallic Co clusters and a small amount of divalent Co ions with disordered magnetic moments. Mn $2p$ XAS and XMCD measurements show that Mn ions at both the octahedral and tetrahedral sites in ${\mathrm{Mn}}_{3-x}{\mathrm{Co}}_x\mathrm{Ga}$ are nearly divalent with weak covalent-bonding nature, reflecting non-Jahn-Teller ions, and that the magnetic moments of Mn ions are polarized along the ordered magnetic moments of the substituted Co ions. Angle-dependent Co $2p$ XMCD data of ${\mathrm{Mn}}_{3-x}{\mathrm{Co}}_x\mathrm{Ga}$ agree with the weak magnetic anisotropy transition from PMA to in-plane magnetic anisotropy (IMA) with increasing $x$. Spin and orbital magnetic moments of Co ions are estimated by using the sum-rule analysis, which reveal negligible Co-content and polar-angle dependencies of the Co orbital to spin magnetic moment ratio. This work reveals that the PMA-to-IMA magnetic transition in ${\mathrm{Mn}}_{3-x}{\mathrm{Co}}_x\mathrm{Ga}$ is determined mainly by ferromagnetic metallic Co clusters.

The recently discovered superconductor, ${\mathrm{UTe}}_2$, has attracted immense scientific interest due to observations that suggest odd-parity superconductivity. It is believed that the material is a heavy-fermion metal at low temperatures although details of the normal state are unclear. Using Fourier transform infrared spectroscopy, we investigated the normal state electronic structure of ${\mathrm{UTe}}_2$ at zero applied magnetic field. Combining the measured reflectivity with the dc resistivity, the complex optical conductivity was obtained over a large frequency range. The frequency dependence of the real part of the optical conductivity exhibits a MIR peak around 4000 ${\mathrm{cm}}^{-1}$ and a narrow Drude peak that develops below 40 K. A combination of density functional and dynamic mean-field theory gives spectra in close correspondence to the experiment. Via this comparison we attribute the prominent MIR peak to interband transitions involving a narrow U $5f$ feature that develops near the Fermi level. In this regard, this comparison along with data that shows the scale of the low-temperature mass renormalization gives spectroscopic evidence for the existence of a low-energy Kondo resonance at temperatures just above the onset of superconductivity and implicates heavy electrons in the formation of the superconducting state. We find that the coherent Kondo resonance is primarily associated with a collapse of scattering and less with a transfer of spectral weight.