Symmetry-protected topological phase is one type of nontrivial quantum disordered many-body state of matter. In this paper we study one class of symmetry-protected topological phases in two-dimensional space with both $\mathrm{PSU}\left(N\right)$ and time-reversal symmetry. These states can be described by a principal chiral model with a topological $\Theta$ term. As long as the time-reversal symmetry and $\mathrm{PSU}\left(N\right)$ symmetry are both preserved, the (1+1)-dimensional boundary of this system must be either gapless or degenerate. We will also construct a wave function of a spin-1 system on the honeycomb lattice, which is a candidate for the symmetry-protected topological phase with both SO(3) and time-reversal symmetry.

Magnetic impurities embedded in inert solids can exhibit long coherence times and interact with one another via their intrinsic anisotropic dipolar interaction. We argue that, as a consequence of these properties, disordered ensembles of magnetic impurities provide an effective platform for realizing a controllable, tunable version of the dipolar quantum spin glass seen in LiHo${}_x$Y${}_{1-x}$F${}_4$. Specifically, we propose and analyze a system composed of dysprosium atoms embedded in solid helium. We describe the phase diagram of the system and discuss the realizability and detectability of the quantum spin glass and antiglass phases.

A collective-variable approach for the study of nonlinear dynamics of magnetic textures in planar nanomagnets is proposed. The variables are just arbitrary parameters (complex or real) in the specified analytical function of a complex variable describing the texture in motion. Starting with such a function, a formal procedure is outlined, allowing a (nonlinear) system of differential equations of motion to be obtained for the variables. The resulting equations are equivalent to Landau-Lifshitz-Gilbert dynamics as far as the definition of collective variables allows it. Apart from the collective-variable specification, the procedure does not involve any additional assumptions (such as translational invariance or steady-state motion). As an example, the equations of weakly nonlinear motion of a magnetic vortex are derived and solved analytically. A simple formula for the dependence of the vortex precession frequency on its amplitude is derived. The results are verified against special cases from the literature and agree quantitatively with experiments and simulations.

We present a comprehensive study of the phonon dispersion in an underdoped, superconducting Ca${}_{2-x}$CuO${}_2$Cl${}_2$ crystal. We interpret the results using lattice dynamical calculations based on a shell model, and we compare the results to other hole-doped cuprates, in particular, to the ones isomorphic to La${}_{2-x}$Sr${}_x$CuO${}_4$ (LSCO). We found that an anomalous dip in the Cu-O bond stretching dispersion develops in oxychlorides with a simultaneous marked broadening of the mode. The broadening is maximum at $\approx$$[\pi /(2a\left)00\right]$, which corresponds to the charge-modulation propagation vector. Our analysis also suggests that screening effects in calculations may cause an apparent cosine-shaped bending of the Cu-O bond-stretching dispersion along both the ($q$ 0 0) and ($q$ $q$ 0) directions, which is not observed on the data close to optimal doping. This observation suggests that the discrepancy between experimental data and ab initio calculations on this mode originates from an overestimation of the doping effects on the mode.

The thermal conductivity of a doped topological crystalline insulator, Sn${}_{0.6}$In${}_{0.4}$Te superconducting single crystal with $T_c=4.1$ K, was measured down to 50 mK. It is found that the residual linear term ${\kappa }_0/T$ is negligible in zero magnetic field. The ${\kappa }_0/T$ shows a slow field dependence at low magnetic field. These results suggest that the superconducting gap is nodeless, unless there exist point nodes with directions perpendicular to the heat current. Due to its high-symmetry fcc crystal structure of Sn${}_{0.6}$In${}_{0.4}$Te, however, such point nodes can be excluded. Therefore we demonstrate that this topological superconductor candidate has a full superconducting gap in the bulk. It is likely the unconventional odd-parity $A_{1u}$ state which supports a surface Andreev bound state.

The switching of rare-earth-based ferrimagnets triggered by thermal excitation is investigated on the basis of an atomistic spin model beyond the rigid-spin approximation, distinguishing magnetic moments due to electrons in $d$ and $f$ orbitals of the rare earth. It is shown that after excitation of the conduction electrons a transient ferromagneticlike state follows from a dissipationless spin dynamics where energy and angular momentum are distributed between the two sublattices. The final relaxation can then lead to a new state with the magnetization switched with respect to the initial state. The time scale of the switching event is to a large extent determined by the exchange interaction between the two sublattices.

Precipitate hardening is commonly used in materials science to control strength by acting on the number density, size distribution, and shape of solute precipitates in the hardened matrix. The Fe-Cu system has attracted much attention over the last several decades due to its technological importance as a model alloy for Cu steels. In spite of these efforts several aspects of its phase diagram remain unexplained. Here we use atomistic simulations to characterize the polymorphic phase diagram of Cu precipitates in body-centered cubic (BCC) Fe and establish a consistent link between their thermodynamic and mechanical properties in terms of thermal stability, shape, and strength. The size at which Cu precipitates transform from BCC to a close-packed 9R structure is found to be strongly temperature dependent, ranging from approximately 4 nm in diameter ($\sim 2700\text{atoms}$) at 200 K to about 8 nm ($\sim 22800\text{atoms}$) at 700 K. These numbers are in very good agreement with the interpretation of experimental data given Monzen et al. [Philos. Mag. A 80, 711 (2000)]. The strong temperature dependence originates from the entropic stabilization of BCC Cu, which is mechanically unstable as a bulk phase. While at high temperatures the transition exhibits first-order characteristics, the hysteresis, and thus the nucleation barrier, vanish at temperatures below approximately 300 K. This behavior is explained in terms of the mutual cancellation of the energy differences between core and shell (wetting layer) regions of BCC and 9R nanoprecipitates, respectively. The proposed mechanism is not specific for the Fe-Cu system but could generally be observed in immiscible systems, whenever the minority component is unstable in the lattice structure of the host matrix. Finally, we also study the interaction of precipitates with screw dislocations as a function of both structure and orientation. The results provide a coherent picture of precipitate strength that unifies previous calculations and experimental observations.

Charge transport through a small topological superconducting island in contact with a normal and a superconducting electrode occurs through a cycle that involves coherent oscillations of Cooper pairs and tunneling in/out the normal electrode through a Majorana bound state, the Josephson-Majorana cycle. We illustrate this mechanism by studying the current-voltage characteristics of a superconductor-topological superconductor-normal metal single-electron transistor. At low bias and temperature the Josephson-Majorana cycle is the dominant mechanism for transport. We discuss a three-terminal configuration where the nonlocal character of the Majorana bound states is emergent.

We develop a model for the gliding of dislocations and plasticity in solid ${}^4$He. This model takes into account the Peierls barrier, multiplication and interaction of dislocations, as well as classical thermally and mechanically activated processes leading to dislocation glide. We specifically examine the dc stress-strain curve and how it is affected by temperature, strain rate, and dislocation density. As a function of temperature and shear strain, we observe plastic deformation and discuss how this may be related to the experimental observation of elastic anomalies in solid hcp ${}^4$He that have been discussed in connection with the possibility of supersolidity or giant plasticity. Our theory gives several predictions for the dc stress strain curves, for example, the yield point and the change in the work-hardening rate and plastic dissipation peak, that can be compared directly to constant strain-rate experiments and thus provide bounds on model parameters.

Magnetophotocurrent with possible spin polarization is generated at room temperature by breaking the time-reversal symmetry of bulk spin-split bands. The band gap excitation in a layered polar semiconductor BiTeI with a large bulk Rashba spin splitting generates a directional photocurrent flowing along the direction orthogonal to both the in-plane magnetic field and crystal polar axis. The direction of this charge current can be flipped by reversing the magnetic field. The photocurrent is found to be independent of light polarization, and ascribed to the $k$-dependent relaxation of nonequilibrium carriers in the Zeeman-shifted spin-split bands.

We present a time-resolved study of energy relaxation and trapping dynamics of polariton condensates in a semiconductor microcavity ridge. The combination of two nonresonant, pulsed laser sources in a GaAs ridge-shaped microcavity gives rise to profuse quantum phenomena where the repulsive potentials created by the lasers allow the modulation and control of the polariton flow. We analyze in detail the dependence of the dynamics on the power of both lasers and determine the optimum conditions for realizing an all-optical polariton condensate transistor switch. The experimental results are interpreted in the light of simulations based on a generalized Gross-Pitaevskii equation, including incoherent pumping, decay, and energy relaxation within the condensate.

We theoretically consider the carrier density dependence of low-temperature electrical conductivity in high-quality and low-disorder two-dimensional (2D) “metallic” electronic systems such as 2D GaAs electron or hole quantum wells or doped/gated graphene. Taking into account resistive scattering by Coulomb disorder arising from quenched random charged impurities in the environment, we show that the 2D conductivity $\sigma \left(n\right)$ varies as $\sigma \sim n^{\beta \left(n\right)}$ as a function of the 2D carrier density $n$ where the exponent $\beta \left(n\right)$ is a smooth, but nonmonotonic, function of density $n$ with possible nontrivial extrema. In particular, the density scaling exponent $\beta \left(n\right)$ depends qualitatively on whether the Coulomb disorder arises primarily from remote or background charged impurities or short-range disorder and can, in principle, be used to characterize the nature of the dominant background disorder. A specific important prediction of the theory is that for resistive scattering by remote charged impurities, the exponent $\beta$ can reach a value as large as 2.7 for $k_{F}d\sim 1$, where $k_F\sim \sqrt{n}$ is the 2D Fermi wave vector and $d$ is the separation of the remote impurities from the 2D layer. Such an exponent $\beta$ ($>$$5/2$) is surprising because unscreened Coulomb scattering by remote impurities gives a limiting theoretical scaling exponent of $\beta =5/2$, and naively one would expect $\beta \left(n\right)\le 5/2$ for all densities since unscreened Coulomb scattering should nominally be the situation bounding the resistive scattering from above. We find numerically and show theoretically that the maximum value of $\alpha$ ($\beta$), the mobility (conductivity) exponent, for 2D semiconductor quantum wells is around 1.7 (2.7) for all values of $d$ (and for both electrons and holes) with the maximum $\alpha$ occurring around $k_{F}d\sim 1$. We discuss experimental scenarios for the verification of our theory.

During the past decade, ab initio methods to calculate the electronic structure of materials based on hybrid functionals have increasingly become widely popular. Here we show that, in the case of small gap transition metal oxides, such as VO${}_2$, with rather subtle physics in the vicinity of the Fermi surface, such hybrid functional schemes without the inclusion of “expensive” fully self-consistent $\mathit{GW}$ corrections fail to yield this physics and incorrectly describe the features of the wave function of states near the Fermi surface. While a fully self-consistent $\mathit{GW}$ on top of a hybrid functional approach does correct these wave functions as expected, and is found to be in general agreement with the results of a fully self-consistent $\mathit{GW}$ approach based on semilocal functionals, it is much more computationally demanding as compared to the latter approach for the benefit of essentially the same results.

Based on a detailed symmetry analysis, we state the general rules to build up the effective low-energy field theory describing a system of electrons weakly interacting with the lattice degrees of freedom. The basic elements in our construction are what we call the “memory tensors,” which keep track of the microscopic discrete symmetries into the coarse-grained action. The present approach can be applied to lattice systems in arbitrary dimensions and in a systematic way to any desired order in derivatives. We apply the method to the honeycomb lattice and reobtain the by-now well-known effective action of Dirac fermions coupled to fictitious gauge fields. As a second example, we derive the effective action for electrons in the kagome lattice, where our approach allows us to obtain in a simple way the low-energy electron-phonon coupling terms.

We report on recombination dynamics and screening effects due to photogenerated carriers in polar ZnO/(Mg,Zn)O single quantum wells. For small well widths ($\le$2.5 nm), the unscreened transition energy and decay time can be determined directly since the influence of the internal electric field is negligible due to strong confinement of the carriers. The energetic dependence of the decay time was reproduced by a relaxation model to describe the localization within potential minima due to well width fluctuations. For larger well widths, screening effects have to be taken into account even for moderate excitation densities in the 1 W/cm${}^{-2}$ range. It is very hard to determine the unscreened single-exciton state of these wells with strong quantum-confined Stark effect within reasonable time scales and a significant signal-to-noise ratio. We present an extended relaxation model to determine the decay time of the unscreened single-exciton state. A difference in polarization between ZnO and Mg${}_x$Zn${}_{1-x}$O of $\Delta P/x=0.026\text{C}/{\text{m}}^2$ is concluded from the dependence of the decay time on the well width.

We employ detuning-dependent decay-rate measurements of a quantum dot in a photonic-crystal cavity to study the influence of phonon dephasing in a solid-state quantum-electrodynamics experiment. The experimental data agree with a microscopic non-Markovian model accounting for dephasing from longitudinal acoustic phonons, and the analysis explains the difference between nonresonant cavity feeding in different nanocavities. From the comparison between experiment and theory we extract the effective phonon density of states experienced by the quantum dot in the nanocavity. This quantity determines all phonon dephasing properties of the system and is found to be described well by a theory of bulk phonons.

Recently, two-dimensional nanostructures consisting of alternating graphene and boron nitride (BN) domains have been synthesized. These systems possess interesting electronic and mechanical properties, with potential applications in electronics and optical devices. Here, we perform a first-principles investigation of models of BN-C hybrid monolayers and nanoribbons deposited on the Cu$\left(111\right)$ surface, a substrate used for their growth in said experiments. For the sake of comparison, we also consider BN and BC${}_2$N nanostructures. We show that BN and BC${}_2$N monolayers bind weakly to Cu$\left(111\right)$, whereas monolayers with alternating domains interact strongly with the substrate at the B-C interface, due to the presence of localized interface states. This binding leads to a deformation of the monolayers and sizable $n$ doping. Nanoribbons exhibit a similar behavior. Furthermore, they also interact significantly with the substrate at the edge, even in the case of passivated edges. These findings suggest a route to tune the band gap and doping level of BN-C hybrid models based on the interplay between nanostructuring and substrate-induced effects.

We have employed the x-ray resonant magnetic scattering (XRMS) technique at the Ru $L_2$ edge of the Ba(Fe${}_{1-x}$Ru${}_x$)${}_2$As${}_2$ ($x=0.205$) superconductor. We show that pronounced resonance enhancements at the Ru $L_2$ edge are observed at the wave vector which is consistent with the antiferromagnetic propagation vector of the Fe in the undoped BaFe${}_2$As${}_2$. We also demonstrate that the XRMS signals at the Ru $L_2$ edge follow the magnetic ordering of the Fe with a long correlation length, ${\xi }_{ab}>2850\pm 400$ Å. Our experimental observation shows that the Ru is spin polarized in Ba(Fe${}_{1-x}$Ru${}_x$)${}_2$As${}_2$ compounds.

This paper reports an inelastic neutron scattering study of superfluid helium confined with highly monodisperse, smooth, and unidimensional silica pores only a few nanometers in diameter, previously studied only by means of macroscopic, thermodynamic techniques. Helium gas sorption isotherms show that the adsorption of helium gas proceeds by film growth, providing quantitative information about the thickness of the adsorbed film and its two-dimensional compressibility. Two different microscopic phases were observed using inelastic neutron scattering. When the adsorbed superfluid helium forms a thin film only a few atomic layers thick, it supports a dramatically modified phonon-roton spectrum as well as a compressed layer roton. The energies of these modified phonon-roton modes are consistent with those of a dilute, low-density film, one in which the average interatomic spacing is greater than in the bulk liquid. In contrast, when the pores are saturated with liquid, the modified phonon-roton spectrum disappears, and instead bulklike modes coexist with the compressed layer mode. The qualitative difference between these two pore-filling regimes is reflected in the effective vibrational density of states.

Inspired by a recent experimental observation of the zero-bias tunneling conductance in superconductor-semiconductor nanowire devices [V. Mourik et al., Science 336, 1003 (2012)], we consider here transport properties of the junctions consisting of a nanowire (Luttinger liquid) coupled to a topological superconductor characterized by the presence of Majorana zero-energy end states. The presence of the Majorana modes leads to a quantization of the zero-bias tunneling conductance at zero temperature. In order to understand this phenomenon, we have developed a framework, based on real-time Keldysh technique, which allows one to compute tunneling conductance at finite temperature and voltage in a realistic experimental setup. Our approach allows one to understand this transport phenomenon from a more general perspective by including the effect of interactions in the nanowire, which sometimes results in a drastic departure from the noninteracting predictions. Thus, our results provide a key insight for the tunneling experiments aiming at detecting Majorana particles in one-dimensional nanowire devices.