The quantum kicked rotor (QKR) driven by $d$ incommensurate frequencies realizes the universality class of $d$-dimensional disordered metals. For $d>3$, the system exhibits an Anderson metal-insulator transition which has been observed within the framework of an atom-optics realization. However, the absence of genuine randomness in the QKR reflects in critical phenomena beyond those of the Anderson universality class. Specifically, the system shows strong sensitivity to the algebraic properties of its effective Planck constant $\tilde{h}\equiv 4\pi /q$. For integer $q$, the system may be in a globally integrable state, in a “supermetallic” configuration characterized by diverging response coefficients, Anderson localized, metallic, or exhibit transitions between these phases. We present numerical data for different $q$ values and effective dimensionalities, with the focus on parameter configurations which may be accessible to experimental investigations.

Multiferroics are materials with coexisting magnetic and ferroelectric orders, which show potential for electrically controlled spintronic devices. A common application of ferroelectrics is in electro-optical modulators exploiting their electric-field-dependent optical indices. The coupling of optical and magnetic degrees of freedom is attractive for designing multifunctional devices, but to date the electro-optical response of multiferroics has hardly been explored. Here we report a joint experimental and theoretical study of this effect in multiferroic ${\mathrm{BiFeO}}_3$ thin films. We confirm the large birefringence present in single crystals and determine the electro-optic coefficients $r_{13}$ and $r_{33}$. We present approaches to increase the obtained coefficients, for instance, by using tetragonal-like ${\mathrm{BiFeO}}_3$, and expand the potential of multiferroics to optical applications.

A sensitive technique for the readout of the state of a qubit is based on the measurement of the conductance through a proximal sensor quantum dot (SQD). Here, we theoretically study the coherent backaction of such a measurement on a coupled SQD-charge-qubit system. We derive Markovian kinetic equations for the ensemble-averaged state of the SQD-qubit system, expressed in the coupled dynamics of two charge-state occupations of the SQD and two qubit isospin vectors, one for each SQD charge state. We find that aside from introducing dissipation, the detection also renormalizes the coherent evolution of the SQD-qubit system. Basically, if the electron on the detector has time to probe the qubit, then it also has time to fluctuate and thereby renormalize the system parameters. In particular, this induces torques on the qubit isospins, similar to the spin torque generated by the spintronic exchange field in noncollinear spin-valve structures. Second, we show that for a consistent description of the detection, one must also include the renormalization effects in the next-to-leading order in the electron tunneling rates, especially at the point of maximal sensitivity of the detector. Although we focus on a charge-qubit model, our findings are generic for qubit readout schemes that are based on spin-to-charge conversion using a quantum dot detector. Furthermore, our study of the stationary current through the SQD, a test measurement verifying that the qubit couples to the detector current, already reveals various significant effects of the isospin torques on the qubit. Our kinetic equations provide a starting point for further studies of the time evolution in charge-based qubit readout. Finally, we provide a rigorous sum rule that constrains such approximate descriptions of the qubit isospin dynamics and show that it is obeyed by our kinetic equations.

By using a different quantum-to-classical mapping from the Trotter-Suzuki decomposition, we identify the entanglement structure of the maximal eigenvectors for the associated quantum transfer matrix. This observation provides a deeper insight into the problem of linear growth of the entanglement entropy in time evolution using conventional methods. Based on this observation, we propose a general method for arbitrary temperatures using the biorthonormal transfer-matrix renormalization group. Our method exhibits a competitive accuracy with a much cheaper computational cost in comparison with two recently proposed methods for long-time dynamics based on a folding algorithm [Phys. Rev. Lett. 102, 240603 (2009)] and a modified time-dependent density-matrix renormalization group [Phys. Rev. Lett. 108, 227206 (2012)].

We consider the string-net model on the honeycomb lattice for Ising anyons in the presence of a string tension. This competing term induces a nontrivial dynamics of the non-Abelian anyonic quasiparticles and may lead to a breakdown of the topological phase. Using high-order series expansions and exact diagonalizations, we determine the robustness of this doubled Ising phase, which is found to be separated from two gapped phases. An effective quantum dimer model emerges in the large tension limit, giving rise to two different translation symmetry-broken phases. Consequently, we obtain four transition points, two of which are associated with first-order transitions whereas the two others are found to be continuous and provide examples of recently proposed Bose condensation for anyons.

It has been demonstrated that atomic force microscopy imaging with CO-functionalized tips provides unprecedented resolution, yet it is subject to strong image distortions. Here we propose a method to correct for these distortions. The lateral force acting on the tip apex is calculated from three-dimensional maps of the frequency shift signal. Assuming a linear relationship between lateral distortion and force, atomic force microscopy images could be deskewed for different substrate systems.

We investigate the ac electric field induced quantum anomalous Hall effect in honeycomb lattices and derive the full phase diagram for arbitrary field amplitude and phase polarization. We show how to induce antichiral edge modes as well as topological phases characterized by a Chern number larger than 1 by means of suitable drivings. In particular, we find that the Chern number develops plateaus as a function of the frequency, providing a time-dependent analog to the ones in the quantum Hall effect.

We study the “Higgs” amplitude mode in the relativistic quantum $O\left(N\right)$ model in two space dimensions. Using the nonperturbative renormalization group we compute the $O\left(N\right)$-invariant scalar susceptibility in the vicinity of the zero-temperature quantum critical point. In the zero-temperature ordered phase, we find a well-defined Higgs resonance for $N=2$ with universal properties in agreement with quantum Monte Carlo simulations. The resonance persists at finite temperature below the Berezinskii-Kosterlitz-Thouless transition temperature. In the zero-temperature disordered phase, we find a maximum in the spectral function which is, however, not related to a putative Higgs resonance. Furthermore we show that the resonance is strongly suppressed for $N\ge 3$.

We investigate signatures of electronic correlations in the narrow-gap semiconductor $\mathrm{Fe}{\mathrm{Ga}}_3$ by means of electrical resistivity and thermodynamic measurements performed on single crystals of $\mathrm{Fe}{\mathrm{Ga}}_3$, ${\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{Ga}}_3$, and $\mathrm{Fe}{\mathrm{Ga}}_{3-y}{\mathrm{Zn}}_y$, complemented by a study of the 4$d$ analog material $\mathrm{Ru}{\mathrm{Ga}}_3$. We find that the inclusion of sizable amounts of Mn and Zn dopants into $\mathrm{Fe}{\mathrm{Ga}}_3$ does not induce an insulator-to-metal transition. Our study indicates that both substitution of Zn onto the Ga site and replacement of Fe by Mn introduces states into the semiconducting gap that remain localized even at highest doping levels. Most importantly, using neutron powder diffraction measurements, we establish that $\mathrm{Fe}{\mathrm{Ga}}_3$ orders magnetically above room temperature in a complex structure, which is almost unaffected by the doping with Mn and Zn. Using realistic many-body calculations within the framework of dynamical mean field theory (DMFT), we argue that while the iron atoms in $\mathrm{Fe}{\mathrm{Ga}}_3$ are dominantly in an $S=1$ state, there are strong charge and spin fluctuations on short-time scales, which are independent of temperature. Further, the low magnitude of local contributions to the spin susceptibility advocates an itinerant mechanism for the spin response in $\mathrm{Fe}{\mathrm{Ga}}_3$. Our joint experimental and theoretical investigations classify $\mathrm{Fe}{\mathrm{Ga}}_3$ as a correlated band insulator with only small dynamical correlation effects, in which nonlocal exchange interactions are responsible for the spin gap of 0.4 eV and the antiferromagnetic order. We show that hole doping of $\mathrm{Fe}{\mathrm{Ga}}_3$ leads, within DMFT, to a notable strengthening of many-body renormalizations.

We have observed a persistent voltage oscillation induced by constant external electric fields in the nonmagnetic narrow-gap semiconductor SmS. This oscillation appears with a large nonlinear conduction in a relatively low electric field of a few kV/cm. We find that the oscillation frequency clearly changes on addition of an external capacitor parallel to the sample, indicating that the oscillation does not arise from the Joule heating effect. This oscillation is analyzed in terms of a carrier charging-discharging model, which is associated with an electric-field-induced insulator-to-metal transition as proposed in the oxide insulator VO${}_2$. This model reasonably explains the observed oscillation phenomenon, implying the existence of a possible instability toward a conductive state driven by the electric field.

In the framework of density functional theory, the geometry, electronic structure, and magnetic properties of the bulk and low index surfaces of $\mathrm{Mo}{\mathrm{Se}}_2$ have been studied. We have carried out calculations with various exchange-correlation functionals to select one which is able to describe the van der Waals (vdW) interactions and gives the best geometry compared with experiments. The inclusion of the vdW forces, however, does not guarantee a reliable description for the geometry of this compound: some vdW functionals strongly overestimate the interlayer distance, similar to GGA functionals. Our investigation shows that the recently introduced optB86b-vdW functional yields the best results for $\mathrm{Mo}{\mathrm{Se}}_2$. The vdW functionals have less impact on the electronic structure: the differences between the band structures of the experimental atomic structure, calculated by the vdW-DF and PBE functionals are marginal. We have tried the HSE06 hybrid functional as well but the results are not satisfactory: the overestimated interlayer distance leads to a significant overestimation of the band gap. The band structure of the bulk and monolayer is calculated and by the analysis of the bands character the indirect to direct band-gap transition is explained. The surface energy, work function and band structure of the surfaces are calculated as well. The role of the $\mathrm{Mo}{\mathrm{Se}}_2$ buffer layer in $\mathrm{Cu}(In,\mathrm{Ga}){\mathrm{Se}}_2$ based solar cells is discussed by considering the work function values.

Electrons on the half-filled honeycomb lattice are expected to undergo a direct continuous transition from the semimetallic into the antiferromagnetic insulating phase with increase of onsite Hubbard repulsion. We attempt to further quantify the critical behavior at this quantum phase transition by means of functional renormalization group (RG), within an effective Gross-Neveu-Yukawa theory for an $\mathrm{SO}\left(3\right)$ order parameter (“chiral Heisenberg universality class”). Our calculation yields an estimate of the critical exponents $\nu \simeq 1.31$, ${\eta }_{\varphi }\simeq 1.01$, and ${\eta }_{\Psi }\simeq 0.08$, in reasonable agreement with the second-order expansion around the upper critical dimension. To test the validity of the present method, we use the conventional Gross-Neveu-Yukawa theory with ${\mathbb{Z}}_2$ order parameter (“chiral Ising universality class”) as a benchmark system. We explicitly show that our functional RG approximation in the sharp-cutoff scheme becomes one-loop exact both near the upper as well as the lower critical dimension. Directly in $2+1$ dimensions, our chiral Ising results agree with the best available predictions from other methods within the single-digit percent range for $\nu$ and ${\eta }_{\varphi }$ and the double-digit percent range for ${\eta }_{\Psi }$. While one would expect a similar performance of our approximation in the chiral Heisenberg universality class, discrepancies with the results of other calculations here are more significant. Discussion and summary of various approaches is presented.

In this paper, we report a first-principles study of the lattice dynamics of small graphene nanoribbon with zigzag edges. Our investigation is based on spin polarized density functional calculations (DFT). Nesting properties in the electronic band structure are very different for nanoribbons with unpolarized, ferromagnetic, and antiferromagnetic configurations. As a result, the phonon spectrum and nesting related softening in phonon frequencies differ in these cases. The unpolarized and ferromagnetic structures show nesting related phonon softening and considerable electron phonon linewidth, while for the antiferromagnetic structure, a band gap at the Fermi energy eliminates these effects. Saturating the nanoribbon edge with hydrogen has negligible effect on the phonon spectra for the magnetic structures while for the unpolarized configuration all structures without hydrogen are unstable due to soft phonon modes. The electron-phonon coupling coefficients have also been calculated and implications for Peierls transition and superconductivity are discussed.

We study the band structure, density of states, and spatial localization of edge states in twisted bilayer graphene nanoribbons. We devise these ribbons by cutting a stripe of commensurate twisted bilayer graphene along a direction with a maximum number of zigzag edge atoms. Due to the spatially inhomogeneous interlayer coupling, edge states stemming from regions with AB stacking are closer to the energy of the Dirac point, whereas those arising from AA-stacked edge states are split in energy due to the stronger interlayer coupling. As opposed to bulk bilayer graphene, for which states near the Dirac point are localized in AA-stacked regions, the interplay of edge and moiré localization produces a distinct spatial distribution of low-energy states in these ribbons.

An accurate description of oxygen dissociation pathways and kinetics for various local adlayer environments is key for an understanding not just of the coverage dependence of oxygen sticking, but also of reactive steady states in oxidation reactions. Density functional theory analysis for M(100) surfaces with $M=\text{Pd}$, Rh, and Ni, where O prefers the fourfold hollow adsorption site, does not support the traditional Brundle-Behm-Barker picture of dissociative adsorption onto second-nearest-neighbor hollow sites with an additional blocking constraint. Rather adsorption via neighboring vicinal bridge sites dominates, although other pathways can be active. The same conclusion also applies for $M=\text{Pt}$ and Ir, where oxygen prefers the bridge adsorption site. Statistical mechanical analysis is performed based on kinetic Monte Carlo simulation of a multisite lattice-gas model consistent with our revised picture of adsorption. This analysis determines the coverage and temperature dependence of sticking for a realistic treatment of the oxygen adlayer structure.

High-energy synchrotron x-ray diffraction was utilized to study the local order of liquid sulfur at high-pressure and high-temperature conditions. A temperature driven structure change in liquid sulfur was observed, signified by an order of magnitude reduction in lengths of sulfur chains. The large change in chain length implies that this is a liquid-liquid phase transition in sulfur. The chain breakage may strongly influence the physical properties, such as the semiconductor-metal transition and a drastic decrease in viscosity across the transition.

The critical behavior of ${\mathrm{Sr}}_{1-x}$${\mathrm{Ca}}_x$${\mathrm{RuO}}_3$ was investigated by a scaling analysis based on the Arrott-Noakes equation of state. The critical exponents β, γ, and δ of the magnetic critical behavior were extracted for samples with 0 $\le x\le 0.6$ . The ferromagnetic system exhibits a smooth suppression of the Curie temperature $T_{\mathrm{C}}$ to zero at a critical concentration $x_c$ ≈ 0.7. The ordered magnetic moment decreases simultaneously as expected for itinerant ferromagnets, however, does not vanish completely at $x_{\mathrm{c}}$, indicating small magnetic clusters or inhomogeneities. For $x$ = 0, mean-field like exponents are observed. With increasing $x$, the critical exponents β, γ, and δ vary nearly linearly from β ≈ 0.5, γ ≈ 1, and δ ≈ 3 for $x$ = 0 to β ≈ 1, γ ≈ 0.9, and δ ≈ 1.6 for $x$ ≈ 0.6. The Widom scaling relation is always met for $x\le 0.6.$ Despite the systematic evolution of the critical exponents as a function of $x$, the exponents cannot be described by any of the universality classes known for classical standard models. The particular trend of the effective critical exponents may be possibly explained by a strong-disorder line of fixed points; the vicinity to $x_c$ suggests that this behavior is caused by the vicinity to a quantum phase transition.

Magnetic excitations in the isostructural spin-dimer systems ${\text{Sr}}_3$${\text{Cr}}_2$${\text{O}}_8$ and ${\text{Ba}}_3$${\text{Cr}}_2$${\text{O}}_8$ are probed by means of high-field electron spin resonance at subterahertz frequencies. Three types of magnetic modes were observed. One mode is gapless and corresponds to transitions within excited states, while two other sets of modes are gapped and correspond to transitions from the ground to the first excited states. The selection rules of the gapped modes are analyzed in terms of a dynamical Dzyaloshinskii-Moriya interaction, suggesting the presence of phonon-assisted effects in the low-temperature spin dynamics of ${\text{Sr}}_3$${\text{Cr}}_2$${\text{O}}_8$ and ${\text{Ba}}_3$${\text{Cr}}_2$${\text{O}}_8$.

The realizations of spin-orbit coupling in cold atoms lead to a burst of research activities in the searching of topological matters in ultracold atom systems. Recent theoretical predictions show that topological Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluids can be realized with appropriate spin-orbit coupling and in-plane Zeeman fields. In this work, a comprehensive understanding of the pairing symmetry, phase diagram, and the edge modes in this topological matter are presented. The center-of-mass momentum of the Cooper pairs plays the role of renormalizing the in-plane Zeeman fields and chemical potential. The in-plane Zeeman fields also induce asymmetry to the effective $p$-wave pairing, apart from a small fraction of higher orbital components. As a result, the phase diagram is composed by different phases, which are determined by the Fermi surface topology and band gap width of the superfluids. Especially, the gapped and gapless topological FFLO phases have totally different finite size effects. These features show that the spin-orbit coupled cold atoms provide an important platform in realizing topological matters which may not be materialized with solids.