Silica phases, $\mathrm{Si}{\mathrm{O}}_2$, have attracted significant attention as important phases in the fields of condensed-matter physics, materials science, and (in view of their abundance in the Earth's crust) geoscience. Here, we experimentally and theoretically demonstrate that coesite undergoes structural modulations under high pressure. Coesite transforms to a distorted modulated structure, coesite-II, at 22–25 GPa with modulation wave vector $q=0.5b^{*}$. Coesite-II displays further commensurate modulation along the $y$ axis at 36–40 GPa and the long-range ordered crystalline structure collapses beyond $\sim 40\mathrm{GPa}$ and starts amorphizing. First-principles calculations illuminate the nature of the modulated phase transitions of coesite and elucidate the modulated structures of coesite caused by modulations along the $y$-axis direction. The structural modulations are demonstrated to result from phonon instability, preceding pressured-induced amorphization. The recovered sample after decompression develops a rim of crystalline coesite structure, but its interior remains low crystalline or partially amorphous. Our results not only clarify that the pressure-induced reversible phase transitions and amorphization in coesite originate from structural modulations along the $y$-axis direction, but also shed light on the densification mechanism of silica under high pressure.

Here, we investigate the Cherenkov emission by an array of moving electric charges in the vicinity of a topologically nontrivial gyrotropic material. It is shown that the nonreciprocal material response may result in a robustly asymmetric Cherenkov emission, such that the spectrum of the emitted radiation and the stopping power depend strongly on the sign of the particle velocity. It is demonstrated that the main emission channels are determined by the unidirectional edge states supported by the topological material. We consider as examples both magnetized plasmas and Weyl semimetals. The latter may exhibit a spontaneous nonreciprocal response without a biasing magnetic field.

We show that the spin-orbit interaction (SOI) arising due to the in-plane electric field of the Coulomb repulsion between electrons in a two-dimensional quantum well produces an attractive component in the pair interaction Hamiltonian that depends on the spins and momenta of electrons. If the Rashba SOI constant of the material is high enough, the attractive component overcomes the Coulomb repulsion and the centrifugal barrier, which leads to the formation of the two-electron bound states. There are two distinct types of two-electron bound states. The relative bound states are formed by the electrons orbiting around their common barycenter. They have a triplet spin structure and are independent of the center-of-mass momentum. In contrast, the convective bound states are formed because of the center-of-mass motion, which couples the electrons with opposite spins. The binding energy in the meV range is attainable for realistic conditions.

We report on the experimental observation of a cyclotron antiresonance in a canonical three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Magnetoreflectance response of single-crystal ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ was studied in 18-T magnetic field, and compared to other topological insulators studied before, the main spectral feature is inverted. We refer to it as an antiresonance. In order to describe this unconventional behavior we propose the idea of an imaginary cyclotron resonance frequency, which on the other hand indicates that the form of the Lorentz force that magnetic field exerts on charge carriers takes an unconventional form.

A range of honeycomb-lattice compounds has been proposed and investigated in the search for a topological Kitaev spin liquid. However, sizable Heisenberg interactions and additional symmetry-allowed exchange anisotropies in the magnetic Hamiltonian of these potential Kitaev materials push them away from the pure Kitaev spin-liquid state. Particularly the Kitaev-to-Heisenberg coupling ratio is essential in this respect. With the help of advanced quantum-chemistry methods, we explore how the magnetic coupling ratios depend on strain and pressure in several honeycomb compounds (${\mathrm{Na}}_2{\mathrm{IrO}}_3$, $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$, and $\alpha -{\mathrm{RuCl}}_3$). We find that the Heisenberg and Kitaev terms are affected differently: For strain, in particular, the Heisenberg component decreases more rapidly than the Kitaev counterpart. This provides a scenario where strain can stabilize a spin liquid in such materials.

We calculate the topological entanglement entropy (TEE) for a three-dimensional hyperhoneycomb lattice generalization of Kitaev's honeycomb lattice spin model. We find that for this model TEE is not directly determined by the total quantum dimension of the system. This is in contrast to general two-dimensional systems and many three-dimensional models, where TEE is related to the total quantum dimension. Our calculation also provides TEE for a three-dimensional toric-code-type Hamiltonian that emerges as the effective low-energy theory for the Kitaev model in a particular limit.

The physical properties of the semiconductor FeSi with very narrow band gap, anomalous behavior of the magnetic susceptibility and metal-insulator transition at elevated temperatures attract great interest due to the still controversial theoretical understanding of their origin. On one side the purely bandlike mechanism of the gap formation in FeSi at low temperature is well established; on the other side a number of experiments and their theoretical interpretation suggest a rich physics of strong correlations at finite temperature. In this work we use an ab initio scheme based on the random-phase approximation and local spin-density approximation (RPA@LSDA) to reveal the role of the electron correlation effects in FeSi extending it by applying a fixed spin moment constraint. In the parameter-free framework we show that correlation effects essentially alter the one-electron LSDA results leading to the formation of an additional state with finite magnetic moment on Fe, whose energy is almost degenerate with the nonmagnetic ground state. This explains the results of high-field experiments, which found a first-order metamagnetic phase transition into a metallic ferromagnetic state. Our results suggest a strongly correlated nature of the low-energy excitations in FeSi. From our supercells calculations we reveal that these excitations are local and exhibit a Kondo-like behavior since a strong antiferromagnetic screening is present.

Formation of a depletion region at the vertically stacked topological insulator $p\text{−}n$ heterostructures is one of the effective approaches to minimize residual carrier density in the bulk regions. Here, we report on characterization of field-effect transistor (FET) based on ${\left(\mathrm{B}{\mathrm{i}}_{0.26}\mathrm{S}{\mathrm{b}}_{0.74}\right)}_2\mathrm{S}{\mathrm{e}}_3/\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ topological insulator heterostructures with a vertical $p\text{−}n$ junction configuration. The thicknesses of the top $p\text{−}{\left(\mathrm{B}{\mathrm{i}}_{0.26}\mathrm{S}{\mathrm{b}}_{0.74}\right)}_2\mathrm{S}{\mathrm{e}}_3$ layer vary from $d=8$ to 25 nm with 3-nm constant thickness of the bottom $n\text{−}\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$. Even at zero gate voltage, increasing thickness of the top layer inverts the dominant conducting carrier from $n$-type to $p$-type. With applying gate voltage, the effect of depletion region at the interface is clearly revealed as a voltage shift of charge neutral point in ambipolar FET operation. The systematic shift of charge neutral point in FET operation clearly indicates that the formation of depletion region can be explained with conventional semiconductor $p\text{−}n$ junction model. In the topological insulator $p\text{−}n$ heterostructures, the thickness tuning of both $p$-type and $n$-type layers is quite important to minimize the residual charge density in the whole device region. The band engineering with $p\text{−}n$ junction will be further applicable to extract the electrical properties of surface state.

Considering the importance of flexoelectric coupling for the physical understanding of the gradient-driven couplings in mesoscale and nanoscale solids, one has to determine its full symmetry and numerical values. The totality of available experimental and theoretical information about the flexocoupling tensor symmetry (specifically the amount of measurable independent components) and numerical values is contradictory. However, the discrepancy between the theory and experiment can be eliminated by consideration of all possible inner symmetries of the flexocoupling tensor and physical limits on its component values. Specifically, this paper reveals the inner “hidden” symmetry of the static flexoelectric tensor that allows minimizing the number of its independent components. Revealed hidden symmetry leads to nontrivial physical sequences, namely, it affects the upper limits of the static flexocoupling constants. Also we analyze the dynamic flexoelectric coupling symmetry and establish the upper limits for the numerical values of its components. These results can help us to understand and quantify the fundamentals of the gradient-type couplings in different solids.

We report a neutron powder diffraction study of $R{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ quadruple perovskite manganites with $R=\mathrm{La}$, Ce, Nd, Sm, and Eu. We show that in all measured compounds concomitant magnetic ordering of the $A$ and $B$ manganese sublattices occurs on cooling below the Néel temperature. The respective magnetic structures are collinear, with one uncompensated ${\mathrm{Mn}}^{3+}$ moment per formula unit as observed in bulk magnetization measurements. We show that both ${\mathrm{LaMn}}_7{\mathrm{O}}_{12}$ and ${\mathrm{NdMn}}_7{\mathrm{O}}_{12}$ undergo a second magnetic phase transition at low temperature, which introduces a canting of the $B$-site sublattice moments that is commensurate in ${\mathrm{LaMn}}_7{\mathrm{O}}_{12}$ and incommensurate in ${\mathrm{NdMn}}_7{\mathrm{O}}_{12}$. This spin canting is consistent with a magnetic instability originating in the $B$-site orbital order. Furthermore, ${\mathrm{NdMn}}_7{\mathrm{O}}_{12}$ displays a third magnetic phase transition at which long-range ordering of the Nd sublattice modifies the periodicity of the incommensurate spin canting. Our results demonstrate a rich interplay between transition-metal magnetism, orbital order, and the crystal lattice, which may be fine-tuned by cation substitution and rare-earth magnetism.

A microscopic understanding of hydrodynamic transport in Dirac and Weyl semimetals has remained elusive in theoretical descriptions and experimental measurements. We investigate the structure and microscopic properties of transport in ${\mathrm{WP}}_2$, a type-II Weyl semimetal exhibiting hydrodynamic transport. Here, we characterize the nature of the microscopic properties of ${\mathrm{WP}}_2$ as a function of temperature through ab initio calculations of the relevant scattering processes, including electron-phonon and electron-electron lifetimes, and provide equivalent calculations for copper as a point of reference. Additionally, we present an approach to calculate phonon drag, a mechanism invoked in recent experiments, through predictions of a phonon-mediated electron-electron lifetime. We show that the resistivity is very well described by the electron-phonon interaction alone, indicating ${\mathrm{WP}}_2$ exhibits conventional metallic electrical behavior in which strong electron-electron correlations do not play a significant role. After establishing the zone-averaged behavior of the calculated lifetimes, we further investigate specific features of the spatial distribution of the electron-phonon lifetime across the Fermi surfaces of ${\mathrm{WP}}_2$ to study possible scattering channels involved in hydrodynamic transport and quantify the degree of lifetime anisotropy in electron and hole pockets. This description of microscopic dynamics in ${\mathrm{WP}}_2$ prompts additional investigation of specific scattering channels and indicates the importance of phonon interactions in understanding connections between transport in hydrodynamic materials and strongly correlated systems including unconventional metals.

We study the zero temperature properties of the sub-Ohmic spin-boson model with quadratic spin-boson coupling. This model describes experimental setups at the optimal working point where the linear coupling between the qubit (spin) and the environmental noise (bosons) is zero and the leading coupling is quadratic. In the strong coupling regime, we find that the existence of spin induces quantum phase transitions (QPTs) between two states of environment: the normal state and a state with local distortions. The phase diagram contains both continuous and the first-order QPTs, with nontrivial critical properties obtained exactly. At the QPTs, the equilibrium state spin dynamics bears power-law $\omega$ dependence in the small frequency limit and a robust coherent Rabi oscillation at high frequency. We discuss the feasibility of observing such environmental QPTs in the qubit-related experiments.

Ultrafast dynamics in three-dimensional topological insulators (TIs) opens new routes for increasing the speed of information transport up to frequencies a thousand times faster than in modern electronics. However, to date, disentangling the exact contributions from bulk and surface transport to the subpicosecond dynamics of these materials remains a difficult challenge. Here, using time- and angle-resolved photoemission, we demonstrate that driving a TI from the bulk-conducting into the bulk-insulating transport regime allows us to selectively switch on and off the emergent channels of ultrafast transport between the surface and the bulk. We thus establish that ultrafast transport is one of the main driving mechanisms responsible for the decay of excited electrons in prototypical TIs following laser excitation. We further show how ultrafast transport strongly affects the thermalization and scattering dynamics of the excited states up to high energies above the Fermi level. In particular, we observe how inhibiting the transport channels leads to a thermalization bottleneck that substantially slows down electron-hole recombination via electron-electron scatterings. Our results pave the way for exploiting ultrafast transport to control thermalization timescales in TI-based optoelectronic applications, and expand the capabilities of TIs as intrinsic solar cells.

In this paper, we report our recent work on the interdot tunneling-dependent Kondo effect for two serially coupled quantum dots (QDs) by adopting the hierarchical equations of motion approach. As is known, for such a system, when interdot tunneling $t$ increases, a continuous crossover from a Kondo singlet to a spin singlet is observed rather than a quantum phase transition, even though an induced antiferromagnetic interaction exists due to the interdot tunneling $t$. Our study focuses on the influence of the tunneling $t$ between two QDs at a finite temperature and especially on the smearing of the quantum phase transition. In this study, we find that, at small $t$, there is an enhanced Kondo effect, which results in the most noticeable Kondo effect appearing at finite $t$ rather than at $t=0$. When $t$ is larger, the Kondo peak splits into two subpeaks. Because of the weak coupling to the leads, the splitting space exactly corresponds to the energy difference between the original degenerated local spin singlet and the local spin triplet of the two isolated QDs. The Kondo peak can be preserved when the energy difference between the local spin singlet and the local spin triplet is smaller than the enhanced Kondo temperature. Finally, interdot tunneling also introduces a doubly occupied charge singlet, which causes charge fluctuations in the ground state at finite temperature and thus, therefore, smears the phase transition. This can also explain why no phase transition is observed in experiments in other QDs systems, even though the phase transition is always predicted in theory.

A powerful mechanism for constructing gauge theories is to start from a theory with a global symmetry, then apply the “gauge principle,” which demands that this symmetry hold locally. For example, the global phase rotation of a system of conserved charges can be promoted to a local phase rotation by coupling to an ordinary $U\left(1\right)$ vector gauge field. More recently, a class of particles has been studied featuring not only charge conservation, but also conservation of higher moments, such as dipole moment, which leads to severe restrictions on the mobility of charges. These particles, called fractons, are known to be intimately connected to symmetric tensor gauge fields. In this work, we show how to derive such tensor gauge theories by applying the gauge principle to a theory of ungauged fractons. We begin by formulating a field theory for ungauged fractons exhibiting global conservation of charge and dipole moment. We show that such fracton field theories have a characteristic non-Gaussian form, reflecting the fact that fractons intrinsically interact with each other even in the absence of a mediating gauge field. We then promote the global higher moment conservation laws to local ones, which requires the introduction of a symmetric tensor gauge field. Finally, we extend these arguments to other types of subdimensional particles besides fractons. This work offers a possible route to the formulation of non-Abelian fracton theories.

The Bernard-LeClair (BL) symmetry classes generalize the Altland-Zirnbauer classes in the absence of Hermiticity. Within the BL scheme, time-reversal and particle-hole symmetry come in two flavors, and “pseudo-Hermiticity” generalizes Hermiticity. We propose that these symmetries are relevant for the topological classification of non-Hermitian single-particle Hamiltonians and Hermitian bosonic Bogoliubov–de Gennes (BdG) models. We show that the spectrum of any Hermitian bosonic BdG Hamiltonian is found by solving for the eigenvalues of a non-Hermitian matrix which belongs to one of the BL classes. We therefore suggest that bosonic BdG Hamiltonians inherit the topological properties of a non-Hermitian symmetry class and explore the consequences by studying symmetry-protected edge instabilities in a one-dimensional system.

Building upon techniques employed in the construction of the Sachdev-Ye-Kitaev model, which is a solvable (0 + 1)-dimensional model of a non-Fermi liquid, we develop a solvable infinite-ranged random-hopping model of fermions coupled to fluctuating $U\left(1\right)$ gauge fields. In a specific large-$N$ limit, our model realizes a gapless non-Fermi-liquid phase, which combines the effects of hopping and interaction terms. We derive the thermodynamic properties of the non-Fermi-liquid phase realized by this model and the charge transport properties of an infinite-dimensional version with spatial structure.

Motivated by recent terahertz (THz) spectroscopy measurements in $\alpha \text{−}{\mathrm{RuCl}}_3$, we develop a theory for magnetoelectric (ME) effects in Mott insulators of $d^5$ transition metal ions in an octahedral crystal field. For $4d$ and $5d$ compounds, the relatively wide-spread orbitals favor charge fluctuations of localized electrons to neighboring ions and a significant ME effect from electronic mechanisms is expected. From a three-orbital Hubbard model with strong spin-orbit coupling, we derive the mechanisms for the electric polarization originating from virtual hopping of the localized holes carrying the spins. We consider the electric polarization generated by pairs of spin operators on nearest-neighbor bonds with either an edge-sharing geometry (i.e., two ligands are shared) or a corner-sharing geometry (i.e., one ligand is shared). The allowed couplings are first derived using a symmetry approach. Then, we explicitly calculate the coupling constants and evaluate the effective polarization operator in the ground state manifold using perturbation theory and exact diagonalization. The results are relevant when considering the THz optical conductivity of magnetic systems such as some perovskite iridates or Kitaev materials. In particular, they help explain the recent THz optical measurements of $\alpha \text{−}{\mathrm{RuCl}}_3$ for which the electric-dipole-induced contribution has been shown to be strong.

Multiband superconductivity is realized in a plethora of systems, from high-temperature superconductors to very diluted superconductors. While several properties of multiband superconductors can be understood as straightforward generalizations of their single-band counterparts, recent works have unveiled rather unusual behaviors unique to the former case. In this regard, a regime that has received significant attention is that near a Lifshitz transition, in which one of the bands crosses the Fermi level. In this paper, we investigate how impurity scattering ${\tau }^{-1}$ affects the superconducting transition temperature $T_c$ across a Lifshitz transition, in the regime where intraband pairing is dominant and interband pairing is subleading. This is accomplished by deriving analytic asymptotic expressions for $T_c$ and $\partial T_c/\partial {\tau }^{-1}$ in a two-dimensional two-band system. When the interband pairing interaction is repulsive, we find that, despite the incipient nature of the band crossing the Fermi level, interband impurity scattering is extremely effective in breaking Cooper pairs, making $\partial T_c/\partial {\tau }^{-1}$ quickly approach the limiting Abrikosov-Gor'kov value of the high-density regime. In contrast, when the interband pairing interaction is attractive, pair-breaking is much less efficient, affecting $T_c$ only mildly at the vicinity of the Lifshitz transition. The consequence of this general result is that the behavior of $T_c$ across a Lifshitz transition can be qualitatively changed in the presence of strong enough disorder: Instead of displaying a sharp increase across the Lifshitz transition, as in the clean case, $T_c$ can actually display a maximum and be suppressed at the Lifshitz transition. These results shed light on the nontrivial role of impurity scattering in multiband superconductors.