We have performed ${}^{31}\mathrm{P}$-NMR measurements on single-crystalline CeRuPO under pressure in order to understand the variation in magnetic character against pressure. The nuclear magnetic resonance (NMR) spectra for $\text{H}\perp c$ and $\text{H}\parallel c$ at 2.15 GPa split below the ordered temperature, which is microscopic evidence of the change in the magnetic ground state from the ferromagnetic state at ambient pressure to the antiferromagnetic (AFM) state under pressure. The analysis of NMR spectra suggests that the magnetic structure in the AFM state is the stripe-type AFM state with the AFM moment $m_{\mathrm{AFM}}\perp c$ axis, and it changes with a magnetic field perpendicular to the $c\mathrm{axis}$. In addition, the dimensionality of magnetic correlations in the spin and $k$ space is estimated. We reveal that three-dimensional magnetic correlations in CeRuPO are robust against pressure, which is quite different from the suppression of the magnetic correlations along the $c\mathrm{axis}$ by Fe substitution in $\text{Ce}\left({\mathrm{Ru}}_{1-x}{\mathrm{Fe}}_x\right)\text{PO}$.

We present a microscopic theory for the current through a tunnel Josephson junction coupled to a nonlinear environment, which consists of an Andreev two-level system coupled to a harmonic oscillator. It models a recent experiment [Bretheau, Girit, Pothier, Esteve, and Urbina, Nature (London) 499, 312 (2013)] on photon spectroscopy of Andreev bound states in a superconducting atomic-size contact. We find the eigenenergies and eigenstates of the environment and derive the current through the junction due to inelastic Cooper pair tunneling. The current-voltage characteristic reveals the transitions between the Andreev bound states, the excitation of the harmonic mode that hybridizes with the Andreev bound states, as well as multiphoton processes. The calculated spectra are in fair agreement with the experimental data.

We report the results of a comprehensive study of the critical dynamics of the classical perovskite antiferroelectric ${\mathrm{PbZrO}}_3$. The combination of inelastic x-ray and diffuse x-ray scattering techniques and Brillouin light scattering was used. It is found that the dispersion of the transverse acoustic (TA) phonons is strongly anisotropic. The dispersion curve of the in-plane polarized TA phonons propagating in [1 1 0] direction demonstrates pronounced softening. Slowing down of the excitations at $R$ point is found, it is manifested in growing of the central peak. This slowing down is too weak to be considered as a primary origin of the corresponding order parameter. Obtained results are treated in terms of TA-transverse optic (TO) flexoelectric mode coupling. It is demonstrated that the structural phase transformation in ${\mathrm{PbZrO}}_3$ can be considered as the result of the only intrinsic instability associated with the ferroelectric soft mode.

The spin structure of an axial next-nearest-neighbor Ising (ANNNI) model in two dimensions (2D) is a renewed problem because different Monte Carlo (MC) simulation methods predicted different spin orderings. The usual equilibrium simulation predicts the occurrence of a floating incommensurate (IC) Kosterlitz-Thouless (KT) type phase, which never emerges in non-equilibrium relaxation (NER) simulations. In this paper, we first examine previously published results of both methods, and then investigate a higher transition temperature $T_{c1}$ between the IC and paramagnetic phases. In the usual equilibrium simulation, we calculate the chain magnetization on larger lattices (up to $512\times 512$ sites) and estimate $T_{c1}\approx 1.16J$ with frustration ratio $\kappa (\equiv -J_2/J_1)=0.6$. We examine the nature of the phase transition in terms of the Binder ratio $g_L$ of spin overlap functions and the correlation-length ratio $\xi /L$. In the NER simulation, we observe the spin dynamics in equilibrium states by means of an autocorrelation function and also observe the chain magnetization relaxations from the ground and disordered states. These quantities exhibit an algebraic decay at $T\lesssim 1.17J$. We conclude that the two-dimensional ANNNI model actually admits an IC phase transition of the KT type.

We present an inelastic neutron scattering study on single-crystalline LiFeAs devoted to the characterization of the incommensurate antiferromagnetic fluctuations at $\mathbf{Q}=(0.5\pm \delta ,0.5\mp \delta ,q_l)$. Time-of-flight measurements show the presence of these magnetic fluctuations up to an energy transfer of 60 meV, while polarized neutrons in combination with a longitudinal polarization analysis on a triple-axis spectrometer prove the pure magnetic origin of this signal. The normalization of the magnetic scattering to an absolute scale yields that magnetic fluctuations in LiFeAs are by a factor of 8 weaker than the resonance signal in nearly optimally Co-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, although a factor of 2 is recovered due to the split peaks owing to the incommensurability. The longitudinal polarization analysis indicates weak spin-space anisotropy with a slightly stronger out-of-plane component between 6 and 12 meV. Furthermore, our data suggest a fine structure of the magnetic signal most likely arising from superposing nesting vectors.

We develop a low-order conserving approximation for the interacting resonant-level model (IRLM), and apply it to (i) thermal equilibrium, (ii) nonequilibrium steady state, and (iii) nonequilibrium quench dynamics. Thermal equilibrium is first used to carefully gauge the quality of the approximation by comparing the results with other well-studied methods, and finding good agreement for small values of the interaction. We analytically show that the power-law exponent of the renormalized level width usually derived using renormalization group approaches can also be correctly obtained in our approach in the weak interaction limit. A closed expression for the nonequilibrium steady-state current is derived and analytically and numerically evaluated. We find a negative differential conductance at large voltages, and the exponent of the power-law suppression of the steady-state current is calculated analytically at zero temperature. The response of the system to quenches is investigated for a single lead as well as for two-lead setup at finite voltage bias at particle-hole symmetry using a self-consistent two-times Keldysh Green function approach, and results are presented for the time-dependent current for different bias and contact interaction strength.

We describe the braiding statistics of topological twist defects in fractional quantum Hall (FQH) states. In particular, we study Abelian bosonic $\left(mmn\right)$ FQH states with bilayer symmetries. Twist defects are staircases that lift orbiting quasiparticles from one layer to another. These point defects exhibit semiclassical non-Abelian characteristics. We develop a procedure to evaluate consistent basis transformations ($F$ symbols) of the defect fusion theory. Unlike quantum anyonic excitations of a topological phase, the exchange and braiding statistics of twist defects is not characterized by modular transformations, but instead a subcollection known as congruent transformations. We also characterize the projective nature of unitary braiding operations and relate it to the global ${\mathbb{Z}}_2$ bilayer symmetry.

A theoretical investigation is carried out on the effect of Coulomb interactions on the optical properties of monolayer transition-metal dichalcogenides (TMDCs) by solving the Bethe-Salpeter equation under the $k·p$ approximation. The calculated absorption spectrum of TMDCs exhibits strong exciton features, and a peculiar optical selection rule is found for circularly polarized light, in agreement with previous theoretical calculations and experimental observations. The exciton-exciton annihilation rate is evaluated by considering the Auger recombination process, which gives rise to the nonlinear optical response. The calculation results agree well with the experimental data, and confirm strong exciton-exciton interaction effects in monolayer TMDCs.

We put forward a theory of the weak localization in two-dimensional graphene layers which explains an experimentally observable transition between positive and negative magnetoresistance. Calculations are performed for the whole range of a classically weak magnetic field with an account on intervalley transitions. Contribution to the quantum correction which stems from closed trajectories with few scatterers is carefully taken into account. We show that intervalley transitions lead not only to the transition from weak antilocalization to the weak localization, but also to the nonmonotonous dependence of the conductivity on the magnetic field.

The surface band bending and electronic properties of ${\mathrm{SnO}}_2$(101) films grown on $r$-sapphire by plasma-assisted molecular beam epitaxy have been studied by Fourier-transform infrared spectroscopy (FTIR), x-ray photoemission spectroscopy (XPS), Hall effect, and electrochemical capacitance-voltage measurements. The XPS results were correlated with density functional theory calculation of the partial density of states in the valence-band and semicore levels. Good agreement was found between theory and experiment with a small offset of the Sn $4d$ levels. Homogeneous Sb-doped ${\mathrm{SnO}}_2$ films allowed for the calculation of the bulk Fermi level with respect to the conduction-band minimum within the $\mathbf{k}·\mathbf{p}$ carrier statistics model. The band bending and carrier concentration as a function of depth were obtained from the capacitance-voltage characteristics and model space charge calculations of the Mott-Schottky plots at the surface of Sb-doped ${\mathrm{SnO}}_2$ films. It was quantitatively demonstrated that ${\mathrm{SnO}}_2$ films have downward band bending and surface electron accumulation. The surface band bending, unoccupied donor surface-state density, and width of the accumulation region all decrease with increasing Sb concentration.

The band structure and intra- and interband scattering processes of the electrons at the surface of a bismuth bilayer on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ have been experimentally investigated by low-temperature Fourier-transform scanning tunneling spectroscopy. The observed complex quasiparticle interference patterns are compared to a simulation based on the spin-dependent joint density of states approach using the surface-localized spectral function calculated from first principles as the only input. Thereby, the origin of the quasiparticle interferences can be traced back to intraband scattering in the bismuth-bilayer valence band and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ conduction band and to interband scattering between the two-dimensional topological state and the bismuth-bilayer valence band. The investigation reveals that the bilayer band gap, which is predicted to host one-dimensional topological states at the edges of the bilayer, is pushed several hundred meV above the Fermi level. This result is rationalized by an electron transfer from the bilayer to ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ which also leads to a two-dimensional electron state in the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ conduction band with a strong Rashba spin splitting, coexisting with the topological state and bilayer valence band.

In a recent paper, Xie et al. [Phys. Rev. B 88, 235128 (2013)], report that the density-functional theory (DFT) with the so-called DFT plus Hubbard $U$ $(\mathrm{DFT}+U)$ modification improves energetics, volumes, and formation enthalpies over the standard form of DFT for uranium metal and U-Zr alloys. Also, spin-orbit coupling (SOC) was argued to advance the aforementioned properties in these systems. We demonstrate, contrarily, that neither the Hubbard $U$ approach nor SOC is necessary for a correct description of uranium metal and U-Zr alloys. We further illustrate that the combination of $\mathrm{DFT}+U$ and SOC in the projector augmented-wave calculations by Xie et al. results in unrealistically large volume expansions, particularly for γ-U, in stark contrast to all previous calculations for elemental uranium. This in turn may also explain why the $\mathrm{DFT}+U$ with SOC model predicts negative enthalpy of mixing in the U-Zr alloy system contradicting conventional DFT as well as one of the main features of the experimental U-Zr phase diagram. The assertion by Xie et al. that $\mathrm{DFT}+U$ is an improvement over DFT for these systems is illustrated to be incorrect.

The recent discovery of a topological phase transition in IV-VI narrow-gap semiconductors has revitalized the decades-old interest in the bulk band inversion occurring in these materials. Here we systematically study the (001) surface states of ${\text{Pb}}_{1-x}{\text{Sn}}_x\text{Se}$ mixed crystals by means of angle-resolved photoelectron spectroscopy in the parameter space $0\le x\le 0.37$ and $300\text{K}\ge T\ge 9\text{K}$. Using the surface-state observations, we monitor directly the topological phase transition in this solid solution and gain valuable information on the evolution of the underlying fundamental band gap of the system. In contrast to common model expectations, the band-gap evolution appears to be nonlinear as a function of the studied parameters, resulting in the measuring of a discontinuous band-inversion process. This finding signifies that the anticipated gapless bulk state is in fact not a stable configuration and that the topological phase transition therefore exhibits features akin to a first-order transition.

The resistivity of vanadium dioxide $\left({\mathrm{VO}}_2\right)$ decreased by over one order of magnitude upon localized illumination with x rays at room temperature. Despite this reduction, the structure remained in the monoclinic phase and had no signature of the high-temperature tetragonal phase that is usually associated with the lower resistance. Once illumination ceased, relaxation to the insulating state took tens of hours near room temperature. However, a full recovery of the insulating state was achieved within minutes by thermal cycling. We show that this behavior is consistent with random local-potential fluctuations and random distribution of discrete recombination sites used to model residual photoconductivity.

We study the Fermi-surface topological transition of the pocket-opening type in a two-dimensional Fermi liquid. We find that the paramagnetic fluctuations in an interacting Fermi liquid typically drive the transition first order at zero temperature. We first gain insight from a calculation using second-order perturbation theory in the self-energy. This is valid for weak interaction and far from instabilities. We then extend the results to stronger interaction, using the self-consistent fluctuation approximation. Experimental signatures are given in light of our results.

We consider scanning gate microscopy imaging of the double slit interference for a pair of quantum point contacts (QPCs) defined within the two-dimensional electron gas. The interference is clearly present in the scattered electron wave functions for each of the incident subbands. Nevertheless, we find that the interference is generally missing in the experimentally accessible conductance maps for many incident subbands. We explain this finding on the basis of the Landauer approach. A setup geometry allowing for observation of the double slit interference by scanning gate microscopy is proposed.

We investigate the origin of the fast recombination dynamics of bound and free excitons in GaN nanowire ensembles by temperature-dependent photoluminescence spectroscopy using both continuous-wave and pulsed excitation. The exciton recombination in the present GaN nanowires is dominated by a nonradiative channel between 10 and 300 K. Furthermore, bound and free excitons in GaN NWs are strongly coupled even at low temperatures resulting in a common lifetime of these states. By solving the rate equations for a coupled two-level system, we show that one cannot, in practice, distinguish whether the nonradiative decay occurs directly via the bound or indirectly via the free state. The nanowire surface and coalescence-induced dislocations appear to be the most obvious candidates for nonradiative defects, and we thus compare the exciton decay times measured for a variety of GaN nanowire ensembles with different surface-to-volume ratio and coalescence degrees. The data are found to exhibit no correlation with either of these parameters, i.e., the dominating nonradiative channel in the GaN nanowires under investigation is neither related to the nanowire surface, nor to coalescence-induced defects. Hence we conclude that nonradiative point defects are the origin of the fast recombination dynamics of excitons in GaN nanowires.

The optical absorption properties of periodically patterned graphene plasmonic resonators are studied experimentally as the graphene sheet is placed near a metallic reflector. By varying the size and carrier density of the graphene, the parameters for achieving a surface impedance closely matched to free-space $(Z_0=377\Omega )$ are determined and shown to result in 24.5% total optical absorption in the graphene sheet. Theoretical analysis shows that complete absorption is achievable with higher doping or lower loss. This geometry, known as a Salisbury screen, provides an efficient means of light coupling to the highly confined graphene plasmonic modes for future optoelectronic applications.