We investigate the topological character of lattice chiral Gaussian fermionic states in two dimensions possessing the simplest descriptions in terms of projected entangled pair states (PEPS). They are ground states of two different kinds of Hamiltonians. The first one, ${\mathcal{H}}_{\mathrm{ff}}$, is local, frustration free, and gapless. It can be interpreted as describing a quantum phase transition between different topological phases. The second one, ${\mathcal{H}}_{\mathrm{fb}}$, is gapped, and has hopping terms scaling as $1/r^3$ with the distance $r$. The gap is robust against local perturbations, which allows us to define a Chern number for the PEPS. As for (nonchiral) topological PEPS, the nontrivial topological properties can be traced down to the existence of a symmetry in the virtual modes that are used to build the state. Based on that symmetry, we construct stringlike operators acting on the virtual modes that can be continuously deformed without changing the state. On the torus, the symmetry implies that the ground state space of the local parent Hamiltonian is twofold degenerate. By adding a string wrapping around the torus, one can change one of the ground states into the other. We use the special properties of PEPS to build the boundary theory and show how the symmetry results in the appearance of chiral modes, and a universal correction to the area law for the zero Rényi entropy.

The dynamics of charge-density fluctuations in a system of two tunnel-coupled wires contains two diffusion modes with dispersion $i\omega =D{q}^2$ and $i\omega =D{q}^2+\frac{2}{{\tau }_t}$, where $D$ is the diffusion coefficient and ${\tau }_t$ is the tunneling time between the wires. The dispersion of corresponding spin-density modes depends on magnetic field as a result of the spin precession with Larmour frequency ${\omega }_L$. The presence of two modes affects the shape of the Hanle curve describing the spin-dependent resistance $R$ between the ferromagnetic strips covering the nonmagnetic wires. We demonstrate that the relative shapes of the $R\left({\omega }_L\right)$ curves, one measured within the same wire and the other measured between the wires, depends on the ratio ${\tau }_t/{\tau }_s$, where ${\tau }_s$ is the spin-diffusion time. If the coupling between the wires is local, i.e., only at the point $x=0$, then the difference of the shapes of intrawire and interwire Hanle curves reflects the difference in statistics of diffusive trajectories, which “switch” or do not switch near $x=0$. When one of the coupled wires is bent into a loop with a radius $a$, the shape of the Hanle curve reflects the statistics of random walks on the loop. This statistics is governed by the dimensionless parameter $a/\sqrt{D{\tau }_s}$.

In the presence of a circularly polarized mid-infrared radiation graphene develops dynamical band gaps in its quasienergy band structure and becomes a Floquet insulator. Here, we analyze how topologically protected edge states arise inside these gaps in the presence of an edge. Our results show that the gap appearing at $\hslash \Omega /2$, where $\hslash \Omega$ is the photon energy, is bridged by two chiral edge states whose propagation direction is set by the direction of the polarization of the radiation field. Therefore, both the propagation direction and the energy window where the states appear can be controlled externally. We present both analytical and numerical calculations that fully characterize these states. This is complemented by simple topological arguments that account for them and by numerical calculations for the case of the semi-infinite sample, thereby eliminating finite-size effects.

Spin-orbit coupling introduces chirality into the electronic structure. This can have profound effects on the magnetization induced by the orbital motion of electrons. Here we derive a formula for the orbital magnetization of interacting electrons in terms of the full Green's function and vertex functions. The formula is applied within dynamical mean-field theory to the Kane-Mele-Hubbard model that allows both topological and trivial insulating phases. We study the insulating and metallic phases in the presence of an exchange magnetic field. In the presence of interactions, the orbital magnetization of the quantum spin Hall insulating phase with inversion symmetry is renormalized by the bulk quasiparticle weight. The orbital magnetization vanishes for the in-plane antiferromagnetic phase with trivial topology. In the metallic phase, the enhanced effective spin-orbit coupling due to the interaction sometimes leads to an enhancement of the orbital magnetization. However, at low doping, magnetization is suppressed at large interaction strengths.

We propose a straightforward and effective approach for quantifying the band inversion induced by spin-orbit coupling in band insulators. In this approach we define a quantity as a function of wave vector in the Brillouin zone (BZ) measuring the mismatch, or “spillage,” between the occupied states of a system with and without SOC. Plots of the spillage throughout the BZ provide a ready indication of the number and location of band inversions driven by SOC. To illustrate the method, we apply this approach to the two-band Dirac model, the 2D Kane-Mele model, a 2D Bi bilayer with applied Zeeman field, and to first-principles calculations of some 3D materials including both trivial and ${\mathbb{Z}}_2$ topological insulators. We argue that the distribution of spillage in the BZ is closely related to the topological indices in these systems. Our approach provides a fresh perspective for understanding topological character in band theory, and should be helpful in searching for new materials with nontrivial band topology.

Measurements of electrical resistivity under applied pressure were performed on single-crystalline samples of LaFeAsO grown in a molten NaAs flux. We observe a smooth suppression of spin-density wave order under nearly hydrostatic applied pressures up to 2.6 GPa and in quasihydrostatic pressures up to 14.7 GPa. Similar to some of the other reports on single and polycrystalline samples of LaFeAsO, these crystals exhibit a resistivity that increases as temperature is lowered. By fitting an Arrhenius law to the the semiconducting-like temperature dependence of the electrical resistivity, we extract an energy gap that is suppressed with pressure and vanishes near 10 GPa. This is accompanied by the emergence of a metallic temperature dependence of the electrical resistivity. A similar behavior is also observed in diamond anvil cell experiments carried out to $\sim 37\mathrm{GPa}$. Complete transitions to a bulk superconducting phase are not observed in any of the experiments.

A tunnel junction between helical edge states, realized via a constriction in a quantum spin Hall system, can be exploited to steer both charge and spin current into various terminals. We investigate the effects of disorder on the transmission coefficient $T_p$ of the junction by modeling disorder with a randomly varying (complex) tunneling amplitude ${\Gamma }_p=\left|{\Gamma }_p\right|exp\left[i{\varphi }_p\right]$. We show that, while for a clean junction $T_p$ is only determined by the absolute value $|{\Gamma }_p|$ and is independent of the phase ${\varphi }_p$, the situation can be quite different in the presence of disorder: phase fluctuations may dramatically affect the energy dependence of $T_p$ of any single sample. Furthermore, analyzing three different models for phase disorder (including correlated ones), we show that not only the amount but also the way the phase ${\varphi }_p$ fluctuates determines the localization length ${\xi }_{\mathrm{loc}}$ and the sample-averaged transmission. Finally, we discuss the physical conditions in which these three models suitably apply to realistic cases.

The band structure of type-I (In,Al)As/AlAs quantum dots with band gap energy exceeding $1.63$ eV is indirect in momentum space, leading to long-lived exciton states with potential applications in quantum information. Optical access to these excitons is provided by mixing of the $\Gamma$- and $X$-conduction-band valleys, for which their spins may be oriented by resonant spin-flip Raman scattering. This access is used to study the exciton spin-level structure by accurately measuring the anisotropic hole and isotropic electron $g$ factors. The spin-flip mechanisms for the indirect exciton and its constituents as well as the underlying optical selection rules are determined. The spin-flip intensity is a reliable measure of the strength of $\Gamma \text{−}X$-valley mixing, as evidenced by both experiment and theory.

The energy scales of interactions that control molecular adsorption and assembly on surfaces can vary by several orders of magnitude, yet the importance of each contributing interaction is not apparent a priori. Tetracyanoquinodimethane (TCNQ) is an archetypal electron acceptor molecule and it is a key component of organic metals. On metal surfaces, this molecule also acts as an electron acceptor, producing negatively charged adsorbates. It is therefore rather intriguing to observe attractive molecular interactions in this system that were reported previously for copper and silver surfaces. Our experiments compared TCNQ adsorption on noble metal surfaces of Ag(100) and Ag(111). In both cases we found net attractive interactions down to the lowest coverage. However, the morphology of the assemblies was strikingly different, with two-dimensional islands on Ag(100) and one-dimensional chains on Ag(111) surfaces. This observation suggests that the registry effect governed by the molecular interaction with the underlying lattice potential is critical in determining the dimensionality of the molecular assembly. Using first-principles density functional calculations with a van der Waals correction scheme, we revealed that the strengths of major interactions (i.e., lattice potential corrugation, intermolecular attraction, and charge-transfer-induced repulsion) are all similar in energy. The van der Waals interactions, in particular, almost double the strength of attractive interactions, making the intermolecular potential comparable in strength to the diffusion potential and promoting self-assembly. However, it is the anisotropy of local intermolecular interactions that is primarily responsible for the difference in the topology of the molecular islands on Ag(100) and Ag(111) surfaces. We anticipate that the intermolecular potential will become more attractive and dominant over the diffusion potential with increasing molecular size, providing new design strategies for the structure and charge transfer within molecular layers.

The x-ray structure factor of molten ${\mathrm{TiO}}_2$ has been measured, enabled by the use of aerodynamic levitation and laser beam heating, to a temperature of $T=2250\left(30\right)\mathrm{K}$. The Ti-O coordination number in the melt is close to $n_{\mathrm{TiO}}=5.0\left(2\right)$, with modal Ti-O bond length $r_{\mathrm{TiO}}=1.881\left(5\right)\AA$, both values being significantly smaller than for the high temperature stable rutile crystal structure $\left(n_{\mathrm{TiO}}=6.0,r_{\mathrm{TiO}}=1.959\AA \right)$. The structural differences between melt and crystal are qualitatively similar to those for alumina, which is rationalized in terms of the similar field strengths of ${\mathrm{Ti}}^{4+}$ and ${\mathrm{Al}}^{3+}$. The diffraction data are used to generate physically and chemically reasonable structural models, which are then compared to the predictions based on various classical molecular dynamics (MD) potentials. Interatomic potentials, suitable for modeling molten ${\mathrm{TiO}}_2$, are introduced, given the inability of existing MD models to reproduce the diffraction data. These potentials have the additional advantage of being able to predict the density and thermal expansion of the melt, as well as solid amorphous ${\mathrm{TiO}}_2$, in agreement with published results. This is of critical importance given the strong correlation between density and structural parameters such as $n_{\mathrm{TiO}}$. The large thermal expansion of the melt is associated with weakly temperature dependent structural changes, whereby simulations show that $n_{\mathrm{TiO}}=5.85\left(2\right)-[3.0\left(1\right)\times {10}^{-4}]T(\mathrm{K},2.75\AA \mathrm{cutoff})$. The ${\mathrm{TiO}}_2$ liquid is structurally analogous to the geophysically relevant high pressure liquid silica system at around 27 GPa. We argue that the predominance of fivefold polyhedra in the melt implies the existence of as-yet-undiscovered ${\mathrm{TiO}}_2$ polymorphs, based on lower-than-octahedral coordination numbers, which are likely to be metastable under ambient conditions. Given the industrial importance of titanium oxides, experimental and computational searches for such polymorphs are well warranted.

An applied stress field ${\sigma }_3$ can reversibly change the temperature of an elastocaloric material under adiabatic conditions, and the temperature change $\Delta T_{{\sigma }_3}$ is usually maximized near phase transitions. Using a thermodynamic approach, we demonstrate that an elastocaloric strength $\alpha =|\Delta T_{{\sigma }_3}|/|{\sigma }_3|$ of 0.016 K/MPa can be achieved benefiting from the full first-order phase transition in ${\mathrm{BaTiO}}_3$ single crystals, which is comparable with typical elastocaloric materials reported in the literature. The elastocaloric temperature change is found to be giant (3.2 K) under a stress of 200 MPa with a temperature span of over 50 K, which can be significantly larger than its electrocaloric counterpart ($\sim 1$ K). Moreover, it is found that the elastocaloric strength can be remarkably enhanced (2.32 K/MPa) as long as the phase transition is triggered even by a modest stress near the sharp first-order phase transition, which is two orders of magnitude larger than those accomplished by full transition. Therefore, even a low stress ($<30$ MPa) can induce a modest elastocaloric effect (1.3 K) comparable with the electrocaloric counterpart, which is accompanied by a reduction of the working temperature span. In addition, it is found that the electrocaloric peak under tensile stresses moves towards higher temperatures with its magnitude slightly enhanced. Hopefully, our study will stimulate further investigations on elastocaloric and stress-mediated electrocaloric effects in ferroelectrics.

We study the ultrafast dynamic process in photoexcited systems and find that the Franck-Condon or Landau-Zener tunneling between the photoexcited state and the ground state is abruptly blocked with increasing the state coupling from nonadiabatic to adiabatic limits. The blockage of the tunneling inhibits the photoexcited state from decaying into the thermalized state and results in an emergence of a metastable state, which represents an entanglement of electronic states with different electron-phonon coupling strengths. Applying this model to the investigation of photoexcited half-doped manganites, we show that the quantum critical transition is responsible for more than a three-order-of-magnitude difference in the ground-state recovery times following photoirradiation. This model also explains some elusive experimental results, such as photoinduced rearrangement of orbital order by the structural rather than electronic process and the structural bottleneck of a one-quarter period of the Jahn-Teller mode. We demonstrate that in the spin-boson model there exist unexplored regions not covered in the conventional phase diagram.

We show that all the bands of the Hofstadter model on the torus have an exactly flat dispersion and Berry curvature when a special system size is chosen. This result holds for any hopping and Chern number. Our analysis therefore provides a simple rule for choosing a particularly advantageous system size when designing a Hofstadter system whose size is controllable, like a qubit lattice or an optical cavity array. The density operators projected onto the flat bands obey exactly the Girvin-MacDonald-Platzman algebra, like for Landau levels in the continuum in the case of $C=1$, or obey its straightforward generalization in the case of $C>1$. This allows a mapping between density-density interaction Hamiltonians for particles in the Hofstatder model and in a continuum Landau level. By using the well-known pseudopotential construction in the latter case, we obtain fractional Chern insulator phases, the lattice counterpart of fractional quantum Hall phases, that are exact zero-energy ground states of the Hofstadter model with certain interactions. Finally, the addition of a harmonic trapping potential is shown to lead to an appealingly symmetric description in which a new Hofstadter model appears in momentum space.

We report on the observation of strong backscattering of charge carriers in the quantum Hall regime of polycrystalline graphene, grown by chemical vapor deposition, which alters the accuracy of the Hall resistance quantization. The temperature and magnetic field dependence of the longitudinal conductance exhibits unexpectedly smooth power-law behaviors, which are incompatible with a description in terms of variable range hopping or thermal activation but rather suggest the existence of extended or poorly localized states at energies between Landau levels. Such states could be caused by the high density of line defects (grain boundaries and wrinkles) that cross the Hall bars, as revealed by structural characterizations. Numerical calculations confirm that quasi-one-dimensional extended nonchiral states can form along such line defects and short circuit the Hall bar chiral edge states.

The Dresselhaus spin-orbit interaction is quantitatively investigated in a (110)-oriented GaAs quantum well by means of time- and spatially resolved Kerr rotation. The experimental results directly demonstrate a unidirectional out-of-plane spin-orbit interaction that linearly depends on the electron momentum along the $\left[1\overline{1}0\right]$ direction and vanishes for the electron momentum along the [001] direction. Spatially resolved measurements of the diffusion-driven spin precession dynamics provide evidence of the formation of a persistent spin-helix state in this system.

We present numerically exact solutions to the problem of a single electron interacting through an extended interaction with optical phonons in two and three dimensions. Comparisons are made with results for the standard Holstein model and with perturbative approaches from both the weak coupling and strong coupling sides. We find, in agreement with earlier work, that the polaron effective mass increases (decreases) in the weak (strong) coupling regime, respectively. However, in two dimensions, the decrease in effective mass still results in too large an effective mass to be relevant in realistic models of normal metals. In three dimensions the decrease can be more relevant but exists only over a very limited range of coupling strengths.

We use a laser-driven single (In,Ga)As quantum dot (QD) in the dressed-state regime of resonance fluorescence $(T=4\text{K})$ to observe the four $D_1$-transition lines of alkali atomic cesium $\left(\mathrm{Cs}\right)$ vapor at room temperature. We tune the frequency of the dressing continuous-wave laser in the vicinity of the bare QD resonance $\sim 335.116\mathrm{THz}$ $(\sim 894.592\mathrm{nm})$ at constant excitation power and thereby controllably tune the center and side channel frequencies of the probe light, i.e., the Mollow triplet. Resonances between individual QD Mollow triplet lines and the atomic hyperfine-split transitions are clearly identified in the $\mathrm{Cs}$ absorption spectrum. Our results show that narrow-band (In,Ga)As QD resonance fluorescence (RF) is suitable to optically address individual transitions of the $D_1$ quadruplet without applying magnetic field or electric field tuning.

In this paper, we use the determinant quantum Monte Carlo method to study the effect of the electric field on the magnetic order in a bilayer Hubbard model on a honeycomb lattice, in which only the direct interlayer hopping energy is included. Our results qualitatively support the layered antiferromagnetic, spin-density wave ground state found in the mean-field theory at the charge neutrality point. The obtained magnetic moments, however, are much smaller than what are estimated in the mean-field theory. As the electric field increases, the magnetic order parameter rapidly decreases.