The effect of graded anisotropy on static and dynamic magnetic properties of ${\mathrm{Ar}}^{+}$-irradiated FePt films has been investigated by static magnetometry, magnetic force microscopy, and Brillouin light scattering from thermally excited spin waves. A gradual variation of magnetic anisotropy with film thickness was obtained by ${\mathrm{Ar}}^{+}$ irradiation. The irradiation incidence angle influences the anisotropy profile: on decreasing $\alpha$, a decreasing thickness of the hard $\mathrm{L}{1}_0$ phase and an increasing thickness of the soft A1 phase were obtained. Accordingly, the zero-field spin-wave frequency gap was found to decrease. In the sample with the highest soft-phase thickness the spin-wave frequency gap takes a substantial value (${\nu }_0\approx 6$ GHz), which could be reproduced assuming the presence of a nonzero “rotatable” anisotropy (i.e., any direction in the film plane can be established as the easy axis by the application of a saturating magnetic field along this direction). The hypothesis is supported by both magnetometry and magnetic force microscopy data.

In the last few years a lot of exotic and anomalous topological phases were constructed by proliferating the vortexlike topological defects on the surface of the $3d$ topological insulator (TI) [Fidkowski et al., Phys. Rev. X 3, 041016 (2013); Chen et al., Phys. Rev. B 89, 165132 (2014); Bonderson et al., J. Stat. Mech. (2013) P09016; Wang et al., Phys. Rev. B 88, 115137 (2013); Metlitski et al., Phys. Rev. B 92, 125111 (2015)]. In this work, rather than considering topological phases at the boundary, we will study quantum critical points driven by vortexlike topological defects. In general, we will discuss a $(2+1)d$ quantum phase transition described by the following field theory: $\mathcal{L}=\overline{\psi }{\gamma }_{\mu }({\partial }_{\mu }-i{a}_{\mu })\psi +|({\partial }_{\mu }-ik{a}_{\mu }){\varphi |}^2+{r\left|\varphi \right|}^2+g{\left|\varphi \right|}^4$, with tuning parameter $r$, arbitrary integer $k$, Dirac fermion $\psi$, and complex scalar bosonic field $\varphi$, which both couple to the same $(2+1)d$ dynamical noncompact U(1) gauge field $a_{\mu }$. The physical meaning of these quantities/fields will be explained in the text. Making use of the new duality formalism developed in [Metlitski  et al., Phys. Rev. B 93, 245151 (2016); Wang et al., Phys. Rev. X 5, 041031 (2015); Wang et al., Phys. Rev. B 93, 085110 (2016); D. T. Son, Phys. Rev. X 5, 031027 (2015)], we demonstrate that this quantum critical point has a quasi-self-dual nature. And at this quantum critical point, various universal quantities such as the electrical conductivity and scaling dimension of gauge-invariant operators, can be calculated systematically through a $1/k^2$ expansion, based on the observation that the limit $k\to +\infty$ corresponds to an ordinary $3d XY$ transition.

We study the effect of surface-induced Dzyaloshinskii-Moriya interaction (DMI) on the ferromagnetic resonance (FMR) spectrum of thickness-modulated one-dimensional magnonic crystals and isolated stripes. The DMI is found to substantially increases the intensity of absorption peaks and shifts the frequencies of the laterally quantized modes. The role of the DMI is determined by analyzing the amplitude and phase distributions of dynamic magnetic excitations calculated with frequency- and time-domain calculation methods. We propose experimentally realizable magnonic crystals and confined structures with multiple FMR absorption peaks. The frequency or magnetic field separation between FMR lines is exploited to propose a method for estimation of the DMI strength.

The x-ray absorption spectra at the Bi-$L_1$ and S-$K$ edges of ${\mathrm{LaO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$ show a decrease of spectral weight close to the absorption thresholds upon F substitution for O, indicating a reduction of the unoccupied Bi $6p$ and S $3p$ states, as expected for electron doping. This doping-induced spectral weight transfer at the S-$K$ edge is similar to the electron-doped cuprate superconductor, ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$, but the polarization-dependent XAS spectra of ${\mathrm{LaO}}_{0.54}{\mathrm{F}}_{0.46}{\mathrm{BiS}}_2$ at the S-$K$ edge is very different from those of ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ at the O-$K$ edge, reflecting different constitution of valence electrons. We found that one-electron band structure calculations cannot describe our experimental S-$K$ XAS spectra of ${\mathrm{LaO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$, suggesting possible correlation effects for $p$ electrons, which is important in transition metal compounds with the dominating $d$ electrons.

Over the past few years, various valleytronic properties have been predicted in graphene like $p$-electron honeycomb structures with staggered sublattice onsite potential; however, most of these studies are based on model calculations, and direct studies on real materials are still rare. In this paper, we propose an experimentally feasible structure based on an epitaxial growth of In/Tl honeycomb lattice on a semihydrogenated graphene/diamond thin film substrate. Our first-principles calculations reveal that due to different absorption sites of the two In/Tl sublattices, the inversion symmetry and quasi-two-dimensional inversion symmetry are broken, yielding inequivalent band gaps at the $K$ and $K^{\prime }$ points of the Brillouin zone. In addition, compared with previous graphene-based models, In/Tl also has a large spin-orbit coupling (SOC). In this regard, such a $p$-electron based system is a good candidate for valley selective circular polarization light absorption. Simultaneously, by integrating its Berry curvatures, we reveal a large SOC induced anomalous Nernst effect, which supports a pure spin current at its Fermi level.

We investigate the logarithmic negativity in strongly disordered spin chains in the random-singlet phase. We focus on the spin-$\frac{1}{2}$ random Heisenberg chain and the random $XX$ chain. We find that for two arbitrary intervals, the disorder-averaged negativity and the mutual information are proportional to the number of singlets shared between the two intervals. Using the strong-disorder renormalization group (SDRG), we prove that the negativity of two adjacent intervals grows logarithmically with the intervals' length. In particular, the scaling behavior is the same as in conformal field theory, but with a different prefactor. For two disjoint intervals the negativity is given by a universal simple function of the cross ratio, reflecting scale invariance. As a function of the distance of the two intervals, the negativity decays algebraically in contrast with the exponential behavior in clean models. We confirm our predictions using a numerical implementation of the SDRG method. Finally, we also implement density matrix renormalization group simulations for the negativity in open spin chains. The chains accessible in the presence of strong disorder are not sufficiently long to provide a reliable confirmation of the SDRG results.

We study the cyclotron resonance in the electron-hole joint Fermi surface of a type-II Weyl semimetal. In magnetic field, the electron and hole pockets touching at the Weyl node are hybridized to form quantized Landau levels corresponding to semiclassical 8-shaped orbits. We calculate the dynamical conductivities for the electric fields oscillating in $x$ and $y$ directions and find that the resonant frequencies in $x$ and $y$ differ by a factor of two, reflecting the figure-of-eight electron motion in real space. The peculiar anisotropy in the cyclotron resonance serves as a unique characteristic of the dumbbell-like Fermi surface.

We report the steady state output polarization characteristics of GaN-based microcavity polariton lasers operated with electrical injection at room temperature. The output is unpolarized below the nonlinear threshold injection current and is linearly polarized above it with a maximum degree of polarization of $\sim 22\%$. The results have been analyzed theoretically, and the calculated results are in agreement with the measured data. We have also measured the polarization-resolved output light current characteristics, wherein a distinct lowering of the non-linear threshold is observed in one device. This is interpreted in terms of spatially inhomogeneous lifting of degeneracy and polarization splitting in the microcavity.

Group-IV monochalcogenides are a family of two-dimensional puckered materials with an orthorhombic structure that is comprised of polar layers. In this article, we use first principles calculations to show the multistability of monolayer SnS and GeSe, two prototype materials where the direction of the puckering can be switched by application of tensile stress or electric field. Furthermore, the two inequivalent valleys in momentum space, which are dictated by the puckering orientation, can be excited selectively using linearly polarized light, and this provides an additional tool to identify the polarization direction. Our findings suggest that SnS and GeSe monolayers may have observable ferroelectricity and multistability, with potential applications in information storage.

We present an experimental study of the microwave power and the linear polarization angle dependence of the microwave-induced magnetoresistance oscillations in the high-mobility GaAs/AlGaAs two-dimensional electron system. Experimental results show the sinusoidal dependence of the oscillatory magnetoresistance extrema as a function of the polarization angle. Yet, as the microwave power increases, the angular dependence includes additional harmonic content, and it begins to resemble the absolute value of the cosine function. We present a theory to explain such peculiar behavior.

Using the Matsubara Green's function formalism we calculate the temperature dependence of spin Hall and spin Nernst conductivities of a two-dimensional electron gas with Rashba spin-orbit interaction in the linear response regime. In the case of the spin Nernst effect we also include the contribution from spin-resolved orbital magnetization, which assures correct behavior of the spin Nernst conductivity in the zero-temperature limit. Analytical formulas for the spin Hall and spin Nernst conductivities are derived in some specific situations. Using the Ioffe-Regel localization criterion, we have also estimated the range of parameters where the calculated results for the spin Hall and spin Nernst conductivities are applicable. Analytical results show that the vertex correction totally suppresses the spin Hall conductivity at arbitrary temperature. The spin Nernst conductivity, in turn, vanishes at $T=0$ when the orbital contribution is taken into account, but generally is nonzero at finite temperatures.

We consider a periodic array of graphene nanoribbons under the action of a strong dc electric field $E_0$ and an external electromagnetic excitation with the frequency $\omega$ and the lateral wave vector $q$. Solving the quasiclassical Boltzmann kinetic equation and calculating the surface dynamic conductivity ${\sigma }_{2\text{D}}(q,\omega ,E_0)$ and the absorption coefficient of such a system we show that the real part of the conductivity and the absorption coefficient may become negative under certain conditions. Physically this corresponds to the amplification of the electromagnetic waves at the expense of the energy of the direct current source. The results are discussed in connection with experiments on the surface acoustic waves and on the Smith-Purcell-type graphene-based terahertz emitter.

Natural oxidation is a common degradation mechanism of both mechanical and electronic properties for most of the new two-dimensional materials. From another perspective, controlled oxidation is an option for tuning material properties, thereby expanding possibilities for real-world applications. Understanding the electronic structure modifications induced by oxidation is highly desirable for new materials like monolayer GeSe, which is a new candidate for near-infrared photodetectors. By means of first-principles calculations, we study the influence of oxygen defects on the structure and electronic properties of the single-layer GeSe. Our calculations show that oxidation is an exothermic process, and it is nucleated in the germanium sites. Oxidation can cause severe local deformations on the monolayer GeSe structure and introduces a deep state in the band gap or a shallow state near the conduction band edge. Furthermore, oxidation increases the band gap by up to 23%, and may induce direct to indirect band-gap transitions. These results suggest that natural or intentionally induced monolayer GeSe oxidation can be a source of new optoelectronic properties, adding another important building block to the two-dimensional layered materials.

We study photonic signatures of symmetry broken and topological phases in a driven, dissipative circuit QED realization of spin-1/2 chains. Specifically, we consider the transverse-field XY model and a dual model with three-spin interactions. The former has a ferromagnetic and a paramagnetic phase, while the latter features, in addition, a symmetry protected topological phase. Using the method of third quantization, we calculate the nonequilibrium steady state of the open spin chains for arbitrary system sizes and temperatures. We find that the bilocal correlation function of the spins at both ends of the chain provides a sensitive measure for both symmetry-breaking and topological phase transitions of the systems, but no universal means to distinguish between the two types of transitions. Both models have equivalent representations in terms of free Majorana fermions, which host zero, one and two topological Majorana end modes in the paramagnetic, ferromagnetic, and symmetry protected topological phases, respectively. The correlation function we study retains its bilocal character in the fermionic representation, so that our results are equally applicable to the fermionic models in their own right. We propose a photonic realization of the dissipative transverse-field XY model in a tunable setup, where an array of superconducting transmon qubits is coupled at both ends to a photonic microwave circuit.

Absolute binding energies of core electrons in molecules and bulk materials can be efficiently calculated by spin paired density-function theory employing a $\mathrm{\Delta }$-Kohn-Sham $\left(\mathrm{\Delta }\mathrm{KS}\right)$ scheme corrected by offsets that are highly transferable. These offsets depend on core level and atomic species and can be determined by comparing $\mathrm{\Delta }\mathrm{KS}$ energies to experimental molecular x-ray photoelectron spectra. We demonstrate the correct prediction of absolute and relative binding energies on a wide range of molecules, metals, and insulators.

We measured the optical conductivity $\tilde{\sigma }\left(\omega \right)$ spectra caused by photocarriers in anatase $\mathrm{Ti}{\mathrm{O}}_2$ by applying ultraviolet-pump terahertz-probe spectroscopy on epitaxially grown thin films. Drude model analyses of $\tilde{\sigma }\left(\omega \right)$ spectra reveal that, in the terahertz region, the reduced mass ($\widehat{m}$) of the carriers is greater and their damping constant ($\gamma$) is smaller than those in the infrared region. With regard to the extended Drude model, these differences can be attributed to the frequency dependence of $\widehat{m}$ and $\gamma$ originating from carrier-phonon scatterings. Furthermore, the interaction between carriers and Ti-O stretching modes in the $500–1000-\mathrm{c}{\mathrm{m}}^{-1}$ region strongly influences the intrinsic carrier scattering mechanism in anatase $\mathrm{Ti}{\mathrm{O}}_2$.

Topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ has shown a number of interesting physical properties. Doping ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with copper or strontium has been demonstrated to make the material superconducting and potentially even a topological superconductor. The recent discovery of superconducting niobium-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ reveals an exciting new physical phenomenon, the coexistence of superconductivity and magnetic ordering, as well as signatures of an odd-parity $p$-wave superconducting order. To understand this new phenomenon, a detailed knowledge of the electronic structure is needed. We present an observation of quantum oscillations in the magnetization (the de Haas-van Alphen effect) of Nb-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. In the fully superconducting crystal, two distinct orbits are observed, in sharp contrast to ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, Cu-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, and Sr-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. The multiple frequencies observed in our quantum oscillations, combined with our electrical transport studies, indicate the multi-orbit nature of the electronic state of Nb-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$.

Motivated by a recent experimental report on in-plane anisotropic resistivity in the double-striped antiferromagnetic phase of FeTe, we theoretically calculate in-plane resistivity by applying a memory function approach to the ordered phase. We find that the resistivity is larger along an antiferromagnetically ordered direction than along a ferromagnetically ordered one, consistent with experimental observation. The anisotropic results are mainly contributed from Drude weight, whose behavior is attributed to Fermi surface topology of the ordered phase.

Understanding optical conductivity data in the optimally doped cuprates in the framework of quantum criticality requires a strongly coupled quantum critical metal which violates hyperscaling. In the simplest scaling framework, hyperscaling violation can be characterized by a single nonzero exponent $\theta$, so that in a spatially isotropic state in $d$ spatial dimensions, the specific heat scales with temperature as $T^{(d-\theta )/z}$, and the optical conductivity scales with frequency as ${\omega }^{(d-\theta -2)/z}$ for $\omega \gg T$, where $z$ is the dynamic critical exponent defined by the scaling of the fermion response function transverse to the Fermi surface. We study the Ising-nematic critical point, using the controlled dimensional regularization method proposed by Dalidovich and Lee [Phys. Rev. B 88, 245106 (2013)]. We find that hyperscaling is violated, with $\theta =1$ in $d=2$. We expect that similar results apply to Fermi surfaces coupled to gauge fields in $d=2$.