We present atomic and electronic structure studies using first-principles calculations of two-dimensional topological insulators, stanene and fluorostanene, deposited on $4H$-SiC(0001) substrates. We demonstrate the stability of H- or F-passivated honeycomb crystals due to a van der Waals interaction between the adsorbate and substrate. Despite destroyed inversion symmetry and biaxial strain the calculations of the band structures and $Z_2$ topological invariants predict that the quantum spin Hall (QSH) phase of stanene on H-passivated SiC as well as fluorostanene on H- and F-passivated SiC survives the interaction with the substrate. Our findings should serve as guidance for the epitaxial growth of tin-based QSH systems on wide-band-gap semiconductors.

We propose a general method to arbitrarily manipulate an electromagnetic wave propagating in a two-dimensional medium, without introducing any scattering. This leads to a whole class of isotropic spatially varying permittivity and permeability profiles that are invisible while shaping the field magnitude and/or phase. In addition, we propose a metamaterial structure working in the infrared that demonstrates deep subwavelength control of the electric field amplitude and strong reduction of the scattering. This work offers an alternative strategy to achieve invisibility with isotropic materials and paves the way for tailoring the propagation of light at the nanoscale.

The main effects of oxygen vacancy defects on the electronic and optical properties of ${\mathrm{V}}_2{\mathrm{O}}_5$ nanowires were studied through in situ Raman, photoluminescence, absorption, and photoemission spectroscopy. Both thermal reduction and electrochemical reduction via lithium insertion leads to the creation of oxygen vacancy defects in the crystal that gives rise to new electronic midgap defect states at energy 0.75 eV below the conduction band edge. The defect formation results in delocalization and injection of excess electrons into the conduction band, as opposed to localized electron injection as previously suggested. Contrary to what is seen in most oxides, the presence of vacancy defects leads to band filling and an increase in the optical band gap of ${\mathrm{V}}_2{\mathrm{O}}_5$ from 1.95 to 2.45 eV, which is attributed to the Burstein-Moss effect. Other observed changes in the optical properties are correlated to the changes in the electronic structure of the oxide as a result of defect formation. Further, in situ Raman measurements during the electrochemical reduction at room temperature show that the oxygen atom that is most readily reduced is the threefold coordinated oxygen (O3).

In this paper we develop a microscopic analysis of the light scattering on a periodic two-level atomic array coupled to an optical nanofiber. We extend the scattering matrix approach for two-level system interaction with nanofiber fundamental guided mode $H{E}_{11}$, which allows us to model the scattering spectra. We support these results by considering the dispersion of the polaritonic states formed by the superposition of the fundamental mode of light $H{E}_{11}$ and the atomic chain states. To illustrate our approach we start by considering a simple model of light scattering over an atomic array in free space. We discuss Bragg diffraction in the atomic array and show that the scattering spectrum is defined by the nonsymmetric coupling of a two-level system with nanofiber and vacuum modes. The proposed method allows consideration of two-level system interactions with a full account of dipole-dipole interactions via both near fields and long-range interaction owing to nanofiber mode coupling.

We study the ground state conduction properties of noninteracting electrons in aperiodic but nonrandom one-dimensional models with chiral symmetry and make comparisons against Anderson models with nondeterministic disorder. The first model we consider is the Fibonacci lattice, which is a paradigmatic model of quasicrystals; the second is the Riemann lattice, which we define inspired by Dyson's proposal on the possible connection between the Riemann hypothesis and a suitably defined quasicrystal. Our analysis is based on Kohn's many-particle localization tensor defined within the modern theory of the insulating state. In the Fibonacci quasicrystal, where all single-particle eigenstates are critical (i.e., intermediate between ergodic and localized), the noninteracting electron gas is found to be an insulator, due to spectral gaps, at various specific fillings $\rho$, including the values $\rho =1/g^n$, where $g$ is the golden ratio and $n$ is any integer; however away from these spectral anomalies, the system is found to be a conductor, including the half-filled case. In the Riemann lattice metallic behavior is found at half filling as well; however, in contrast to the Fibonacci quasicrystal, the Riemann lattice is generically an insulator due to single-particle eigenstate localization, likely at all other fillings. Its behavior turns out to be alike that of the off-diagonal Anderson model, albeit with different system-size scaling of the band-center anomalies. The advantages of analyzing the Kohn's localization tensor instead of other measures of localization familiar from the theory of Anderson insulators (such as the participation ratio or the Lyapunov exponent) are highlighted.

According to Lieb's theorem the ferromagnetic interaction in graphene-based materials with bipartite lattice is a result of disbalance between the number of sites available for $p_z$ electrons in different sublattices. Here we report on another mechanism of the ferromagnetism in functionalized graphene that is the direct exchange interaction between spin orbitals. By the example of the single-side semihydrogenated (${\mathrm{C}}_2\mathrm{H}$) and semifluorinated (${\mathrm{C}}_2\mathrm{F}$) graphene we show that such a coupling can partially or even fully compensate antiferromagnetic character of indirect exchange interactions reported earlier [Phys. Rev. B 88, 081405(R) (2013)]. As a result, ${\mathrm{C}}_2\mathrm{H}$ is found to be a two-dimensional material with the isotropic ferromagnetic interaction and negligibly small magnetic anisotropy, which prevents the formation of the long-range magnetic order at finite temperature in accordance with the Mermin-Wagner theorem. This gives a rare example of a system where direct exchange interactions play a crucial role in determining a magnetic structure. In turn, ${\mathrm{C}}_2\mathrm{F}$ is found to be at the threshold of the antiferromagnetic-ferromagnetic instability, which in combination with the Dzyaloshinskii-Moriya interaction can lead to a skyrmion state.

We employ resonant soft x-ray diffraction (RSXD) to isolate the signal from the CE-type antiferromagnetic phase of ${{\left(\mathrm{La},\mathrm{Pr}\right)}_{1-}}_x\mathrm{C}{\mathrm{a}}_x\mathrm{Mn}{\mathrm{O}}_3$ (with $x\approx 3/8$), and follow only this phase through the known phases of the material in the $B\text{−}T$ phase diagram. This material is known to exhibit a range of electronic ordering phenomena, most notably a metal-insulator transition (associated with colossal magnetoresistance) and phase separation between the antiferromagnetic phase and a ferromagnetic phase. Bulk magnetization measurements under the same $B\text{−}T$ conditions were also conducted, giving a full picture of both phases for direct side-by-side comparison. The comparison specifically focuses on the metal-insulator transition. Upon magnetic field ramping to this transition, we find that the CE-type order undergoes a sharp quench at high temperatures (above phase coexistence temperatures) but that at lower temperatures, where the CE order is metastable, the transition broadens significantly. At the lowest temperatures, where a spin glass-type phase is expected, a slow annihilation of remanent CE domains is observed. Finally, a refined phase diagram is presented.

We report systematic measurements of anomalous Hall effect (AHE) and spin-orbit torques (SOTs) in MnGa/IrMn films, in which a single $L{1}_0\text{−}\mathrm{MnGa}$ epitaxial layer reveals obvious orbital two-channel Kondo (2CK) effect. As increasing the thickness of the antiferromagnet IrMn, the strong spin Hall effect (SHE) has gradually suppressed the orbital 2CK effect and modified the AHE of MnGa. A scaling involving multiple competing scattering mechanisms has been used to distinguish different contributions to the modified AHE. Finally, the sizeable SOT in the MnGa/IrMn films induced by the strong SHE of IrMn have been investigated. The IrMn layer also supplies an in-plane exchange bias field and enables nearly field-free magnetization reversal.

We study the origin of broadening of superconducting transition in sputtered Nb films at high magnetic fields. From simultaneous tunneling and transport measurements we conclude that the upper critical field $H_{c2}$ always corresponds to the bottom of transition $R\sim 0$, while the top $R\sim R_n$ occurs close to the critical field for destruction of surface superconductivity $H_{c3}\simeq 1.7H_{c2}$. The two-dimensional nature of superconductivity at $H>H_{c2}$ is confirmed by cusplike angular dependence of magnetoresistance. Our data indicates that surface superconductivity is remarkably robust even in disordered polycrystalline films and, surprisingly, even in perpendicular magnetic fields. We conclude that surface superconductivity, rather than flux-flow phenomenon, inhomogeneity, or superconducting fluctuations, is the primary cause of broadening of superconducting transition in magnetic field.

The interplay between superconductivity and charge-density-wave (CDW) order plays a central role in the layered transition-metal dichalcogenides. $1T-\mathrm{Ti}{\mathrm{Se}}_2$ forms a prime example, featuring superconducting domes on intercalation as well as under applied pressure. Here, we present high energy-resolution inelastic x-ray scattering measurements of the CDW soft phonon mode in intercalated ${\mathrm{Cu}}_x\mathrm{Ti}{\mathrm{Se}}_2$ and pressurized $1T-\mathrm{Ti}{\mathrm{Se}}_2$ along with detailed ab-initio calculations for the lattice dynamical properties and phonon-mediated superconductivity. We find that the intercalation-induced superconductivity can be explained by a solely phonon-mediated pairing mechanism, while this is not possible for the superconducting phase under pressure. We argue that a hybridization of phonon and exciton modes in the pairing mechanism is necessary to explain the full observed temperature-pressure-intercalation phase diagram. These results indicate that $1T-\mathrm{Ti}{\mathrm{Se}}_2$ under pressure is close to the elusive state of the excitonic insulator.

We present a method for precisely measuring the tunnel splitting in single-molecule magnets (SMMs) using electron-spin resonance, and use these measurements to precisely and independently determine the underlying transverse anisotropy parameter, given a certain class of transitions. By diluting samples of the SMM ${\text{Ni}}_4$ via cocrystallization in a diamagnetic isostructural analog we obtain markedly narrower resonance peaks than are observed in undiluted samples. Using custom loop-gap resonators we measure the transitions at several frequencies, allowing a precise determination of the tunnel splitting. Because the transition under investigation occurs at zero field, and arises due to a first-order perturbation from the transverse anisotropy, we can determine the magnitude of this anisotropy independent of any other Hamiltonian parameters. This method can be applied to other SMMs with tunnel splittings arising from first-order transverse anisotropy perturbations.

Under the conditions of superfluid–Mott-insulator criticality in two dimensions, the trapping centers—i.e., local potential wells and bumps—are generically characterized by an integer charge corresponding to the number of trapped particles (if positive) or holes (if negative). Varying the strength of the center leads to a transition between two competing ground states with charges differing by $\pm 1$. The hallmark of the transition scenario is a splitting of the number density distortion $\delta n\left(r\right)$ into a half-integer core and a large halo carrying a complementary charge of $\pm \frac{1}{2}$. The sign of the halo changes across the transition and the radius of the halo $r_0$ diverges on the approach to the critical strength of the center, $V=V_c$, by the law $r_0\propto {|V-V_c|}^{-\tilde{\nu }}$, with $\tilde{\nu }\approx 2.33\left(5\right)$.

We perform ab initio molecular dynamics simulations to study structural and transport properties in liquid $\mathrm{A}{\mathrm{l}}_{1-x}\mathrm{C}{\mathrm{u}}_x$ alloys, with copper composition $x\le 0.4$, in relation to the applicability of the Stokes-Einstein (SE) equation in these melts. To begin, we find that self-diffusion coefficients and viscosity are composition dependent, while their temperature dependence follows an Arrhenius-type behavior, except for $x=0.4$ at low temperature. Then, we find that the applicability of the SE equation is also composition dependent, and its breakdown in the liquid regime above the liquidus temperature can be related to different local ordering around each species. In this case, we emphasize the difficulty of extracting effective atomic radii from interatomic distances found in liquid phases, but we see a clear correlation between transport properties and local ordering described through the structural entropy approximated by the two-body contribution. We use these findings to reformulate the SE equation within the framework of Rosenfeld's scaling law in terms of partial structural entropies, and we demonstrate that the breakdown of the SE relation can be related to their temperature dependence. Finally, we also use this framework to derive a simple relation between the ratio of the self-diffusivities of the components and the ratio of their partial structural entropies.

The nature of the dynamical quantum phase transition between the many-body localized (MBL) phase and the thermal phase remains an open question, and one line of attack on this problem is to explore this transition numerically in finite-size systems. To maximize the contrast between the MBL phase and the thermal phase in such finite-size systems, we argue one should choose a Floquet model with no local conservation laws and rapid thermalization to “infinite temperature” in the thermal phase. Here we introduce and explore such a Floquet spin chain model and show that standard diagnostics of the MBL-to-thermal transition behave well in this model even at modest sizes. We also introduce a physically motivated space-time correlation function, which peaks at the transition in the Floquet model, but is strongly affected by conservation laws in Hamiltonian models.

Optical excitation of apical oxygen vibrations in $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6+\delta }$ has been shown to enhance its $c$ axis superconducting-phase rigidity, as evidenced by a transient blueshift of the equilibrium interbilayer Josephson plasma resonance. Surprisingly, a transient $c$ axis plasma mode could also be induced above $T_c$ by the same apical oxygen excitation, suggesting light activated superfluid tunneling throughout the pseudogap phase of $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6+\delta }$. However, despite the similarities between the transient plasma mode above $T_c$ and the equilibrium Josephson plasmon, alternative explanations involving high-mobility quasiparticle transport should be considered. Here, we report an extensive study of the relaxation of the light induced plasmon into the equilibrium incoherent phase. These new experiments allow for a critical assessment of the nature of this mode. We determine that the transient plasma relaxes through a collapse of its coherence length rather than its carrier (or superfluid) density. These observations are not easily reconciled with quasiparticle interlayer transport and rather support transient superfluid tunneling as the origin of the light induced interlayer coupling in $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6+\delta }$.

We study Ising antiferromagnets that have nearest-neighbor interactions on multilayer triangular lattices with frustrated ($abc$ and $abab$) stacking, and make comparisons with the unfrustrated ($aaa$) stacking. If interlayer couplings are much weaker than in-plane ones, the paramagnetic phase of models with frustrated stackings has a classical spin-liquid regime at low temperature, in which correlations are strong both within and between planes, but there is no long-range order. We investigate this regime using Monte Carlo simulations and by mapping the spin models to coupled height models, which are treated using renormalization group methods and an analysis of the effects of vortex excitations. The classical spin-liquid regime is parametrically wide at small interlayer coupling in models with frustrated stackings. By contrast, for the unfrustrated stacking there is no extended regime in which interlayer correlations are strong without three-dimensional order.

The conductivity of vortex lattices in multiband superconductors with high concentration of impurities is calculated based on microscopic kinetic theory at temperatures significantly smaller than the critical one. Both the limits of high and low fields are considered, when the magnetic induction is close to or much smaller than the critical field strength $H_{c2}$, respectively. It is shown that in contrast to single-band superconductors, the resistive properties are not universal but depend on the pairing constants and ratios of diffusivities in different bands. The low-field magnetoresistance can strongly exceed the Bardeen-Stephen estimation in a quantitative agreement with experimental data for the two-band superconductor ${\mathrm{MgB}}_2$.

We present the results of a high-resolution valence-band photoemission spectroscopic study of ${\mathrm{SmB}}_6$ which shows evidence for a V-shaped density of states of surface origin within the bulk gap. The spectroscopy data are interpreted in terms of the existence of heavy $4f$ surface states, which may be useful in resolving the controversy concerning the disparate surface Fermi-surface velocities observed in experiments. Most importantly, we find that the temperature dependence of the valence-band spectrum indicates that a small feature appears at a binding energy of about $-9$ meV at low temperatures. We attribute this feature to a resonance caused by the spin-exciton scattering in ${\mathrm{SmB}}_6$ which destroys the protection of surface states due to time-reversal invariance and spin-momentum locking. The existence of a low-energy spin exciton may be responsible for the scattering, which suppresses the formation of coherent surface quasiparticles and the appearance of the saturation of the resistivity to temperatures much lower than the coherence temperature associated with the opening of the bulk gap.

We study nonlocal correlations in a three-orbital Hubbard model defined on an extended one-dimensional chain using determinant quantum Monte Carlo and density matrix renormalization group methods. We focus on a parameter regime with robust Hund's coupling, which produces an orbital selective Mott phase (OSMP) at intermediate values of the Hubbard $U$, as well as an orbitally ordered ferromagnetic insulating state at stronger coupling. An examination of the orbital- and spin-correlation functions indicates that the orbital ordering occurs before the onset of magnetic correlations in this parameter regime as a function of temperature. In the OSMP, we find that the self-energy for the itinerant electrons is momentum dependent, indicating a degree of nonlocal correlations while the localized electrons have largely momentum independent self-energies. These nonlocal correlations also produce relative shifts of the holelike and electronlike bands within our model. The overall momentum dependence of these quantities is strongly suppressed in the orbitally ordered insulating phase.