The phenomenological textbook equations for charge and heat transport are extensively used in a number of fields ranging from semiconductor devices to thermoelectricity. We provide a rigorous derivation of transport equations by solving the Boltzmann equation in the relaxation-time approximation and show that the currents can be rigorously represented by an expansion in terms of the “driving forces”. Besides the linear and nonlinear response to the electric field, the gradient of the chemical potential and temperature, there are also terms that give the response to the higher-order derivatives of the potentials. These nonlocal responses might play an important role for some materials and/or under certain conditions, such as extreme miniaturization. Our solution provides the general solution of the Boltzmann equation in the relaxation-time approximation (or, equivalently, the particular solution for the specific boundary conditions). It differs from the Hilbert expansion, which provides only one of infinitely many solutions which may or may not satisfy the required boundary conditions.

We discuss monolayer and bilayer quantum Hall systems in which each layer is a half-filled Landau level (LL) system. In the mean field approximation of the Son's formalism, there is a common pairing structure that underlines the possibilities for paired ground states in both systems. We argue that the particle-hole (PH) Pfaffian state in the (particle-hole symmetric) half-filled LL of a monolayer, and analogous state in the PH symmetric bilayer (in which each layer is half-filled LL) can be considered as critical states, i.e., states that cannot describe a phase under PH symmetry. We point out that the inclusion of a PH symmetry breaking (like LL mixing) may stabilize the PH Pfaffian in a monolayer. In the bilayer case, in numerical experiments on a sphere, by choosing the PH symmetric shift, we can stabilize the interlayer correlated (111) excitonic state or critical state, for any distance between the layers, but in general, with no bias, the evolution of the bilayer includes other phases.

Magnetic moments (MMs) of electrons in topological insulator quantum dots (TI-QDs) are investigated using a model system, namely a multiorbital honeycomb lattice. Their nature and orientation with respect to the spin are studied. We show that large MMs are not specific to edge states in nontrivial gaps, as band states can host even larger MMs. However, we demonstrate that edge-state and band-state MMs have a totally different sensitivity to disorder. Measuring the MMs in TI-QDs is therefore a direct way to probe the nontrivial to trivial topological transition under increasing disorder.

Circularly symmetric graphene nanostructures exist in the form of graphene rings, dots, and antidots. For realistic structure sizes, atomistic studies typically become prohibitively demanding. Analytical results, however, can be found within the Dirac-equation approach even in the presence of a perpendicular magnetic field. We model nanostructure confinement by using a circularly symmetric mass term and analyze the influence of geometry, magnetic field, and mass term on the eigenstates. Excellent agreement with atomistic models for small structures is demonstrated. In addition, we find good agreement with recent magneto-transport measurements for large graphene rings and investigate their valley-dependent density of states.

High pressure resistance, susceptibility, and Fe $K\beta$ x-ray emission spectroscopy measurements were performed on Fe-ladder compound ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$. Pressure-induced superconductivity was observed which is similar to the previously reported superconductivity in the ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ samples. The slope of local magnetic moment versus pressure shows an anomaly across the insulator-metal transition pressure in the ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ samples. The local magnetic moment is continuously decreasing with increasing pressure, and the superconductivity appears only when the local magnetic moment value is comparable to the one in the iron-pnictide superconductors. Our results indicate that the compressed ${\mathrm{BaFe}}_{2}C{h}_3$ ($Ch=\mathrm{S}$, Se) is a new family of iron-based superconductors. Despite the crystal structures completely different from the known iron-based superconducting materials, the magnetism in this Fe-ladder material plays a critical role in superconductivity. This behavior is similar to the other members of iron-based superconducting materials.

We report a high-temperature ($T=0.3$ K) indicator of the excitation gap ${\mathrm{\Delta }}_{5/2}$ at the filling factor $\nu =\frac{5}{2}$ fractional quantum Hall state in ultrahigh-quality AlGaAs/GaAs two-dimensional electron gases. As the lack of correlation between mobility $\mu$ and ${\mathrm{\Delta }}_{5/2}$ has been well established in previous experiments, we define, analyze, and discuss the utility of a different metric ${\rho }_{5/2}$, the resistivity at $\nu =\frac{5}{2}$, as a high-temperature predictor of ${\mathrm{\Delta }}_{5/2}$. This high-field resistivity reflects the scattering rate of composite fermions. Good correlation between ${\rho }_{5/2}$ and ${\mathrm{\Delta }}_{5/2}$ is observed in both a density-tunable device and in a series of identically structured wafers with similar density but vastly different mobility. This correlation can be explained by the fact that both ${\rho }_{5/2}$ and ${\mathrm{\Delta }}_{5/2}$ are sensitive to long-range disorder from remote impurities, while $\mu$ is sensitive primarily to disorder localized near the quantum well.

Monolayer transition metal dichalcogenides (TMDs) feature a valley degree of freedom, giant spin-orbit coupling, and spin-valley locking. These exotic natures have stimulated efforts of exploring potential applications in conceptual spintronics, valleytronics, and quantum computing. Among all the exotic directions, a long relaxation time of spin and/or valley polarization is critical. The present valley dynamics studies concentrate on the band edge excitons which predominate the optical response due to an enhanced Coulomb interaction in two dimensions. The valley relaxation time of free carriers remains ambiguous. In this Rapid Communication, we use time-resolved Kerr rotation spectroscopy to probe the valley dynamics of excitons and free carriers in monolayer tungsten diselenide. The valley relaxation time of free carriers is found around 2 ns at 70 K, about three orders of magnitude longer than the excitons of about 2 ps, and 15 times larger than that of trions (130 ps). The extended valley relaxation time of free carriers evidences that an exchange interaction dominates the valley relaxation in optical excitations. The pump-probe spectroscopy also reveals an exciton binding energy of 0.60 eV in monolayer ${\mathrm{WSe}}_2$.

The multiflavor $\mathit{BF}$ theories in (3+1) dimensions with cubic or quartic coupling are the simplest topological quantum field theories that can describe fractional braiding statistics between looplike topological excitations (three-loop or four-loop braiding statistics). In this paper, by canonically quantizing these theories, we study the algebra of Wilson loop and Wilson surface operators, and multiplets of ground states on the three-torus. In particular, by quantizing these coupled $\mathit{BF}$ theories on the three-torus, we explicitly calculate the $\mathcal{S}$ and $\mathcal{T}$ matrices, which encode fractional braiding statistics and the topological spin of looplike excitations, respectively. In the coupled $\mathit{BF}$ theories with cubic and quartic coupling, the Hopf link and Borromean ring of loop excitations, together with pointlike excitations, form composite particles.

Photoelectrons produced from the excitation of spin-degenerate states in solids can have a sizable spin polarization, which is related to the phase of interfering channels in the photoemission matrix elements. Such spin polarization can be measured by spin-resolved photoemission spectroscopy to gain information about the transitions and the Wigner time delay of the process. Incorporating strongly correlated electron systems into this paradigm could yield a novel means of extracting phase information crucial to understanding the mechanism of their emergent behavior. In this work, we present, as a case study, experimental measurements of the cuprate superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ by spin-resolved photoemission while maintaining full angular and energy resolution. A spin polarization of at least $10\%$ is observed, which is related to the phase of the photoelectron wave function.

The electronic structure of stoichiometric tin dioxide ($\mathrm{Sn}{\mathrm{O}}_2$) is studied by probing its unoccupied states using the fine structure in the electron energy-loss spectra (EELS) at the oxygen-$K$ (O-$K$) edge. The spectral measurements were performed both at room and at high temperatures (773 K) and compared to ab initio calculations carried out using the real-space multiple-scattering and linearized augmented-plane-wave methods. Important many-body effects are included via quasiparticle corrections calculated within the many-pole $GW$ self-energy approximation. An additional energy-dependent damping is calculated to account for vibrational effects. Results from this paper demonstrated that quantitative agreement between theoretical and experimental spectra can be obtained when nonspherical potentials and quasiparticle self-energy effects are considered and vibrational broadening is included. Modifications of the electronic structure by single oxygen vacancies, both in the bulk and at the (110) surface, also are predicted. Our predictions support the use of O-$K$ EELS as a probe of the defect structures in $\mathrm{Sn}{\mathrm{O}}_2$ surfaces and nanoparticles.

The temperature dependence of the absorption-edge energy has been determined in mixed layered transition-metal dichalcogenides of $\mathrm{Zr}{\mathrm{S}}_x\mathrm{S}{\mathrm{e}}_{2-x}$, for $x=0.5$, 1.0, 1.5, and 2.0 in the range of 32–300 K. The single crystals of $\mathrm{Zr}{\mathrm{S}}_x\mathrm{S}{\mathrm{e}}_{2-x}$ samples have been grown by the chemical vapor transport technique and characterized with the help of different methods. The absorption edge is found to shift toward lower energy with increasing the temperature. The experimental spectra and the parameters that describe the temperature dependence of the absorption edges are evaluated and described in the framework of the model proposed by Manoogian and Woolley [Can. J. Phys. 62, 285 (1984)]. The presence of the Urbach tail just below the band edge of $\mathrm{Zr}{\mathrm{S}}_x\mathrm{S}{\mathrm{e}}_{2-x}$ was observed. The steepness parameter σ and its functional temperature and composition dependence are discussed. Additionally, the values of the Urbach energies as well as the phonon energies are deduced. The obtained results support that the thermal phonon interactions and the structural and compositional disorders contribute to the Urbach tail in $\mathrm{Zr}{\mathrm{S}}_x\mathrm{S}{\mathrm{e}}_{2-x}$ crystals.

Multiple line nodes in energy-band gaps are found in semimetals preserving mirror-reflection symmetry. We classify possible configurations of multiple line nodes with crossing points (crossing line nodes) under point-group symmetry. Taking the spin-orbit interaction (SOI) into account, we also classify topological phase transitions from crossing-line-node Dirac semimetals to other topological phases, e.g., topological insulators and point-node semimetals. This study enables one to find crossing-line-node semimetal materials and their behavior in the presence of SOI from the band structure in the absence of SOI without detailed calculations. As an example, the theory applies to hexagonal rare-earth trihydrides with the ${\mathrm{HoD}}_3$ structure and clarifies that it is a crossing-line-node Dirac semimetal hosting three line nodes.

In this work, we studied the adsorption behavior of deposited Si atoms along with their diffusion and other dynamic processes on a Pb monolayer-covered Si(111) surface from 125 to 230 K using a variable-temperature scanning tunneling microscope. The Pb-covered Si(111) surface forms a low-symmetry rowlike ($\sqrt{7}\times \sqrt{3}$) structure in this temperature range and the Si atoms bind favorably to two specific on-top sites ($T_{1A}$ and $T_{1B}$) on the trimer row after deposition at the sample temperature of $\sim 125\mathrm{K}$. The Si atoms were immobile at low temperatures and started to switch between the two neighboring $T_{1A}$ and $T_{1B}$ sites within the same trimer when the temperature was raised to $\sim 150\mathrm{K}$. When the temperature was raised above $\sim 160\mathrm{K}$, the adsorbed Si atoms could hop to other trimers along the same trimer row. Below $\sim 170\mathrm{K}$, short hops to adjacent trimers dominated, but long hops dominated at temperatures above $\sim 170\mathrm{K}$. The activation energy and prefactor for the Si atoms diffusion were derived through analysis of continuous-time imaging at temperatures from 160 to 174 K. In addition, irreversible aggregation of single Si atoms into Si clusters started to occur at the phase boundaries or defective sites at temperatures above $\sim 170\mathrm{K}$. At temperature above $\sim 180\mathrm{K}$, nearly all Si atoms aggregated into clusters, which may have important implications for the atomic mechanism of epitaxial growth of Si on the Pb-covered Si(111) surface. In addition, our study provides strong evidence for breaking in the mirror symmetry in the ($\sqrt{7}\times \sqrt{3}$)-Pb structure, which has implications for the atomic model of this controversial structure.

Topologically nontrivial states reveal themselves in strongly spin-orbit coupled systems by Dirac cones. However, their appearance is not a sufficient criterion for a topological phase. In topological insulators, where these states protect surface metallicity, they are straightforwardly assigned based on bulk-boundary correspondence. On metals, where these states are suspected to have tremendous impact as well, e.g., in catalysis, their topological protection is difficult to assess due to the lacking band gap and the frequent assignment to topological properties appears unjustified. Here, we discover by angle-resolved photoemission a state with the dispersion of a Dirac cone at a low-symmetry point of W(110). Our ab initio calculations predict this feature with a linear band crossing and high spin polarization. However, instead of being born by topology, the states arise from Rashba split bands and do not fundamentally depend on the opening of a spin-orbit gap. On the other hand, we find that the [001] mirror plane protects the band crossing point and renormalizes the dispersion towards a Dirac-cone shape. In this sense, the discovered state is the metal counterpart of the surface state of a topological crystalline insulator. The Dirac cone is tilted due to its origin in an accidental band crossing away from high symmetry points. Tilted Dirac cones have recently been predicted for two- and three-dimensional materials and were observed in three-dimensional Weyl semimetals. Accordingly, the protection and renormalization by mirror symmetry uncovered here are a potentially much wider spread phenomenon which does not require topological properties. Our results also indicate why the massive gapless crossing predicted for topological crystalline insulators has never been observed.

We analyze and give estimates for the long-distance coupling via floating metallic gates between different types of spin qubits in quantum dots made of different commonly used materials. In particular, we consider the hybrid, the singlet-triplet, and the spin-$\frac{1}{2}$ qubits, and the pairwise coupling between each type of these qubits with another hybrid qubit in GaAs, InAs, Si, and ${\mathrm{Si}}_{0.9}{\mathrm{Ge}}_{0.1}$. We show that hybrid qubits can be capacitively coupled strongly enough to implement two-qubit gates, as long as the distance of the dots from the metallic gates is small enough. Thus, hybrid qubits are good candidates for scalable implementations of quantum computing in semiconducting nanostructures.

We perform oscillatory shear simulations to determine the loss modulus for three solids with identical interaction yet distinct structures: ordered, random, and glassy alloys. Random and glassy alloys show more pronounced high-frequency loss in the THz regime than the ordered alloy. Ordered and random alloys exhibit a power-law decay in damping strength as frequency decreases over nearly five decades. Glassy alloy, with a limited frequency range of power-law decay, retains significant damping at low frequencies extending down to $\sim 100\mathrm{MHz}$ due to slow irreversible deformation of local clusters.

We experimentally examine the topological nature of a strongly coupled spin-photon system induced by damping. The presence of both spin and photonic losses results in a non-Hermitian system with a variety of exotic phenomena dictated by the topological structure of the eigenvalue spectra and the presence of an exceptional point (EP), where the coupled spin-photon eigenvectors coalesce. By controlling both the spin resonance frequency and the spin-photon coupling strength we observe a resonance crossing for cooperativities above one, suggesting that the boundary between weak and strong coupling should be based on the EP location rather than the cooperativity. Furthermore, we observe dynamic mode switching when encircling the EP and identify the potential to engineer the topological structure of coupled spin-photon systems with additional modes. Our work therefore further highlights the role of damping within the strong coupling regime, and demonstrates the potential and great flexibility of spin-photon systems for studies of non-Hermitian physics.

We present a high-pressure study of $\mathrm{M}{\mathrm{g}}_{1–x}\mathrm{F}{\mathrm{e}}_x\mathrm{O}$ (ferropericlase, Fp) single crystals $0.04\left(1\right)\le x\le 0.33\left(1\right)$ with a focus on ferrous iron spin-state crossover and the material behavior preceding it. Using Vegard's law and highly accurate high-pressure single-crystal experimental data, we extract the lattice parameter $a_{\mathrm{FeO}}^{\mathrm{HS}}$ of the “FeO high spin lattice contribution” in the MgO-FeO solid solution. We find that ferropericlases with a wide range of compositions share the same critical parameter $a_C$ (defined as the minimum $a_{\mathrm{FeO}}^{\mathrm{HS}}$ after which spin crossover starts). Furthermore, we discuss the effect of composition on spin crossover in ferrous iron in ferropericlase in the limits of low and moderate concentrations of $\mathrm{F}{\mathrm{e}}^{2+}$.

A resonant inelastic x-ray scattering study of overdamped spin excitations in slightly underdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO) with $x=0.12$ and 0.145 is presented. Three high-symmetry directions have been investigated: (1) the antinodal $(0,0)\to (\frac{1}{2},0)$, (2) the nodal $(0,0)\to (\frac{1}{4},\frac{1}{4})$, and (3) the zone-boundary direction $(\frac{1}{2},0)\to (\frac{1}{4},\frac{1}{4})$ connecting these two. The overdamped excitations exhibit strong dispersions along (1) and (3), whereas a much more modest dispersion is found along (2). This is in strong contrast to the undoped compound ${\mathrm{La}}_2{\mathrm{CuO}}_4$ (LCO) for which the strongest dispersions are found along (1) and (2). The $t-t^{\prime }-t^{\prime \prime }-U$ Hubbard model used to explain the excitation spectrum of LCO predicts—for constant $U/t$—that the dispersion along (3) scales with ${(t^{\prime }/t)}^2$. However, the diagonal hopping $t^{\prime }$ extracted on LSCO using single-band models is low ($t^{\prime }/t\sim -0.16$) and decreasing with doping. We therefore invoked a two-orbital ($d_{x^2-y^2}$ and $d_{z^2}$) model which implies that $t^{\prime }$ is enhanced. This effect acts to enhance the zone-boundary dispersion within the Hubbard model. We thus conclude that hybridization of $d_{x^2-y^2}$ and $d_{z^2}$ states has a significant impact on the zone-boundary dispersion in LSCO.

We study, within the fluctuation-exchange approximation, the spin-fluctuation-mediated superconductivity in Hubbard-type models possessing electron and hole bands, and compare them with a model on a square lattice with a large Fermi surface. In the large Fermi surface model, superconductivity is more enhanced for better nesting for a fixed band filling. By contrast, in the models with electron and hole bands, superconductivity is optimized when the Fermi surface nesting is degraded to some extent, where finite-energy spin fluctuations around the nesting vector develop. The difference lies in the robustness of the nesting vector, namely, in models with electron and hole bands, the wave vector at which the spin susceptibility is maximized is fixed even when the nesting is degraded, whereas when the Fermi surface is large, the nesting vector varies with the deformation of the Fermi surface. We also discuss the possibility of realizing in actual materials the bilayer Hubbard model, which is a simple model with electron and hole bands, and is expected to have a very high $T_c$.