Rubidium, like the other alkali metals, exhibits multiple structural transformations in its solid phase upon application of pressure. Recent experiments and calculations suggest it also goes through a phase transformation in its liquid state, along the 573 K isotherm, slightly above its melting curve. We employ ab initio molecular dynamic simulations to study the structural and electronic characteristics of this liquid-liquid transition. Applying the quasicrystalline model, we find that the short-range order of liquid Rb changes abruptly from bcc-like to $\beta$-Sn-like at a pressure significantly lower than the bcc to $\beta$-Sn transition in the solid phase at lower temperatures. In contrast to previous claims, the transition in the liquid cannot be related to the Peierls transition, as the liquid does not exhibit formation of a pseudogap at the Fermi level, nor to the transfer of charge from the $s$ state to the $d$ state, which is found to progress monotonically with pressure. Instead, a qualitative change in the shape of the electronic $d$ band is shown to occur at the same pressure range as the structural phase transition, which leads to the conclusion that the $\mathit{sd}$ hybridization must be accompanied by a significant change in the electronic band structure, observed in the solid phase, in order to produce a phase transition.

The present study utilizes synchrotron-based x-ray diffraction (XRD), photoluminescence (PL), and x-ray absorption near edge structure (XANES) spectroscopic techniques to comprehend the evolution of optical intense blue-green emission in 100 keV nitrogen (N) ion implanted $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ (STO) thin films deposited by RF magnetron sputtering technique. The XRD pattern shows a shift in reflections at lower N ion fluences and the amorphization of the films at higher fluences. A disordered phase induced by implantation in the STO films leads to an intense blue-green emission due to oxygen (O) vacancies and N ($2p$) bound states. A schematic diagram of energy levels has been proposed to explain the origin of PL emission. The XANES spectra at Ti $K$ edge reflect a change in the valency of Ti ions and the local atomic structure of ordered and disordered phases of STO with an increase in N ion fluence. The splitting of peak assigned to $e_g$ orbitals, and discrepancy in ratio $d_{z^2}/d_{x^2-y^2}$ observed in the Ti $L$- and O $K$-edge spectra, confirm a distortion in $\mathrm{Ti}{\mathrm{O}}_6$ octahedral structure and modifications in O $2p$-Ti $3d$ hybridization states. The synchrotron-based techniques reveal that N ion implanted STO can be a good photoluminescent material exhibiting a variety of emissions through bound states of O vacancies and implanted N ions.

Crystal and electronic structure of ${\mathrm{Co}}_3{\mathrm{O}}_4$ spinel have been investigated by x-ray absorption near edge structure (XANES) at the Co $K$ edge up to 58.5 GPa and Raman scattering up to 65 GPa. Several transitions have been observed upon pressurization, and the original structure was recovered on decompression. Experimental and theoretical XANES and Raman data are compatible with the occurrence of a monoclinic $P{2}_1/c$ phase above $\sim 52.7$ GPa. Vibrational modes analyzed in details by Raman scattering indicate that two other subtle transitions take place above $\sim 21.9$ GPa (orthorhombic $Fddd$) and $\sim 43.0$ GPa (monoclinic $C2/m$), in agreement with the previous x-ray diffraction experiments. Our combined experimental XANES and multiple scattering calculations indicate clear evidence of a tetrahedral to octahedral coordination crossover at the ${\mathrm{Co}}^{2+}$ sites, being completed upon transition to the monoclinic $P{2}_1/c$ phase. The valence and spin states at two different $\mathrm{Co}$ sites remained unchanged at least up to the transition onset to the monoclinic $P{2}_1/c$ phase ruling out the possibility of charge transfer and spin crossover in the intermediate phases as proposed previously.

When a group of compactly packed free fermions is allowed to spread over an empty one-dimensional lattice, the spreading particles can create entanglement between different parts of the lattice. We show, although breaking of translational invariance (TI) of the lattice by disorder slows down the spreading of local observables, the entanglement entropy of a subsystem can nonetheless receive a remarkable enhancement as long as the subsystem lies within the single-particle localization length. We show the main mechanism behind this enhancement is the reentrant exchange of particles between the subparts due to transport of mutual information due to backscattering. We discuss the length and timescales relevant to the phenomenon. We study the phenomenon for breaking of TI by both quasiperiodic and random potentials. We further explore the effect of randomness only in the initial state. This also exhibits a similar enhancement effect even in a TI lattice. We also touch upon the special case of periodic potential where qualitatively similar phenomenology emerges, although the coherence in the backscattering in this case leads to effects not captured by our simple yet generic picture.

We report a study based on first-principles calculations complemented by a tight-binding approach and a $\mathit{k}·\mathit{p}$ model of the spin-orbit effects in epitaxially strained $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$. The imposed biaxial tensile strain allows us to rotate the electrical polarization of $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$ from out-of-plane to in-plane directions through structural phase transitions between a tetragonal, monoclinic, and orthorhombic phase. It is found that Ti $d$-state conduction bands exhibit two strong Rashba-like spin-splitting parameters of (1) 0.39 eV Å at a tensile strain $\eta =1.55$% due to a polarization rotation in the monoclinic phase and (2) 1.08 eV Å under large tensile strain of 3.8% due to band anticrossing in the orthorhombic phase. Remarkably, we also identified the presence of a quasipersistent spin texture for the band with a $d_{yz-zx}$ character in the orthorhombic phase. We then conclude that using strain could be an interesting way to tune the spin-orbit effects in ferroelectric materials with technological interests.

We perform a theoretical study of the effects of electronic correlations on the superconducting gap structure of multiband superconductors. In particular, by comparing standard RPA-based spin-fluctuation mediated gap structures to those obtained within the FLEX formalism for an iron-based superconductor, we obtain directly the feedback effects from electron-electron interactions on the momentum-space gap structure. We show how self-energy effects can lead to an orbital inversion of the orbital-resolved spin susceptibility, and thereby invert the hierarchy of the most important orbitals channels for superconducting pairing. This effect has important consequences for the detailed gap variations on the Fermi surface. We expect such self-energy feedback on the pairing gap to be generally relevant for superconductivity in strongly correlated multiorbital systems.

The dynamics of an active, finite-size, and immiscible impurity in a dilute quantum fluid at finite temperature is characterized by means of numerical simulations of the Fourier truncated Gross-Pitaevskii equation. The impurity is modeled as a localized repulsive potential and described with classical degrees of freedom. It is shown that impurities of different sizes thermalize with the fluid and undergo a stochastic dynamics compatible with an Ornstein-Uhlenbeck process at sufficiently large time lags. The velocity correlation function and the displacement of the impurity are measured and an increment of the friction with temperature is observed. Such behavior is phenomenologically explained in a scenario where the impurity exchanges momentum with a dilute gas of thermal excitations, experiencing an Epstein drag.

We study the subgap spectrum of a 2D Abrikosov vortex in an $s$-wave superconductor in the absence and presence of a point impurity. By solving the Eilenberger equations without impurity for two models of the vortex (including a self-consistent one), we find multiple subgap spectral branches. The number of these branches may be arbitrarily large provided that the magnetic field screening length is large enough. The quasiclassical spectrum of the vortex has a local gap with a width of the order of the bulk gap and a spatial extent of several coherence lengths. The existence of such a gap is the prerequisite for the appearance of discrete impurity-induced states. Within the Gor'kov equations formalism, we find that a single impurity induces up to four discrete quasiparticle states in the vortex. The energies and wave functions of the impurity states are calculated for different parameters. We claim that most of the predicted spectral features can be observed in scanning tunnel spectroscopy experiments.

We develop a “self-avoiding walks” approximation and use it to calculate the spectral function of a single hole introduced into a two-dimensional square lattice Ising antiferromagnet. The local spectral function obtained qualitatively agrees with the exact diagonalization result and is largely incoherent. Such a result stays in contrast with the spectrum obtained on a Bethe lattice, which consists of the well-separated quasiparticle-like peaks and stems from the motion of a hole in an effective linear potential. We determine that this onset of the incoherent spectrum on a square lattice (i) is not triggered by the so-called Trugman loops but (ii) originates in the warping of the linear potential by the interactions between magnons created along the tangential paths of the moving hole.

We develop a stochastic description of the topological properties in an interacting Chern insulator. We confirm the Mott transition's first-order nature in the interacting Haldane model on the honeycomb geometry from a mean-field variational approach supported by density matrix renormalization group results and Ginzburg-Landau arguments. From the Bloch sphere, we make predictions for circular dichroism of light related to the quantum Hall conductivity on the lattice and in the presence of interactions. This analysis shows that the topological number can be measured from the light response at the Dirac points. Electron-electron interactions can also produce a substantial number of particle-hole pairs above the band gap, which leads us to propose a stochastic Chern number as an interacting measure of the topology. The stochastic Chern number can describe disordered situations with a fluctuating staggered potential, and we build an analogy between interaction-induced particle-hole pairs and temperature effects. Our stochastic approach is physically intuitive, easy to implement, and leads the way to further studies of interaction effects.

A recent study has revealed that the low carrier density electron gas (2DEG) induced at the interface of EuO and (111) ${\mathrm{KTaO}}_3$ exhibits a broken symmetry phase with a strong in-plane anisotropy of the resistivity. We present a minimal tight-binding model of this (111) 2DEG, including the large spin-orbit coupling from the Ta ions, which reveals a hexagonal Fermi surface with a highly enhanced $2k_F$ electronic susceptibility. We argue that repulsive electronic interactions, together with a ferromagnetic EuO substrate, favor a magnetic stripe instability leading to a partially gapped Fermi surface. Such a stripe state, or its vestigial nematicity, could explain the observed transport anisotropy. We propose a $k·p$ theory for the low energy $j=3/2$ states, which captures the key results from our tight-binding study, and further reveals the intertwined dipolar and octupolar modulations underlying this magnetic stripe order. We conclude by speculating on the relation of this stripe order to the superconductivity seen in this material.

We propose theoretically an effect, i.e., the nonlinear planar Nernst effect (NPNE), in nonmagnetic topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ in the presence of an in-plane magnetic field. We find that the Nernst current scales quadratically with temperature gradient but linearly with magnetic field and exhibits a cosine dependence on the orientation of the magnetic field with respect to the direction of the temperature gradient. The NPNE has a quantum origin arising from the conversion of a nonlinear transverse spin current to a charge current due to a joint result of hexagonal warping effect, spin-momentum locking, and the time-reversal symmetry breaking induced by the magnetic field.

Layered materials with robust magnetic ordering have been attracting significant research interest. In recent experiments, a new layered material $1T-{\mathrm{CrTe}}_2$ has been synthesized and exhibits ferromagnetism above the room temperature. Here, based on first-principles calculations, we investigate the electronic, magnetic, and transport properties of $1T-{\mathrm{CrTe}}_2$, both in the bulk and in the two-dimensional limit. We show that $1T-{\mathrm{CrTe}}_2$ can be stable in the monolayer form, and has a low exfoliation energy. The monolayer structure is an intrinsic ferromagnetic metal, which maintains a relatively high Curie temperature above 200 K. Particularly, we reveal interesting features in the anomalous Hall transport. We show that in the ground state, both bulk and monolayer $1T-{\mathrm{CrTe}}_2$ possess vanishing anomalous Hall effect, because the magnetization preserves one vertical mirror symmetry. The anomalous Hall conductivity can be made sizable by tuning the magnetization direction or by uniaxial strains that break the mirror symmetry.

A change in the composition of the ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ IV–VI semiconductor or in its lattice parameter can drive a transition from a topologically trivial to a topological crystalline insulator (TCI), crossing a region where the alloy is in the Weyl semimetal phase. Incorporation of the magnetic Mn ions induces strong perturbations of the electronic structure, which act on both orbital and spin variables. Our first principles calculations show that the presence of Mn shifts the TCI and the Weyl region towards higher Sn contents in ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$. When the Mn spin polarization is finite, the spin perturbation, like the orbital part, induces changes in band energies comparable to the bandgap, which widens the Weyl region. The effect opens the possibility of driving transitions between various topological phases of the system by a magnetic field or by spontaneous Mn magnetization. We also propose a new method to calculate topological indices for systems with a finite spin polarization defined based on the concept of the Chern number. These valid topological characteristics enable the identification of the three distinct topological phases of the ${\mathrm{Pb}}_{1-x-y}{\mathrm{Sn}}_x{\mathrm{Mn}}_y\mathrm{Te}$ alloy.

We study the delta-$T$ noise in the Kondo regime, which implies the charge current noise under the temperature bias for the SU(2) Kondo quantum dot. We propose an experimentally measurable quantity to quantify the low-temperature properties in the delta-$T$ noise, $S_{\ell }=S(T_{\mathrm{L}},T_{\mathrm{R}})-(1/2)[S(T_{\mathrm{L}},T_{\mathrm{L}})+S(T_{\mathrm{R}},T_{\mathrm{R}})]$, which yields the shot noise expression in the noninteracting limit. We calculate this quantity for the SU(2) Kondo quantum dot in the particle-hole symmetric case. We found that the $S_{\ell }$ exhibits qualitatively the same behavior in both the electrochemical potential-biased case and the temperature-biased case. The quantitative difference appears as a difference of the coefficients of the noises, which reflects the difference of the Fermi distribution function: electrochemical potential biased or temperature biased.

In this work, ab initio calculations based on density functional theory and the Landauer formalism are carried out to investigate ballistic thermoelectric properties of $T\text{−}\mathrm{Hf}{\mathrm{Se}}_2$ nanoribbons (NRs). The zigzag-edged NRs are metallic, and they are not included in this study. The armchair NRs possess two types of edge symmetries depending on the number of atoms present in a row; odd-numbered NRs have mirror symmetry, whereas the even-numbered NRs have glide reflection symmetry. The armchair-edged NRs are dynamically stable and show semiconducting properties with varying band gap values in the infrared and visible regions. Detailed transport analyses show that the $n$-type Seebeck coefficient and the power factor differ because of the structural symmetry, whereas the $p$-type thermoelectric coefficients are not significantly influenced. It is shown that the phonon thermal conductance is reduced to a third of its two-dimensional value via nanostructuring. The $p$-type Seebeck coefficient and the power factor for $T$-phase $\mathrm{Hf}{\mathrm{Se}}_2$ are enhanced in NRs. We report that the $p$-type $ZT$ value of $\mathrm{Hf}{\mathrm{Se}}_2$ NRs at 300 and 800 K are enhanced by factors of 4 and 3, respectively.

We study a non-Hermitian Aubry-André-Harper model with both nonreciprocal hoppings and complex quasiperiodical potentials, which is a typical non-Hermitian disordered system. We introduce boundary-dependent self-dualities in this model and obtain analytical results to describe its Asymmetrical Anderson localization and topological phase transitions. We find that the Anderson localization is not necessarily in accordance with the topological phase transitions, which are characteristics of localization of states and topology of energy spectrum, respectively. Furthermore, in the localized phase, single-particle states are asymmetrically localized due to non-Hermitian skin effect and have energy-independent localization lengths. We also discuss possible experimental detections of our results in electric circuits.

We report a first principles analysis of the linear electro-optic (EO) or Pockels effect in rhombohedral $\mathrm{BaTi}{\mathrm{O}}_3$ (BTO), the lowest temperature phase of this ferroelectric perovskite. Si-integrated BTO is a promising material for EO modulators in Si photonics, with applications in optical quantum computing. This requires a fundamental understanding of the Pockels effect in BTO at low temperature. Through density functional perturbation theory, we explore the origins of a strong Pockels response by examination of the components that make up the ionic contribution to the EO tensor. We identify the phonon modes dominating the EO response and establish their relation to the electronic structure and optical properties of the material.

The structural stability of monolayer SnSe at finite temperature is investigated by the phonon quasiparticle approach that combines first-principles molecular dynamics and lattice dynamics. At 300 K, we witness the dynamic stability of monolayer Pmmn SnSe that is originated from the Cmcm bulk phase. The pair correlation and atomic displacement distribution functions of Sn and Se atoms confirm the lattice stability at high temperature. The hardening of Raman activated ${\mathrm{A}}_g^2$ and ${\mathrm{B}}_{2g}$ modes at the $\mathrm{\Gamma }(0,0,0)$ point and normal modes at the boundary $M(0,\frac{1}{2},\frac{1}{2})$ point of the Brillouin zone is observed with giant frequency shifts, revealing the heavy anharmonicity in monolayer SnSe. The calculated anharmonic phonon dispersions and density of states show significant temperature dependence and some medium-frequency phonon modes adopt giant frequency shifts. Due to the lattice anharmonicity, the predicted lattice thermal conductivity within the framework of a phonon gas model is very low for monolayer SnSe, close to the value of the bulk phase along the out-of-plane direction. As temperature increases, the characteristic of covalent bonds between Sn and Se atoms in plane and the normal direction of monolayer SnSe becomes isotropic. This results in a high degeneracy of the electronic band structure, beneficial to the electronic transport properties. A thermoelectric $ZT$ of $\sim 0.52$ for $n$ doping is predicted at 300 K, hinting that the monolayer SnSe is a fairly good thermoelectric material suitable for application at room temperature.