van der Waals (vdW) materials have always been a fertile ground for exploring pressure-tuned dimensional crossover and novel physical properties, such as superconductivity. Nevertheless, few of the dimensional crossovers with an accompanying superconductivity occur at a low pressure close to environmental conditions. Here, we predicted a novel $C2/m$ structure of ${\mathrm{LiP}}_{15}$ which can be achieved by compressing the experimentally synthesized $P\overline{1}$ structure above 2 GPa. Under compression, ${\mathrm{LiP}}_{15}$ goes through a one- to quasi-two-dimensional crossover and an accompanying semiconducting to superconducting electronic transition. The $C2/m$ structure is a superconductor with an increasing transition temperature under pressure that reaches up to 14.2 K at 15 GPa. In addition, we found that the phosphorus allotrope obtained by removing lithium atoms in the $C2/m$ structure is dynamically stable, and its energy is almost equivalent to that of black phosphorus at ambient pressure. These findings not only pave a new way to design a new vdW material by means of pressure-induced dimensional crossover, but also provide a possibility for searching a new vdW superconductor in similar systems.

Our understanding of pressure-induced anomalies in the physical and mechanical properties of Nb with a simple body-centered cubic structure remains limited both experimentally and theoretically. In this work we experimentally investigate the sound velocity of the compressional, bulk, and shear waves of Nb up to about 69 GPa and 1100 K under shock compression. These measurements reveal a significant softening in both the compressional and shear wave sound velocities in Nb compressed at 50 to 60 GPa. First-principles computations indicate that this anomalous behavior is most likely due to a pressure-induced Lifshitz-type electronic topological transition at 50 to 60 GPa. This study thus indicates that high-order phase transformations associated with the changes of electronic structures play an important role in the physical modification of transition metals.

The twist angle in transition metal dichalcogenide heterobilayers is a compelling degree of freedom that determines electron correlations and the period of lateral confinement of moiré excitons. Here we perform polarization-resolved second harmonic generation (SHG) spectroscopy of $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{W}{\mathrm{Se}}_2$ heterostructures. We demonstrate that by choosing suitable laser energies the twist angle between two monolayers can be measured directly on the assembled heterostructure. We show that the amplitude and polarization of the SHG signal from the heterostructure are determined by the twist angle between the layers and exciton resonances at the SH energy. For heterostructures with close to zero twist angle, we observe changes of exciton resonance energies and the appearance of new resonances in the linear and nonlinear susceptibilities.

In this paper, we discuss the optical response due to the excitonic effect of two types of hBN bilayers: AB and ${\mathrm{AA}}^{\prime }$. Understanding the properties of these bilayers is of great utility to the study of twisted bilayers at arbitrary angles since these two configurations correspond to the limit cases of $0^{\circ }$ and ${60}^{\circ }$ rotation. To obtain the excitonic response, we present a method to solve a four-band Bethe-Salpeter equation by casting it into a one-dimensional problem, thus greatly reducing the numerical burden of the calculation when compared with strictly two-dimensional methods. We find results in good agreement with ab initio calculations already published in the literature for the ${\mathrm{AA}}^{\prime }$ bilayer, and predict the excitonic conductivity of the AB bilayer, which remains largely unstudied. The main difference in the conductivity of these two types of bilayers is the appearance of a small, yet well-resolved resonance between two larger ones in the AB configuration. This resonance is due to a mainly interlayer exciton, and is absent in the ${\mathrm{AA}}^{\prime }$ bilayer. Also, the conductivity of the AB bilayer is due to both intralayer and interlayer excitons and is dominated by $p$-states, while intralayer $s$-states are the relevant ones for the ${\mathrm{AA}}^{\prime }$ configuration, like in a monolayer. The effect of introducing a bias in the ${\mathrm{AA}}^{\prime }$ bilayer is also discussed.

In this paper, we study the PXP Hamiltonian with an external magnetic field that exhibits both quantum scar states and quantum criticality. It is known that this model hosts a series of quantum many-body scar states violating quantum thermalization at zero magnetic field, and it also exhibits an Ising quantum phase transition driven by a finite magnetic field. Although the former involves the properties of generic excited states and the latter concerns the low-energy physics, we discover two surprising connections between them, inspired by the observation that both states possess subvolume law entanglement entropies. First, we show that the quantum many-body scar states can be tracked to a set of quantum critical states, whose nature can be understood as pairwisely occupied Fermi sea states. Second, we show that the partial violation of quantum thermalization diminishes in the quantum critical regime. We envision that these connections can be extended to general situations and readily verified in existing cold atom experiment platforms.

We investigate the thermal transport properties of a quantum anomalous Hall insulator nanoribbon covered by superconductors. Due to the peculiar properties of chiral Majorana fermion (CMF) edge states, we found a half-integer quantized thermal conductance plateau. Different from the ordinary superconductor which conducts electricity but does not conduct heat, in the quantum anomalous Hall insulator–topological superconductor junction, the CMF occurs at the boundary of the topological superconductor which carries heat from left to right. A chiral topological superconductor with Chern number $\mathcal{N}=\pm 1$ has a single CMF, which is equivalent to half an ordinary fermion propagating along the edge, leading to a half-integer quantized thermal conductance plateau. When the topological superconducting edge has two CMFs, it is equivalent to ejecting an ordinary fermion, resulting in an integer quantized thermal conductance plateau. Moreover, the half-integer quantized thermal conductance can also be used to study the properties of Majorana Kramer pairs in a helical topological superconductor. Finally, we also find that the half-integer quantized plateau appears against moderate disorder and with the metal leads besides the quantum anomalous Hall insulator, which is promising to be realized in experiments.

The search for topological superconductors is one of the most pressing and challenging questions in condensed-matter and material research. Despite some early suggestions that doping a topological insulator might be a successful recipe to find topological superconductors, to date there is no general understanding of the relationship of the topology of the superconductor and the topology of its underlying normal state system. One of the major obstacles is the strong effect of the Fermi surface and its subsequent pairing tendencies within the Hubbard model framework, which usually prevents a detailed comparison between different topological superconducting systems. Here we present an analysis of doped insulators—topological and trivial—where the dominant Fermi surface effects have been equalized. Our approach allows us to study and compare superconducting instabilities of different normal state systems and present rigorous results about the influence of the normal state system's topology.

Understanding magnetic phase transitions from stripe to skyrmion textures provide fundamental scientific insights into the details of the intermediate topologies through which the system evolves. The solitonic nature of spin texture in both stripe and skyrmion phases has been observed in materials with Dzyaloshinskii-Moriya interaction. Here we show that the field evolution in a dipolar interaction mediated Fe/Gd multilayer that exhibits achiral stripe textures behaves similar to a finite-sized chiral soliton lattice. As a function of magnetic field, the stripes exhibit discrete jumps in periodicity indicative of intermediate topologies as the stripes get wound into skyrmions.

The superconducting behavior of the topological semimetal $\mathrm{Ca}{\mathrm{Sn}}_3$ was investigated by means of magnetotransport and muon spectroscopy ($\mu \mathrm{S}\mathrm{R}$) measurements, both providing strong evidence of nematic behavior. Magnetotransport detects an anisotropic upper critical field, characterized by a twofold symmetry about $C_4$ axis, thus breaking the rotational symmetry of the underlying cubic lattice. Transverse-field $\mu \mathrm{S}\mathrm{R}$ data support such picture, with the muon depolarization rate depending strongly on the magnetic field direction, here applied along the [110] or [001] crystal directions. In the former case, the absence of any additional muon depolarization suggests an unconventional vortex lattice. In the latter case, a vortex lattice encompassing a sample volume of at least 52% indicates the bulk nature of $\mathrm{Ca}{\mathrm{Sn}}_3$ superconductivity. The resulting superfluid density in the (001) planes shows a gapped low-temperature behavior, with a superconducting gap value $\mathrm{\Delta }\left(0\right)\simeq 0.61\left(7\right)$ meV. Additional zero-field $\mu \mathrm{S}\mathrm{R}$ results indicate that the superconducting state is time-reversal invariant. This fact and the breaking of rotational symmetry in a fully gapped superconductor are consistent with an unconventional pairing state in a multidimensional representation, thus making $\mathrm{Ca}{\mathrm{Sn}}_3$ an important example of nematic superconductor.

X-ray diffraction and Raman spectroscopy of ${\mathrm{H}}_2\mathrm{O}$ (ice) structures are measured under static compression in combination with grain normalizing heat treatment via direct laser heating. We report the transition from cubic ice-VII to a structure of tetragonal symmetry, ice-${\mathrm{VII}}_{\text{t}}$ at $5.1\pm 0.5\mathrm{GPa}$. This is succeeded by the H-bond symmetrization transition occurring at a pressure of $30.9\pm 3\mathrm{GPa}$. Both experimental observations are supported by simulated Raman spectra from density-functional theory quantum calculations. The transition to H-bond symmetrization is evidenced by the reversible emergence of its characteristic Raman mode and a 2.5-fold increase in bulk modulus, implying a significant increase in bonding strength.

The unsteady, nonlinear magnetization dynamics induced by spin injection in an easy-plane ferromagnetic channel subject to an external magnetic field are studied analytically. Leveraging a dispersive hydrodynamic description, the Landau-Lifshitz equation is recast in terms of hydrodynamic-like variables for the magnetization's perpendicular component (spin density) and azimuthal phase gradient (fluid velocity). Spin injection acts as a moving piston that generates nonlinear, dynamical spin textures in the ferromagnetic channel with downstream quiescent spin density set by the external field. In contrast to the classical problem of a piston accelerating a compressible gas, here, variable spin injection and field lead to a rich variety of nonlinear wave phenomena from oscillatory spin shocks to solitons and rarefaction (expansion) waves. A full classification of solutions is provided using nonlinear wave modulation theory by identifying two key aspects of the fluid-like dynamics: subsonic/supersonic conditions and convex/nonconvex hydrodynamic flux. Familiar waveforms from the classical piston problem such as rarefaction waves and shocks manifest in their spin-based counterparts as smooth and highly oscillatory transitions, respectively, between two distinct magnetic states. The spin shock is an example of a dispersive shock wave, which arises in many physical systems. New features without a gas dynamics counterpart include composite wave complexes with “contact” spin shocks and rarefactions. Magnetic supersonic conditions lead to two pronounced piston edge behaviors including a stationary soliton and an oscillatory wave train. These coherent wave structures have physical implications for the generation of high frequency spin waves from pulsed injection and persistent, stable stationary and/or propagating solitons in the presence of magnetic damping. The analytical results are favorably compared with numerical simulations.

The nature of magnetism in the intercalated honeycomb iridate ${\mathrm{Ag}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$ has been a subject of recent intensive debate, where the absence or presence of antiferromagnetic order has been reported to be related to possible structural disorder effects and an enhanced Ir-O hybridization and itinerancy with respect to the parent $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ has been suggested as the origin of distinct x-ray spectroscopy features. In the present work we investigate the microscopic nature of the electronic and magnetic properties of ${\mathrm{Ag}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$ via a combination of density functional theory combined with exact diagonalization of ab initio derived models for various experimental and theoretical structures. We evaluate two possible scenarios, the itinerant quasimolecular framework (QMO) on the one hand, and the localized relativistic $j_{\mathrm{eff}}=1/2$ and $j_{\mathrm{eff}}=3/2$ picture on the other hand, and find that the second description is still viable for this system. We further calculate resonant inelastic x-ray scattering spectra and show that agreement with experimental observations can be obtained if the presence of Ag vacancies leading to changes in Ir filling and structural disorder is assumed. Finally, we show that the experimentally observed antiferromagnetic spiral magnetic order is reproduced by our ab initio derived magnetic models.

We investigate a system consisting of one or two topological-insulator leads which are tunnel coupled to a single dot level. The leads are described by the one-dimensional Su-Schrieffer-Heeger model. We show that (topological) edge states cause characteristic features in the dot spectral function, the dot occupation, and the finite-bias current across the dot. As the kinetic energy is quenched in the dot region, local two-particle interactions are of particular relevance there. This motivates us to test whether the aforementioned edge-state features are robust against such interactions; we report here that they are either robust or even enhanced. We conclude that the characteristic features can be used to determine if the leads are in their topologically nontrivial or trivial phase.

Contemporary studies of magnetic topological insulators (TIs) aimed at investigating Dirac point (DP) gaps by angle-resolved photoemission spectroscopy (ARPES) require ultrahigh energy, momentum, and spatial resolution. Despite satisfying all these conditions, accurate measurements can be spoiled by the well-known, but often neglected, effect of the energy shift of bands, in particular the topological surface state (TSS), due to sample contamination and irradiation. Thus, in this work we focus on the effect of TI irradiation with laser light ($h\nu =6.3$ eV) on the measurability of its electronic structure using the ARPES method. We find that the distribution of the shifted TSS is sufficiently localized and remains stable over time under constant conditions. Therefore, it can be used to store information. The TSS retains its spin pattern regardless of irradiation, as demonstrated by dichroic ARPES measurements. However, irradiation leads to a significant broadening of the TSS, accompanied by a drop in its intensity. All these factors complicate the analysis of the states near the DP and the DP gap size. Finally, using a simple model, we demonstrate the variability of the DP gap over time in the resulting ARPES image, caused by both shift and broadening effects. Thus, accurate values of the DP gap can be obtained by measuring over a relatively small period of time, which is determined by a certain time constant of the energy shift.

The superconducting gap structures in the transition-metal-based kagome metal $A{\text{V}}_3{\mathrm{Sb}}_5$ ($A=\text{K}$, Rb, Cs), the first family of quasi-two-dimensional kagome superconductors, remain elusive as there is strong experimental evidence for both nodal and nodeless gap structures. Here we show that the dichotomy can be resolved because of the coexistence of time-reversal symmetry breaking with a conventional fully gapped superconductivity. The symmetry protects the edge states which arise on the domains of the lattice symmetry breaking order to remain gapless in proximity to a conventional pairing. We demonstrate this result in a four-band tight-binding model using the V $d_{X^2\text{a}-Y^2}$-like and the in-plane Sb $p_z$-like Wannier functions that can faithfully capture the main feature of the materials near the Fermi level.

We present a systematic theory of acoustic-phonon-mediated superconductivity which incorporates Coulomb repulsion, explaining the recent experiment in Bernal bilayer graphene under a large displacement field. The acoustic-phonon mechanism predicts that $s$-wave spin-singlet and $f$-wave spin-triplet pairings are degenerate and dominant. Assuming a spin-polarized valley-unpolarized normal state, we obtain $f$-wave spin-triplet superconductivity with $T_c\sim 20$ mK near $n_e=-0.6\times {10}^{12} {\mathrm{cm}}^{-2}$ for hole doping, in approximate agreement with the experiment. We further predict the existence of superconductivity for larger doping in both electron-doped and hole-doped regimes. Our results indicate that the observed spin-triplet superconductivity in Bernal bilayer graphene arises from acoustic phonons.

Parity breaking gives rise to rich superconducting properties through the admixture of even- and odd-parity Cooper pairs. New light has been shed on parity-breaking superconductors by recent observations of nonreciprocal responses such as nonlinear optical responses and the superconducting diode effect. In this Letter, we demonstrate that nonreciprocal responses are characterized by a unidirectional correction to the superfluid density, which we call nonreciprocal superfluid density. This correction leads to the nonreciprocal Meissner effect, namely, the asymmetric screening of magnetic fields due to the nonreciprocal magnetic penetration depth. Performing a microscopic analysis of an exotic superconductor ${\mathrm{UTe}}_2$ and examining the temperature dependence and renormalization effect, we show that the nonreciprocal Meissner effect is useful to probe parity-mixing properties and gap structures in superconductors.