Quantum spin Hall (QSH) insulator materials feature topologically protected edge states that can drastically reduce dissipation and are useful for the next-generation electronics. However, the nonvolatile control of topological edge state is still a challenge. In this paper, based on first-principles calculations, the switchable topological states are found in the van der Waals (vdW) heterostructures consisting of two-dimensional (2D) Bi(111) bilayer (BL) and $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ by reversing the electric polarization of the ferroelectric $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$. The topological switching results from the different charge transfer associated with the two opposite polarization states of $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$. This new topological switching mechanism has the unique advantages of being fully electrical as well as nonvolatile. Our finding provides an unprecedented approach to realize ferroelectric control of topological states in 2D materials, which will have great potential for applications in topological nanoscale electronics.

This paper presents a method of calculating the valence electron contribution to the stopping and interaction cross sections of swift ions in matter using linear response calculations of the dielectric function in crystals. The ab initio response function is used to calculate excitations at low energy and momentum transfers to account for material-specific effects, while higher energy and momentum transfers use a free electron gas response for increased computational efficiency. The ECPSSR method for computing core cross sections is modified to allow predictions of the core contribution to electronic stopping. The charge distribution by entrained electrons is explicitly modeled to account for the additional screening beyond linear response. We use the methods developed to predict the electronic stopping of protons and $\alpha$ particles in silicon and compare to measured values.

Inspired by two recent experiments in Nature [Fornieri et al., Nature (London) 569, 89 (2019); Ren et al., Nature (London) 569, 93 (2019)], we numerically studied the transport properties of topological superconductivity in a planar Josephson junction. Our numerical calculations suggest that the origin of a zero-bias conductance peak is not unique. In addition to the single pair of Majorana zero modes, multiple pairs of Majorana zero modes can also exist at the ends of the junction. Although the bulk energy spectra in these two regions are distinguished, the signal measured through conductance cannot reveal the true information of the bulk energy spectrum. Thus the zero-bias conductance peaks measured in both two regions are quite similar and are hard to distinguish. In order to reveal the right information regarding the topological region, more methods in addition to the local conductance measurements are needed. Moreover, these multiple Majorana zero modes are not stable and fragile for topological quantum computation. To realize a topological Josephson junction with stable Majorana zero modes, the junction width should be smaller than half of the superconducting coherence length, and the superconducting bulk width should be larger than the coherence length.

We report on the measurements of 9.6 MHz ultrasound propagation down to 15 mK in polycrystalline quantum solid ${}^4\mathrm{He}$ containing 0.3 and 20 ppm ${}^3\mathrm{He}$ impurities. The attenuation and speed of ultrasound are strongly affected by the dislocation vibration. The observed increase in attenuation from 1.2 K to a peak near 0.3 K is independent of drive amplitude and reflects crossover from overdamped to underdamped oscillation of dislocations pinned at network nodes. Below 0.3 K, amplitude-dependent and hysteretic variations are observed in both attenuation and speed. The attenuation decreases from the peak at 0.3 K to a very small constant value below 70 mK at sufficiently low drive amplitudes of ultrasound, while it remains a high value down to 15 mK at the highest drive amplitude. The behaviors at low drive amplitudes can be well described by the effects of the thermal pinning and unpinning of dislocations by the impurities. The binding energy between a dislocation line and a ${}^3\mathrm{He}$ atom is estimated to be 0.35 K. The nonlinear and hysteretic behaviors at intermediate drive amplitudes are analyzed in terms of stress-induced unpinning which may occur catastrophically within a network dislocation segment. The relaxation time for pinning at 15 mK is very short ($<4$ s), while more than 1000 s is required for unpinning.

Entanglement measures are useful tools in characterizing otherwise unknown quantum phases and indicating transitions between them. Here we examine the concurrence and entanglement entropy in quantum spin chains with random long-range couplings, spatially decaying with a power-law exponent $\alpha$. Using the strong disorder renormalization group (SDRG) technique, we find by analytical solution of the master equation a strong disorder fixed point, characterized by a fixed point distribution of the couplings with a finite dynamical exponent, which describes the system consistently in the regime $\alpha >\frac{1}{2}$. A numerical implementation of the SDRG method yields a power-law spatial decay of the average concurrence, which is also confirmed by exact numerical diagonalization. However, we find that the lowest-order SDRG approach is not sufficient to obtain the typical value of the concurrence. We therefore implement a correction scheme which allows us to obtain the leading-order corrections to the random singlet state. This approach yields a power-law spatial decay of the typical value of the concurrence, which we derive both by a numerical implementation of the corrections and by analytics. Next, using numerical SDRG, the entanglement entropy (EE) is found to be logarithmically enhanced for all $\alpha$, corresponding to a critical behavior with an effective central charge $c=\ln \left(2\right)$, independent of $\alpha$. This is confirmed by an analytical derivation. Using numerical exact diagonalization (ED), we confirm the logarithmic enhancement of the EE and a weak dependence on $\alpha$. For a wide range of partition size $l$, the EE fits a critical behavior with a central charge close to $c=1$, which is the same as for the clean Haldane-Shastry model with a power-law-decaying interaction with $\alpha =2$. Only for small $l\ll L$, in a range which increases with the number of spins $N$, we find deviations which are rather consistent with the strong disorder fixed point central charge $c=\ln \left(2\right)$. Furthermore, we find using ED that the concurrence shows power-law decay, albeit with smaller power exponents than obtained by SDRG. We also present results obtained with DMRG and find agreement with ED for sufficiently small $\alpha <2$, whereas for larger $\alpha$ DMRG tends to underestimate the entanglement entropy and finds a faster decaying concurrence.

Optomagnetics emerges as a growing field of research cross-linking optics, magnetism, and material science. Here we provide a microscopic quantum-mechanical and a macroscopic classical model to describe an optomagnetic medium, i.e., optical gyration coefficient from a nonlinear optics point of view. Our self-consistent quantum-mechanical formulation considers all orders of perturbing field and results not only in finding generalized Pitaevskii's relationship, where photoinduced magnetization can be expanded in terms of light power, but also provides compact and analytical expressions for optical gyration vector coefficients. Classical treatment is then developed based on the anharmonic Drude-Lorentz model showing that the photoinduced DC magnetization is proportional to odd harmonics of the light power. The difference in quantum and classical results is revealed and discussed. Having a pump-probe setup in mind, we describe how a probe light signal can propagate down an optomagnetic medium, i.e., a medium that is magnetized by intense circularly polarized pump light, via its permittivity tensor and find light propagation characteristics. Inverse Faraday and Cotton-Mouton effects are discussed as a result of circular and linear birefringences and their Verdet constants have been analytically found.

${\mathrm{TaSnS}}_2$ single crystal and polycrystalline samples are investigated in detail by magnetization, electrical resistivity, and specific heat as well as Raman spectroscopy and nuclear magnetic resonance (NMR). Studies are focused on the temperature and magnetic field dependence of the superconducting state. We determine the critical fields for both directions $B\parallel c$ and $B\perp c$. Additionally, we investigate the dependence of the resistivity, the critical temperature, and the structure through Raman spectroscopy under high pressure up to 10 GPa. At a pressure of $\approx 3\mathrm{GPa}$ the superconductivity is suppressed below our minimum temperature. The Sn NMR powder spectrum shows a single line which is expected for the ${\mathrm{TaSnS}}_2$ phase and confirms the high sample quality. Pronounced de Haas-van Alphen oscillations in the ac susceptibility of polycrystalline sample reveal two pairs of frequencies indicating coexisting small and large Fermi surfaces. The effective mass of the smaller Fermi surface is $\approx 0.5m_{\mathrm{e}}$. We compare these results with the band structures from DFT calculations. Our findings on ${\mathrm{TaSnS}}_2$ are discussed in terms of a quasi-two-dimensional BCS superconductivity.

We describe degenerate square spin as an ensemble of magnetic monopoles coupled via an emergent entropic field that subsumes the effect of the underlying spin vacuum. We compute their effective free energy, entropic interaction, correlations, screening, and structure factor, and find that they coincide with the experimental ones. Unlike in pyrochlore ices, a dimensional mismatch between real and entropic interactions leads to weak singularities at the pinch points and algebraic correlations at long distances. This algebraic screening can be, however, camouflaged by a pseudoscreening regime.

We compute the quantum Fisher information (QFI) associated with two different local operators in the ground state of the Kitaev honeycomb model, and find divergent behavior in the second derivatives of these quantities with respect to the driving parameter at the quantum phase transition between the gapped and gapless phases for both fully antiferromagnetic and fully ferromagnetic exchange couplings, thus demonstrating that the second derivative a locally defined, experimentally accessible, QFI can detect topological quantum phase transitions. The QFI associated with a local magnetization operator behaves differently from that associated with a local bond operator depending on whether the critical point is approached from the gapped or gapless side. We show how the behavior of the second derivative of the QFI at the critical point can be understood in terms of the diverging length scales associated to the two and four point correlators of the Majorana degrees of freedom. We present critical exponents associated with the divergences of these length scales.

Here we revisit the thermodynamics of the Kitaev quantum spin liquid realized on the honeycomb lattice. We address two main questions: First, we investigate whether there are observable thermodynamic signatures of the topological Majorana boundary modes of the Kitaev honeycomb model. We argue that for the time-reversal invariant case the residual low-temperature entropy is the primary thermodynamic signature of these Majorana edge modes. Using large-scale Monte Carlo simulations, we verify that this residual entropy is present in the full Kitaev model. When time-reversal symmetry is broken, the Majorana edge modes are potentially observable in more direct thermodynamic measurements such as the specific heat, though only at temperatures well below the bulk gap. Second, we study the energetics, and the corresponding thermodynamic signatures, of the flux excitations in the Kitaev model. Specifically, we study the flux interactions on both cylinder and torus geometries numerically and quantify their impact on the thermodynamics of the Kitaev spin liquid by using a polynomial fit for the average flux energy as a function of flux density and extrapolating it to the thermodynamic limit. By comparing this model to Monte Carlo simulations, we find that flux interactions have a significant quantitative impact on the shape and the position of the low-temperature peak in the specific heat.

Noncollinear magnetism opens exciting possibilities to generate topological superconductivity. Here, we focus on helical and cycloidal magnetic textures in magnet-superconductor hybrid structures in a background magnetic field. We demonstrate that this system can enter a topological phase which can be understood as a set of parallel topological wires. We explore and confirm this idea in depth with three different approaches: a continuum model, a tight-binding model based on the magnetic unit cell, and exact diagonalization on a finite two-dimensional lattice. The key signature of this topological state is the presence of Majorana bound states at certain disclination defects in the magnetic texture. Based on the $C_2$ symmetry imposed by the helical or cycloidal texture, we employ the theory of topological crystalline superconductors with rotation invariants to obtain the Majorana parity at disclinations. Furthermore, we consider a 90-deg helimagnet domain wall, which is formed by a string of alternating disclinations. We discuss how the resulting chain of disclination bound states hybridizes into two chiral modes with different velocities. We suggest that hybrid systems of chiral magnets and superconductors are capable of hosting Majorana modes in various spatial configurations with potentially far less nanoengineering than in, e.g., semiconductor wires.

We discuss twisted bilayer graphene (TBG) based on a theorem of flat-band ferromagnetism put forward by Mielke and Tasaki. According to this theorem, ferromagnetism occurs if the single-particle density matrix of the flat-band states is irreducible and we argue that this result can be applied to the quasi-flat-bands of TBG that emerge around the charge-neutrality point for twist angles around the magic angle $\theta \sim 1.{05}^{\circ }$. We show that the density matrix is irreducible in this case, thus predicting a ferromagnetic ground state for neutral TBG ($n=0$). We then show that the theorem can also be applied only to the flat conduction or valence bands, if the substrate induces a single-particle gap at charge neutrality. Also in this case, the corresponding density matrix turns out to be irreducible, leading to ferromagnetism at half filling ($n=\pm 2$).

Substitutional replacement of N with O in epitaxial CrN(001) layers is achieved by reactive sputter deposition in a mixed ${\mathrm{N}}_2$ and $\mathrm{Ar}+{\mathrm{O}}_2$ atmosphere. O incorporation facilitates Cr vacancies, yielding a rocksalt-structure solid solution ${\mathrm{Cr}}_{1-x/2}{\mathrm{N}}_{1-x}{\mathrm{O}}_x$ with a single compositional parameter $x$ and a measured lattice constant that decreases from $a=0.4175$ to 0.4116 nm for $x=0$ to 0.59. First-principles calculations predict $da/dx=+0.0200$, −0.0018, and −0.0087 nm for $\mathrm{Cr}{\mathrm{N}}_{1-x}{\mathrm{O}}_x, {\mathrm{Cr}}_{1-x/3}{\mathrm{N}}_{1-x}{\mathrm{O}}_x$, and ${\mathrm{Cr}}_{1-x/2}{\mathrm{N}}_{1-x}{\mathrm{O}}_x$, respectively, and confirm, in combination with ion-beam compositional analyses and x-ray diffraction results, a vacancy concentration of $x/2$ per cation site. The room-temperature resistivity decreases by over two orders of magnitudes from $\rho =1.5\times {10}^{-3}$ to $7.7\times {10}^{-6}\mathrm{\Omega }\mathrm{m}$ for $x=0–0.26$. This is accompanied by a transition from a negative to a positive temperature coefficient of resistivity, an increase in the Hall mobility from ${\mu }_{\mathrm{H}}=0.37$ to $1.7{\mathrm{cm}}^2/\mathrm{Vs}$, an increase in the carrier concentration $n=1.1\times {10}^{20}$ to $4.9\times {10}^{21}{\mathrm{cm}}^{-3}$, and a decrease in the calculated band gap from 0.54 to 0 eV. However, $\rho$ increases again to $6.9\times {10}^{-5}\mathrm{\Omega }\mathrm{m}$ for $x=0.34$ and to $>10\mathrm{\Omega }\mathrm{m}$ for $x\ge 0.59$, with a corresponding drop in ${\mu }_{\mathrm{H}}$ and the opening of a gap. The maximum in the measured carrier density and conductivity at $x=0.26$ is attributed to a maximum in the density of states near the Fermi level, in perfect agreement with calculations using supercells with randomly distributed O substitutions and Cr vacancies that predict a maximum at $x=0.25\pm 0.05$. The measurements indicate insulator-to-metal and subsequent metal-to-insulator transitions at $x=0.04\pm 0.03$ and $0.30\pm 0.03$, respectively. The transition to metallic conduction is attributed to substitutional O on anion sites acting as donors, while the measured $n$ for $x=0.01–0.26$ quantitatively indicates charge compensation by cation vacancies that act as acceptors. The insulating properties at large $x$ are likely caused by increased electron correlation effects.

Popular models of the phenomenological relaxation operators that are widely used in the master equation formalism for open condensed-matter systems have significant flaws ranging from limited applicability to violation of fundamental physical principles. We propose a relatively simple universal model of the relaxation operator which is free from these flaws, has a correct static limit, has a correct direct-current limit in a uniform electric field, includes both interband and intraband transitions, and is valid for an arbitrary dispersion of quasiparticles in a solid. We use the proposed operator to generalize the Lindhard formula and derive explicit expressions for the relaxation operator for Dirac materials with an unconventional energy spectrum of quasiparticles, such as graphene and Weyl semimetals. We compare the linear susceptibility spectra for graphene obtained with different relaxation models and show that the proposed relaxation operator leads to physically meaningful behavior of the susceptibility at low frequencies, whereas the existing models become completely invalid.

We present a combined oxygen $K$-edge x-ray absorption spectroscopy and resonant inelastic x-ray scattering (RIXS) study of the bilayer ruthenate ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$. Our RIXS experiments on ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ were carried out on the overlapping planar and interplanar oxygen resonances, which are distinguishable from the apical one. Comparison to equivalent oxygen $K$-edge spectra recorded on band-Mott insulating ${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$ is made. In contrast to ${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$ spectra, which contain excitations linked to Mott physics, ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ spectra feature only intra-$t_{2g}$ ones that do not directly involve the Coulomb energy scale. As found in ${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$, we resolve two intra-$t_{2g}$ excitations in ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$. Moreover, the lowest lying excitation in ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ shows a significant dispersion, revealing a collective character different from what is observed in ${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$. Theoretical modeling supports the interpretation of this lowest energy excitation in ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ as a magnetic transverse mode with multiparticle character, whereas the corresponding excitation in ${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$ is assigned to combined longitudinal and transverse spin modes. These fundamental differences are discussed in terms of the inequivalent magnetic ground-state manifestations in ${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$ and ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$.

We numerically investigate the quantum criticality of the chiral Heisenberg universality class with the total number of fermion components $N=8$ in terms of the Gross-Neveu theory. Auxiliary-field quantum Monte Carlo simulations are performed for the square lattice Hubbard model in the presence of a $d$-wave pairing field, inducing Dirac cones in the single-particle spectrum. This property makes the model particularly interesting because it turns out to belong to the same universality class of the Hubbard model on the honeycomb lattice, which is the canonical model for graphene, despite the unit cells being apparently different (e.g., they contain one and two sites, respectively). We indeed show that the two phase transitions, expected to occur on the square and on the honeycomb lattices, have the same quantum criticality. We also argue that details of the models, i.e., the way of counting $N$ and the anisotropy of the Dirac cones, do not change the critical exponents. The present estimates of the exponents for the $N=8$ chiral Heisenberg universality class are $\nu =1.05\left(5\right)$, ${\eta }_{\varphi }=0.75\left(4\right)$, and ${\eta }_{\psi }=0.23\left(4\right)$, which are compared with the previous numerical estimations.

We construct few-body, interacting, nonlocal Hamiltonians with a quantum scar state in an otherwise thermalizing many-body spectrum. In one dimension, the embedded state is a critical state, and in two dimensions, the embedded state is a chiral topologically ordered state. The models are defined on slightly disordered lattices, and the scar state appears to be independent of the precise realization of the disorder. A parameter allows the scar state to be placed at any position in the spectrum. We show that the level spacing distributions are Wigner-Dyson and that the entanglement entropies of the states in the middle of the spectrum are close to the Page value. Finally, we confirm the topological order in the scar state by showing that one can insert anyons into the state.

Organic light-emitting diode (OLED) devices in the archetype small-molecule fluorescent guest-host system tris(8-hydroxyquinolinato) aluminum ($\mathrm{Al}{\mathrm{q}}_3$) doped with 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran (DCM2) displays a redshift in light-emission frequency which is extremely sensitive to the dopant concentration. This effect can be used to tune the emission frequency in this particular class of OLEDs. In this work, a model is proposed to describe this effect using a combination of density functional theory quantum-chemical calculations and stochastic simulations of exciton diffusion via a Förster mechanism. The results show that the permanent dipole moments of the $\mathrm{Al}{\mathrm{q}}_3$ molecules generate random electric fields that are large enough to cause a nonlinear Stark shift in the band gap of neighboring DCM2 molecules. As a consequence of these nonlinear shifts, a non-Gaussian probability distribution of highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) gaps for the DCM2 molecules in the $\mathrm{Al}{\mathrm{q}}_3$ matrix is observed, with long exponential tails to the low-energy side. Surprisingly, this probability distribution of DCM2 HOMO-LUMO gaps is virtually independent of DCM2 concentration into the $\mathrm{Al}{\mathrm{q}}_3$ matrix, at least up to a fraction of 10%. This study shows that this distribution of gaps, combined with out-of-equilibrium exciton diffusion among DCM2 molecules, is sufficient to explain the experimentally observed emission redshift.

We predict and analyze mechanical instability and corresponding self-sustained mechanical oscillations occurring in a nanoelectromechanical system composed of a metallic carbon nanotube (CNT) suspended between two superconducting leads and coupled to a scanning tunneling microscope (STM) tip. We show that such phenomena are realized in the presence of both the coherent Andreev tunneling between the CNT and superconducting leads, and an incoherent single electron tunneling between the voltage biased STM tip and CNT. Treating the CNT as a single-level quantum dot, we demonstrate that the mechanical instability is controlled by the Josephson phase difference, relative position of the electron energy level, and the direction of the charge flow. It is found numerically that the emergence of the self-sustained oscillations leads to a substantial suppression of DC electric current.