We study the superconducting pairing symmetry in twisted bilayer graphene by solving the Bogoliubov-de Gennes equation for all electrons in moiré supercells. With increasing the pairing potential, the system evolves from the mixed nontopological $d+id$ and $p+ip$ phase to the $s+p+d$ phase via the first-order phase transition. In the time-reversal symmetry breaking $d+id$ and $p+ip$ phase, vortex and antivortex lattices accompanying spontaneous supercurrent are induced by the twist. The superconducting order parameter is nonuniform in the moiré unit cell. Nevertheless, the superconducting gap in the local density of states is identical in the unit cell. The twist-induced vortices and nontopological nature of the mixed $d+id$ and $p+ip$ phase are not captured by the existing effective models. Our results suggest the importance of long-range pairing interaction for effective models.

The electronic structures of cerium-based ternary 122 compounds $\mathrm{Ce}{M}_2{\mathrm{Si}}_2$, where $M=\mathrm{Ru}$, Rh, Pd, and Ag, are investigated systematically by using the density functional theory in combination with the single-site dynamical mean-field theory. The momentum-resolved spectral functions, total and $4f$ partial density of states, self-energy functions, and valence state fluctuations are calculated. The obtained results are in good accord with the available experimental data. It is suggested that, upon increasing atomic number from Ru to Ag, the $4f$ electrons should become increasingly localized. An itinerant-localized crossover for $4f$ electrons driven by chemical substitution may emerge when $M$ changes from Pd to Ag. Particularly, according to the low-frequency behaviors of $4f$ self-energy functions, we identify an orbital-dependent $4f$ insulating state in ${\mathrm{CeAg}}_2{\mathrm{Si}}_2$, which is totally unexpected.

Near-field heat radiation and transfer are rich in various exciting effects, in particular, regarding the amplification due to the geometrical configuration of the system. In this paper, we study heat exchange in situations where the objects are confined by additional objects so that the dimensionality of heat flow is reduced. In particular, we compute the heat transfer for spherical point particles placed between two parallel plates. The presence of the plates can enhance or reduce the transfer compared to the free case and provides a slower power-law decay for large distance. We also compute the heat radiation of a sphere placed inside a spherical cavity, finding that it can be larger or smaller compared to the radiation of a free sphere. This radiation shows strong resonances as a function of the cavity's size. For example, the cooling rate of a nanosphere placed in a cavity varies by a factor of ${10}^5$ between cavity radii 2 $\mu \text{m}$ and 5 $\mu \text{m}$.

We derive full analytic expressions of three-body interactions from Landau-level (LL) mixing in fractional quantum Hall systems with Schrieffer-Wolff transformation. The formalism can be applied to any LL, and to very general systems without rotational or Galilean invariance. We illustrate how three-body pseudopotentials can be readily computed from the analytical expressions for a wide variety of different systems, and show that for realistic systems, softening the bare Coulomb interactions (e.g., finite thickness or screening) can significantly suppress three-body interactions. More interestingly, for experimental systems without Galilean invariance (which is common for real materials), there is strong evidence that higher orders in band dispersion can drive the Moore-Read state from anti-Pfaffian to Pfaffian phase. Our analysis points to the importance of the realistic band structure details to the non-Abelian topological phases, and the analytical expressions we derived can also be very useful for high fidelity numerical computations.

We present ab initio calculations of electronic structure and Coulomb interaction parameters for lanthanides, from Ce to Lu. We use a self-consistent scheme based on the density functional theory (DFT)+$U$ and constrained random-phase approximation (cRPA) methods, as implemented in the abinit code. We study more particularly the self-consistent values for the interaction parameters. We show that a realistic self-consistent calculation of the electronic structure and interaction parameters is not always possible. We discuss the possible improvements.

A maximum entropy method for analytic continuation is extended by introducing quantum relative entropy. This method is formulated in terms of matrix-valued functions and therefore invariant under arbitrary unitary transformation of input matrix. As a result, the continuation of off-diagonal elements becomes straightforward. Without introducing any further ambiguity, the Bayesian probabilistic interpretation is maintained just as in the conventional maximum entropy method. The applications of our generalized formalism to a model spectrum and a real material demonstrate its usefulness and superiority.

Electron-doped $\mathrm{SrTi}{\mathrm{O}}_3$ (where the dopant can be Nb or La) has been widely investigated for both its fundamental interest in condensed matter physics and for industrial applications. Its electronic properties are closely related to the Ti-O bonding states in the $\mathrm{SrTi}{\mathrm{O}}_3$ crystal. To further develop and control these properties, it is crucial to understand the factors controlling the change in lattice parameters and local lattice distortion upon doping with various atoms. Herein, we report the changes in lattice parameters and local lattice distortion in Nb- and La-doped $\mathrm{SrTi}{\mathrm{O}}_3$ single crystals, investigated by in-plane x-ray diffraction and first-principles calculations. The lattice parameter of Nb- and La-doped $\mathrm{SrTi}{\mathrm{O}}_3$ single crystals increased with dopant concentration. The broad intensities around the Bragg peak observed in the in-plane x-ray-diffraction experiments indicated that the local lattice expansion and contraction, or local lattice distortions, in the crystal were caused by the dopant atoms. First-principles calculations similarly showed that the lattice expansion and local lattice distortions in the $\mathrm{SrTi}{\mathrm{O}}_3$ crystals were caused by doped Nb and La atoms. Atoms surrounding Schottky pairs of O and Sr vacancies were displaced both away from and towards the vacancies, resulting in a reduction in the lattice expansion of donor-doped $\mathrm{SrTi}{\mathrm{O}}_3$. Donor atoms and Schottky pairs thus play an important role in determining the lattice parameters of $\mathrm{SrTi}{\mathrm{O}}_3$ crystals. These fundamental structural analyses provide a useful basis to further investigate the electronic conductivity of electron-doped $\mathrm{SrTi}{\mathrm{O}}_3$.

We study the return probability for the Anderson model on the random regular graph and give evidence of the existence of two distinct phases: a fully ergodic and nonergodic one. In the ergodic phase, the return probability decays polynomially with time with oscillations, being the attribute of the Wigner-Dyson-like behavior, while in the nonergodic phase the decay follows a stretched exponential decay. We give a phenomenological interpretation of the stretched exponential decay in terms of a classical random walker. Furthermore, comparing typical and mean values of the return probability, we show how to differentiate an ergodic phase from a nonergodic one. We benchmark this method first in two random matrix models, the power-law random banded matrices, and the Rosenzweig-Porter matrices, which host both phases. Second, we apply this method to the Anderson model on the random regular graph to give further evidence of the existence of the two phases.

We present a semiclassical formalism for antiferromagnetic (AFM) magnonics which promotes the central ingredient of spin wave chirality, encoded in a quantity called magnonic isospin, to a first-class citizen of the theory. We use this formalism to unify results of interest from the field under a single chirality-centric formulation. Our main result is that the isospin is governed by unitary time evolution, through a Hamiltonian projected down from the full spin wave dynamics. Because isospin is SU(2) valued, its dynamics on the Bloch sphere are precisely rotations, which, in general, do not commute. Consequently, the induced group of operations on AFM spin waves is nonabelian. This is a paradigmatic departure from ferromagnetic magnonics, which operates purely within the abelian group generated by spin wave phase and amplitude. Our investigation of this nonabelian magnonics in AFM insulators focuses on studying several simple gate operations, and offering in broad strokes a program of study for interesting new logic families in antiferromagnetic spin wave systems.

Polyakov et al. [Phys. Rev. B 95, 180202(R) (2017)] have presented a comparison of experimental versus calculated photoemission for gold deposited on $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, with the calculation based on a substitutional model of gold replacing bismuth, derived from an extended x-ray absorption fine-structure (EXAFS) analysis of the gold $L_{\mathrm{III}}$ edge. This Comment examines the coordination environment of gold in solid-state selenides in comparison to the model used, deficiencies within the EXAFS treatment presented by Polyakov et al., and provides a more thorough examination and interpretation of the x-ray absorption near-edge structure (XANES) and the EXAFS that invalidates the claims by the authors of gold being substitutional in topological insulator $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$.

In the Comment on our publication [Phys. Rev. B 95, 180202(R) (2017)], R. A. Gordon claims that our main conclusion is not valid, namely that gold atoms deposited in situ on the (0001) surface of single-crystalline ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ reside in substitutional sites, i.e., replacing bismuth atoms within the topmost quintuple layer (QL). Based on x-ray absorption near-edge (XANES) spectra and a re-evaluation of extended x-ray absorption fine structure (EXAFS) data above the Au $L_{\mathrm{III}}$ edge, R. A. Gordon concludes that Au resides in a twofold environment as a result of an interface reaction leading to an ${\mathrm{Au}}_2\mathrm{S}$-type local structure, in which gold adopts an Au(I) state and is linearly coordinated by selenium atoms. In this Reply, we will confirm the results published in the original paper and their interpretation that Au atoms reside in the substitutional site.

Understanding the superheating of crystals may serve as essential information for unraveling the mechanisms of homogeneous melting. Superheated crystals have been observed in experiments for decades and have broad implications in nanoscale embedded devices; however, the full extent of the metastable superheated crystalline state within equilibrium thermodynamic changes remains uncertain. Here, we investigate this problem from a geometrical perspective of the Gibbs's volume-entropy-internal energy thermodynamic surface. We find that in a homogeneous melting process, the limit of the superheated crystal can be defined as the state at which the crystal's internal energy or enthalpy, depending on whether the constraint condition is constant volume or pressure, equals the value of this property at the state where heterogeneous freezing begins. We demonstrate that the thermodynamic foundations of several different melting simulation methods, which previously were understood as mostly independent from each other, can be unified and elucidated from the same rigorous and quantitative perspective of the Gibbs surface. By tracking the trajectories of atoms relative to their equilibrium positions, we have identified the mechanisms of cooperative diffusion in the superheated face-centered-cubic Lennard-Jones crystal. Such diffusive motion is undergone in a manner that hops toward the first-nearest neighbor while keeping the crystalline structure unchanged.

The transition-metal dichalcogenide compounds ${\mathrm{MoTe}}_2$ and ${\mathrm{WTe}}_2$ are polymorphic with both semiconducting and metallic phases. The thermodynamically stable phase of ${\mathrm{WTe}}_2$ at room temperature is orthorhombic and metallic and displays a wide range of interesting phenomena including type-II Weyl fermions, titanic magnetoresistance and superconductivity in bulk, and quantum spin Hall insulator behavior in the monolayer case. On the other hand, the stable phase of ${\mathrm{MoTe}}_2$ at room temperature is a trigonal prismatic semiconductor that has a direct gap in the monolayer with strong spin-orbit coupling. The alloy series ${\mathrm{Mo}}_{1-x}{\mathrm{W}}_x{\mathrm{Te}}_2$ thus offers the possibility for tuning the structural and, consequently, electronic phases via tuning of the composition. Here, we report comprehensive studies of the electronic structure of ${\mathrm{Mo}}_{1-x}{\mathrm{W}}_x{\mathrm{Te}}_2$ alloys using angle-resolved photoemission spectroscopy and first-principles calculations as a function of composition. At room temperature, we find a sharp boundary between the orthorhombic and the trigonal prismatic phases at $x=0.10$ using structural characterization. We also show that by compositional tuning it is possible to control the band inversion in this series of compounds thus yielding important consequences for topological surface states.

In $A_3{\mathrm{Cr}}_2{\mathrm{O}}_8$, where $A$ = Sr or Ba, the ${\mathrm{Cr}}^{5+}$ ions surrounded by oxygen ions in a tetrahedral coordination are Jahn-Teller active. The Jahn-Teller distortion leads to a structural transition and a related emergence of three twinned monoclinic domains below the structural phase transition. This transition is highly dynamic over an extended temperature range for $A$ = Sr. We have investigated mixed compounds ${\mathrm{Ba}}_{3-x}{\mathrm{Sr}}_x{\mathrm{Cr}}_2{\mathrm{O}}_8$ with $x=2.9$ and $x=2.8$ by means of x-ray and neutron diffraction, Raman scattering, and calorimetry. Based on the obtained evolution of the phonon frequencies, we find a distinct suppression of the orbital-lattice fluctuation regime with increasing Ba content. This stands in contrast to the linear behavior exhibited by unit cell volumes, atomic positions, and intradimer spin-spin exchange interactions.

Magnetic order on the spatially anisotropic triangular lattice of $\alpha -{\mathrm{NaMnO}}_2$ is studied via neutron diffraction measurements. The transition into a commensurate, collinear antiferromagnetic ground state with $\mathbf{k}=(0.5,0.5,0)$ was found to occur below $T_{\mathrm{N}}=22$ K. Above this temperature, the transition is preceded by the formation of a coexisting, short-range ordered, incommensurate state below $T_{\mathrm{IC}}=45$ K whose two-dimensional propagation vector evolves toward $\mathbf{k}=(0.5,0.5)$ as the temperature approaches $T_{\mathrm{N}}$. At high temperatures ($T>T_{\mathrm{IC}}$), quasielastic scattering reveals one-dimensional spin correlations along the nearest-neighbor Mn-Mn “chain direction” of the ${\mathrm{MnO}}_6$ planes. Our data are consistent with the predictions of a mean-field model of Ising-like spins on an anisotropic triangular lattice, as well as the predominantly one-dimensional Heisenberg spin Hamiltonian reported for this material.

The effect of thermal fluctuations on the phase structure of monoaxial helimagnets with external magnetic field parallel to the chiral axis is analyzed by means of a saddle point expansion of the free energy. The phase transition that separates the conical and forced ferromagnetic phases is changed to first order by the thermal fluctuations. In a purely monoaxial system the pitch of the conical state remains independent of temperature and magnetic field, as in mean-field theory, even when fluctuations are taken into account. However, in the presence of weak Dzyaloshinskii-Moriya interactions in the plane perpendicular to the chiral axis, thermal fluctuations induce a dependence of the pitch on temperature and magnetic field. This may serve to determine the nature of magnetic interactions in such systems.

We study the formation of magnetic clusters in frustrated magnets in their cooperative paramagnetic regime. For this purpose, we consider the $J_1\text{−}{J}_2\text{−}{J}_3$ classical Heisenberg model on kagome and pyrochlore lattices with $J_2=J_3=J$. In the absence of farther-neighbor couplings, $J=0$, the system is in the Coulomb phase with magnetic correlations well characterized by pinch-point singularities. Farther-neighbor couplings lead to the formation of magnetic clusters, which can be interpreted as a counterpart of topological-charge clusters in Ising frustrated magnets [T. Mizoguchi, L. D. C. Jaubert, and M. Udagawa, Phys. Rev. Lett. 119, 077207 (2017)]. The concomitant static and dynamical magnetic structure factors, respectively $\mathcal{S}\left(\mathit{q}\right)$ and $\mathcal{S}(\mathit{q},\omega )$, develop half-moon patterns. As $J$ increases, the continuous nature of the Heisenberg spins enables the half-moons to coalesce into connected “star” structures spreading across multiple Brillouin zones. These characteristic patterns are a dispersive complement of the pinch-point singularities, and signal the proximity to a Coulomb phase. Shadows of the pinch points remain visible at finite energy $\omega$. This opens the way to observe these clusters through (in)elastic neutron scattering experiments. The origin of these features are clarified by complementary methods: large-$N$ calculations, semiclassical dynamics of the Landau-Lifshitz equation, and Monte Carlo simulations. As promising candidates to observe the clustering states, we revisit the origin of “spin molecules” observed in a family of spinel oxides $A{B}_2{\mathrm{O}}_4(A=\text{Zn},\text{Hg},\text{Mg},B=\text{Cr},\text{Fe})$.

${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been carried out to investigate the magnetic and electronic properties of the filled skutterudite metallic compound ${\mathrm{SrFe}}_4{\mathrm{As}}_{12}$. The temperature dependence of Knight shift $K$ determined by the NQR spectrum under a small magnetic field $\left(\le 0.5\mathrm{T}\right)$ shows the similar temperature dependence of the magnetic susceptibility $\chi$ which exhibits a broad maximum at $T^{*}\sim 50$ K. The nuclear spin-lattice relaxation rate divided by temperature $1/T_{1}T$ increases with decreasing temperature and exhibits a broad maximum at $T\sim 70$ K, similar to the case of $\chi$. The temperature dependence of $K$ and $1/T_{1}T$ is reasonably explained by a simple model where we assume a concave-shaped band structure near the Fermi energy. Based on a Korringa ratio analysis using the $T_1$ and $K$ data, ferromagnetic spin fluctuations are found to exist in ${\mathrm{SrFe}}_4{\mathrm{As}}_{12}$. These results indicate that ${\mathrm{SrFe}}_4{\mathrm{As}}_{12}$ can be characterized to be a metal with ferromagnetic correlations and also the peculiar band structure responsible for the suppression of $1/T_{1}T$ and $K$ at low temperatures.

The Dresselhaus spin-orbit interaction is expected to perturb the quantum spin Hall phase predicted to arise within InAs/GaSb coupled quantum wells. To gain a greater understanding of this interaction, the spin-orbit coupling in two InAs/GaSb coupled quantum well samples, grown along the [001] axis, was investigated along three different in-plane crystallographic axes. By measuring the crystallographic axis dependence of the Dresselhaus spin-orbit coupling, we can deconvolute this coupling from the spin splitting arising from axis-invariant Rashba spin-orbit coupling. We find that the Dresselhaus parameter is robust against an external gate bias and small changes in growth conditions, with an associated Dresselhaus parameter of $(0.20\pm 0.08)\times {10}^{-11}\mathrm{eV}\mathrm{m}$ being measured across all samples and top gate bias conditions. In addition, we show that the asymmetries associated with the coupled quantum well structure, leading to Rashba spin-orbit coupling, are likely to play a dominant role in determining the spin-orbit interaction experienced by a quantum spin Hall state as the system is tuned towards charge neutrality.

We investigate the magnetotransport properties of a two-dimensional electron gas with anisotropic $k$-cubic Rashba interaction at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface. The Landau levels and density of states of the system as well as the magnetotransport coefficients are evaluated. A somehow anomalous beating pattern in low magnetic field regime is found in both the density profile and magnetoresistivity. We discuss the impact of electron density, Landau level broadening, and Rashba spin-orbit constant on the appearance of the beatings in low magnetic fields and find that at low electron concentrations and not very strong spin-orbit interactions the beatings smooth out. On the other hand, as the magnetic field increases, the Zeeman term becomes the dominant splitting mechanism, leading to the spin-split peaks in Shubnikov–de Haas oscillations. We also show that the observation of the beatings in low magnetic fields needs a system with rather higher carrier concentration so that the beatings persist up to sufficiently large fields where the oscillations are not smoothed out by Landau level broadening. The quantum Hall plateaus are evaluated, and we show the Chern number with both even and odd values is replaced by the odd numbers when two subband energies are close to spin-degenerate energy levels. Along with the numerical evaluation of the magnetotransport properties, a perturbative calculation is performed which can be used in the case of low densities and not very large filling factors.