Very recently, carbon-boron clathrate $\mathrm{Sr}{\mathrm{B}}_3{\mathrm{C}}_3$ has been successfully synthesized, in which carbon and boron atoms form $s{p}^3$-bonded truncated octahedral cages. Interestingly, the $s{p}^3$-hybridized $\sigma$-bonding bands are partially occupied. This may drive $\mathrm{Sr}{\mathrm{B}}_3{\mathrm{C}}_3$ into a superconducting state, like boron-doped diamond. By means of density functional first-principles calculations and Wannier interpolation technique, we have investigated the electron-phonon coupling and phonon-mediated superconductivity in $\mathrm{Sr}{\mathrm{B}}_3{\mathrm{C}}_3$. Our calculations reveal that there exists strong coupling between $s{p}^3$-hybridized $\sigma$-bonding bands and boron-associated $E_g$ phonon modes. Based on the Migdal-Eliashberg theory, we self-consistently solve the anisotropic Eliashberg equations. It is found that $\mathrm{Sr}{\mathrm{B}}_3{\mathrm{C}}_3$ is a single-gap superconductor, with superconducting transition temperature being 40 K. The anisotropic ratio of superconducting energy gap is computed to be 32.8%. Further replacing Sr with Ba, the transition temperature can be boosted to 43 K in $\mathrm{Ba}{\mathrm{B}}_3{\mathrm{C}}_3$ due to phonon softening. These findings suggest that $\mathrm{Sr}{\mathrm{B}}_3{\mathrm{C}}_3$ and $\mathrm{Ba}{\mathrm{B}}_3{\mathrm{C}}_3$ are phonon-mediated high-temperature anisotropic $s$-wave superconductors.

For a number of skutterudite-related cage compounds, such as cubic $R_3{M}_4{\mathrm{Sn}}_{13}$ ($R=\mathrm{Ca}$ or La, $M=\text{transition}$ $d$-electron metal) or tetragonal ${\mathrm{Y}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$, we have previously reported the impact of various atomic defects in the crystal lattices on the normal-state and superconducting properties of these clathrates. In these quasiskutterudites the nanoscale disorder and/or local inhomogeneity in composition lead to an abnormal increase in the superconducting transition temperature $T_c$. Here we investigate the impact of atomic defects in the superconductors ${\mathrm{Lu}}_{5-\delta }{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ (tetragonal symmetry $I{4}_1/acd$). We have documented that ${\mathrm{Lu}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ does not crystallize in the assumed stoichiometry but forms a 5-6-18 phase with a Lu deficiency ($\delta \approx 0.5$). Our comprehensive investigations of electronic structure and thermodynamic and electrical transport properties documented a $T_c$ increase in the more disordered sample (sample 1). The quality of two investigated samples (sample 1 and sample 2) with similar stoichiometry but different local inhomogeneity was obtained by microanalysis and electron transmission microscopy observations. The band structure calculations show a hybridization pseudogap in the electronic bands of stoichiometric ${\mathrm{Lu}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ at about 0.3 eV in respect to the Fermi level ${\epsilon }_F$. The ab initio calculations predict the scenario that vacancies could shift this pseudogap towards ${\epsilon }_F$, which is manifested by the Mott variable-range hopping behavior in the resistivity $\rho \sim T^{-1/4}$ of more homogeneous sample 2.

Local topological markers have proven to be a valuable tool for investigating systems with topologically nontrivial bands. Due to their local nature, such markers can treat translationally invariant systems and spatially inhomogeneous systems on an equal footing. Among the most prevalent of these is the so-called Chern marker, which is available for systems in two spatial dimensions. In this paper we describe how to generalize this marker to 1D and 3D systems by showing that the relevant expressions accurately describe the phenomenon of topological pumping given by the first and second Chern numbers in 1D and 3D, respectively. In addition to providing general derivations, we verify the markers by numerically considering model Hamiltonians. These results will open the door for future studies, including the influence of disorder on topological pumping and topological phase transitions in odd-dimensional systems.

Population control is an essential component of any projector Monte Carlo algorithm. This control mechanism usually introduces a bias in the sampled quantities that is inversely proportional to the population size. In this paper, we investigate the population control bias in the full configuration interaction quantum Monte Carlo method. We identify the precise origin of this bias and quantify it in general. We show that it has different effects on different estimators and that the shift estimator is particularly susceptible. We derive a reweighting technique, similar to the one used in diffusion Monte Carlo, for correcting this bias and apply it to the shift estimator. We also show that by using importance sampling, the bias can be reduced substantially. We demonstrate the necessity and the effectiveness of applying these techniques for sign-problem-free systems where this bias is especially notable. Specifically, we show results for large one-dimensional Hubbard models and the two-dimensional Heisenberg model, where corrected FCIQMC results are comparable to the other high-accuracy results.

A theoretical study of Bragg diffraction by an enantiomorphic pair of structures should apply to patterns gathered on β-Mn using resonance-enhanced x-ray diffraction. The chiral polymorph of manganese, and structurally related compounds, will be more convenient than trigonal crystals of low quartz, tellurium, and berlinite used in previous structural studies of this type, because cubic symmetry removes some complications of correction for absorption and macroscopic birefringence. Intensity of a Bragg spot engaged by circular polarization in the primary beam of photons $\mathrm{{\rm Y}}\left(h,k,l\right)$ is proposed as a chiral signature of the illuminated material. The partial intensity requires a knowledge of scattering amplitudes in all four channels of polarization, which are reported as functions of an azimuthal angle (rotation of the crystal about the axis of the reflection vector). Unlike trigonal chiral crystals, $\mathrm{{\rm Y}}\left(0,0,l\right)=0$ for β-Mn and it is ($h, k$, 0), and symmetry-related Bragg spots, that epitomize structural chirality. Specifically, reflection vectors ($h, k$, 0) with odd Miller indices and the chiral axis of β-Mn mesh in terms of helicity, with the corresponding Υ($h, k$, 0) equal in magnitude and opposite in sign for partners in the enantiomorphic pair. Dependence of Υ($h, k$, 0) on the azimuthal angle does not mirror the dyad or tetrad axes of rotation symmetry in the cubic crystal structure.

In this paper, we report scanning tunneling microscopy/scanning tunneling spectroscopy measurements of the evolution of the electronic state from the Mott phase to the nearly commensurate phase in Nb-substituted $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$. Although substitution of 1% Nb for Ta reduced the transition temperature, the sample was still in the Mott phase at the measurement temperature of 4.2 K. In this sample, domain walls were generated, and the electronic state changed from the Mott state at and around the domain walls. At substitution of 5% Nb, the sample did not undergo the phase transition to the Mott phase down to 4.2 K. In this sample, the number of the walls increased and the size of the domains largely reduced. The increase in the number of walls caused the overlap of the changed electronic states near the walls. This overlap induced the overall change in the electronic state of the sample.

The theory of massive photons in the presence of axions is studied as the effective theory describing the electromagnetic response of semimetals when a particular quartic fermionic pairing perturbation triggers the formation of charged chiral condensates, giving rise to an axionic superconductor. We investigate corrections to the Yukawa-like potential mediated by massive photons due to axion excitations up to one-loop order and compute the modifications of the London penetration length.

We consider the one-dimensional interacting Bose gas in the presence of time-dependent and spatially inhomogeneous contact interactions. Within its attractive phase, the gas allows for bound states of an arbitrary number of particles, which are eventually populated if the system is dynamically driven from the repulsive to the attractive regime. Building on the framework of generalized hydrodynamics, we analytically determine the formation of bound states in the limit of adiabatic changes in the interactions. Our results are valid for arbitrary initial thermal states and, more generally, generalized Gibbs ensembles.

In this paper we investigate strain and local polarization field effects in zinc-blende indium gallium nitride (InGaN) alloys and quantum wells. To do so we parametrize and establish a Stillinger-Weber potential with parameters fitted to hybrid functional density functional theory data. The developed model gives very good agreement with quantities to which it has not been fitted, such as Kleinman parameters of cubic III-N materials or the composition dependence of the lattice constant in InGaN alloys. Equipped with this model, we extract the composition dependence of elastic constants $C_{11}$ and $C_{12}$ in InGaN alloys, including bowing parameters for these quantities, which may form input for continuum-based calculations. Furthermore, applying this model to InGaN alloys and wells reveals that random alloy fluctuations can lead to strong local strain field fluctuations. Building on this information, we present a model that allows for the calculation of connected local built-in field fluctuations at the microscopic level, accounting for first- and second-order piezoelectric effects. The approach is general and can be applied to any zinc-blende III–V alloy or heterostructure investigated in the frame of semiempirical models (e.g., valence force field models) targeting strain fields on an atomistic level. Here, building on our Stillinger-Weber potential we show that local strain fluctuations in zinc-blende InGaN quantum wells can lead to strong piezoelectric built-in field fluctuations. This contribution has been widely overlooked in previous theoretical studies of these systems. Finally, we briefly discuss the impact of these polarization field fluctuations on carrier localization effects in such quantum well systems.

An all-optical approach to control interlayer charge transfer and interlayer exciton dynamics is demonstrated using transition metal dichalcogenide heterostructures of ${\mathrm{MoSe}}_2/{\mathrm{MoS}}_2$ and ${\mathrm{MoSe}}_2/{\mathrm{WS}}_2$ as examples. In the three-pulse pump-probe experiments, a control pulse injects photocarriers and produces an electric field due to the interlayer charge separation. A pump pulse excites new carriers and their dynamics under the influence of the control-injected carriers and field is time-resolved by measuring differential reflectance of a third probe pulse. In ${\mathrm{MoSe}}_2/{\mathrm{MoS}}_2$, the interlayer electron transfer time from ${\mathrm{MoSe}}_2$ to ${\mathrm{MoS}}_2$ decreases with increasing the control-injected carrier density and electric field. The dipole moment of the interlayer excitons is reduced by the control pulse, which can be utilized for ultrafast and all-optical control of interlayer excitons. The recombination of the interlayer excitons is enhanced by the control pulse, which increases the spatial overlap of the electron and hole wave functions. The effect of the control pulse on the interlayer excitons is confirmed in the ${\mathrm{MoSe}}_2/{\mathrm{WS}}_2$ heterostructure, although its effect on the electron transfer time is not resolvable. These results reveal useful information to understand and control interlayer charge transfer dynamics and provide tools for ultrafast manipulation of electrons in van der Waals heterostructures.

Two-dimensional polarity is intriguing but remains in the early stage. Here a structural evolution diagram is established for monolayer GeS, which leads to a noncollinear ferrielectric $\delta$ phase energetically as stable as the ferroelectric $\alpha$ phase. Its ferrielectricity is induced by the phonon frustration, i.e., the competition between ferroelectric and antiferroelectric modes, providing more routes to tune its polarity. Besides its prominent properties such as large band gap, large polarization, and high Curie temperature, more interestingly, the morphotropic phase boundary between $\alpha$ and $\delta$ phases is highly possible, which is crucial to obtain giant piezoelectricity for lead-free applications.

We consider the Ising chain driven by oscillatory transverse magnetic fields. For certain parameter regimes, we reveal a hidden integrable structure in the problem, which allows access to the exact time evolution in this driven quantum system. We compute time-evolved one- and two-point functions following a quench that activates the driving. It is shown that this model does not heat up to infinite temperature, despite the absence of energy conservation, and we further discuss the generalization to a family of driven Hamiltonians that do not appear to suffer heating to infinite temperature, despite the absence of integrability and disorder. The particular model studied in detail also presents a route for realizing exotic physics (in this case, signatures of the $E_8$ structure associated with perturbing a critical quantum Ising chain with a small longitudinal magnetic field) by suitably tuning the driving frequency. In particular, we numerically confirm that the ratio of the masses of the two lowest meson excitations is given by the golden ratio.

In a one-dimensional (1D) system with degenerate ground states, their domain boundaries, dubbed solitons, emerge as topological excitations often carrying unconventional charges and spins; however, the soliton excitations are vital in only the nonordered regime. Then a question arises: How do the solitons conform to a three-dimensional (3D) ordered state? Here, using a quasi-1D organic ferroelectric, tetrathiafulvalene-$p$-chloranil (TTF-CA), with degenerate polar dimers, we pursue the fate of spin-soliton charge-soliton composite matter in a 1D polar-dimer liquid upon its transition to a 3D ferroelectric order by resistivity, nuclear magnetic resonance (NMR), and nuclear quadrupole resonance (NQR) measurements. We demonstrate that the soliton matter undergoes neutral spin-spin soliton pairing and spin-charge soliton pairing to form polarons, coping with the 3D order. Below the ferroelectric transition, the former contributes to the magnetism through triplet excitations, which rapidly fade out on cooling, whereas the latter carries electrical current with paramagnetic spins that more moderately decrease with temperature. The nearly perfect scaling between NMR and NQR relaxation rates in the ferroelectric phase evidences that spin carriers diffuse with lattice distortion, namely, in the form of polarons. From the combined analyses of conductivity and NMR relaxation rate, we derive the excitation energies of polaron excitations and diffusion. Our results reveal the whole picture of soliton matter that condenses into the 3D ordered state.

Zircon-type $\mathrm{PrV}{\mathrm{O}}_4$ has been studied at high pressures and room temperature by means of synchrotron powder x-ray diffraction. At room temperature, we observed the previously known zircon-to-monazite phase transition at 5.5(4) GPa and a second phase transition from monazite to a monoclinic structure at 12.7(8) GPa, which we identified as a $\mathrm{PbW}{\mathrm{O}}_4$-III-type phase. This conclusion is supported by our ab initio calculations, which also predict a scheelite-type phase to be stable at high pressure. Motivated by this finding, we subjected zircon-type $\mathrm{PrV}{\mathrm{O}}_4$ samples to high pressure (7 GPa) and temperature (600, 800, and 1000 °C) in a Paris-Edinburgh large volume press cell. As a result, we could synthesize a scheelite-type phase of $\mathrm{PrV}{\mathrm{O}}_4$, which was recovered in a metastable state at room conditions, and the unit-cell parameters and the atomic positions have been determined. We also report a substantial increase of the scheelite proportion with time by comparing synthesis of different durations, at the same pressure-temperature conditions. The transition mechanisms of the zircon-, monazite-, and scheelite-type structures as well as the role of kinetic barriers will be discussed. Information on the compressibility of different polymorphs will be reported too.

We discover magnetic phases hosting highly elusive and technologically important antiferromagnetic skyrmion quasiparticles in a model for a Rashba-coupled Hund insulator. The results are based on unbiased simulations of a classical superexchange model derived, in this work, from a realistic microscopic electronic Hamiltonian. We also discover a skyrmion density wave ground state characterized by a $(Q,Q)$ modulation in the local skyrmion density map. A unique inhomogeneous state characterized by a circular pattern in the spin structure factor and filamentary real-space textures is identified as the parent of sparse antiferromagnetic skyrmions. We predict that the magnetic states reported here can be realized in thin films of multiorbital systems involving $4d$ or $5d$ transition metals.

First-principles density functional theory, combined with the Monte Carlo method, predicts that the ${\mathrm{Fe}}_2{\mathrm{B}}_2$ monolayer of the MBene family has a stable columnar antiferromagnetic (AFM) ground state. Below the critical temperature, $T_c=115$ K in equilibrium, the spins rotate by the same amount in every other column of Fe atoms, but they retain the same direction in the same column. Under applied tensile strains, $T_c$ and the order parameter can increase nonmonotonically. The onset of the columnar order can result in a transition from two dimension (2D) to 1D in magnetic, electronic, and conduction properties. The ordered magnetic state itself can be tuned by external magnetic field, whereby the columnar magnetic order changes to ferromagnetic order with a double hysteresis behavior. When terminated by Fluorine atoms, the columnar order changes to the AFM order with $T_c$ rising above room temperature. This situation is rather unusual and insofar is fundamental for a realistic, strictly 2D monolayer and can have critical consequences in spin conduction.

Using a stimulated Brillouin gain spectroscopy technique, we have measured the equation of state of bulk liquid helium-4 at $\sim 1$ K between 0 and 10 bars. Our results are in very good agreement with previous measurements and with theoretical equations of state. However, our specific method allows one to determine it on space/timescales as narrow as 35 $\mu \mathrm{m}$/190 ns. This technique is of particular interest for solving a remaining debate about the value of the homogeneous cavitation pressure of liquid ${}^4\mathrm{He}$ at $T\sim 1$ K.

We proposed a framework for the topological characterization of non-Hermitian band structures. Different from previous $K$-theoretical approaches, our approach is homotopical, which enables us to see more topological invariants. Specifically, we considered the classification of non-Hermitian systems with separable band structures. We found that the whole classification set is decomposed into several sectors, based on the braiding of energy levels. Each sector can be further classified based on the topology of eigenstates (wave functions). Due to the interplay between energy levels braiding and eigenstates topology, we found some torsion invariants, which only appear in the non-Hermitian world via homotopical approach. We further proved that these new topological invariants are unstable, in the sense that adding more bands will trivialize these invariants.

We define a class of tensor network states for spin systems where the individual tensors are functionals of fields. The construction is based on the path-integral representation of correlators of operators in quantum field theory. These tensor network states are infinite-dimensional versions of matrix product states and projected entangled pair states. We find the field tensor that generates the Haldane-Shastry wave function and extend it to two dimensions. We give evidence that the latter underlies the topological chiral state described by the Kalmeyer-Laughlin wave function.

Magnetic topological insulators and semimetals have a variety of properties that make them attractive for applications, including spintronics and quantum computation, but very few high-quality candidate materials are known. In this paper, we use systematic high-throughput density functional theory calculations to identify magnetic topological materials from the ≈40 000 three-dimensional materials in the JARVIS-DFT database. First, we screen materials with net magnetic moment $>0.5\mu \mathrm{B}$ and spin-orbit spillage (SOS) $>0.25$, resulting in 25 insulating and 564 metallic candidates. The SOS acts as a signature of spin-orbit-induced band-inversion. Then we carry out calculations of Wannier charge centers, Chern numbers, anomalous Hall conductivities, surface band structures, and Fermi surfaces to determine interesting topological characteristics of the screened compounds. We also train machine learning models for predicting the spillages, band gaps, and magnetic moments of new compounds, to further accelerate the screening process. We experimentally synthesize and characterize a few candidate materials to support our theoretical predictions.