We analyze the gapped phase of the Kitaev honeycomb model perturbatively in the isolated-dimer limit. Our analysis is based on the continuous unitary transformations method, which allows one to compute the spectrum as well as matrix elements of operators between eigenstates at high order. The starting point of our study consists of an exact mapping of the original honeycomb spin system onto a square-lattice model involving an effective spin and a hard-core boson. We then derive the low-energy effective Hamiltonian up to order 10 which is found to describe an interacting-anyon system, contrary to the order 4 result which predicts a free theory. These results give the ground-state energy in any vortex sector and thus also the vortex gap, which is relevant for experiments. Furthermore, we show that the elementary excitations are emerging free fermions composed of a hard-core boson with an attached spin- and phase-operator string. We also focus on observables and compute, in particular, the spin-spin correlation functions. We show that they admit a multiplaquette expansion that we derive up to order 6. Finally, we study the creation and manipulation of anyons with local operators, show that they also create fermions, and discuss the relevance of our findings for experiments in optical lattices.

We present results of a comprehensive ab initio investigation of the atomic and electronic structure of a variety of reconstruction models for the $4H\text{-SiC}\left(1\overline{1}02\right)\text{−}c\left(2\times 2\right)$ surface. The basic structural building blocks are Si adatoms residing in $H3$ or $T4$ sites above the surface and carbon dimers in the top layer, respectively. These reduce the number of surface dangling bonds per unit cell from twelve at the ideal surface to only two in the different reconstruction models investigated. Several arrangements of triple-bonded carbon dimers bridging two second-layer Si atoms or double-bonded carbon dimers are considered. A configuration with double-bonded carbon dimer pairs and Si adatoms in $H3$ sites turns out to be most favorable. It is 2.1 eV per unit cell lower in total energy than the reconstruction model suggested earlier solely on the basis of experimental data. A reaction pathway of the surface from an initial configuration with Si adatoms in metastable $T4$ sites to the final adsorption configuration with Si adatoms in the most stable $H3$ sites is investigated, revealing an energy barrier of about 0.4 eV between the two. The electronic structure of several reconstruction models is analyzed by calculated surface band structures, charge-density distributions, and scanning tunneling microscopy images. The results are discussed in comparison with most recent experimental data.

We study transport properties of a two-dimensional electron gas, placed in a classically strong perpendicular magnetic field and in constant and oscillating in-plane electric fields. The analysis is based on a quantum Boltzmann equation derived for a weakly disordered two-dimensional electron gas. We consider disordered potential with both long- and short-range correlations. Electron scattering off such disorder is not limited to small change in momentum direction, but occurs on an arbitrary angle, including the backscattering. The nonlinearity of the transport in the considered system is a consequence of two coexisting effects: formation of a nonequilibrium distribution function of electrons and modification of the scattering rate off the disorder in the presence of dc and ac electric fields. This work describes both effects in a unified way. The calculated dissipative component of electric current oscillates as a function of the electric-field strength and frequency of microwave radiation in qualitative agreement with experiments.

A recently proposed six-parameter mixed power-exponential expression is tested against density-functional theory binding-energy curves and equations of state across the $4d$ and $5d$ transition-metal series. It is shown to remove known failures of the popular Birch-Murnaghan, extended Rydberg, and generalized Morse expressions in that it is able to not only reproduce the observed hard-core repulsion of the early transition metals under compression but also remove predicted spurious oscillations in the binding-energy relations under expansion. However, it is unable to fit the well-known anomalous behavior of the equation of state for lanthanum.

We present the design, fabrication, and characterization of a metamaterial absorber which is resonant at terahertz frequencies. We experimentally demonstrate an absorptivity of 0.97 at 1.6 THz. Importantly, our absorber is only $16\mu \text{m}$ thick, resulting in a highly flexible material that, further, operates over a wide range of angles of incidence for both transverse electric and transverse magnetic radiation.

Topological insulators supporting non-Abelian anyonic excitations are in the center of attention as candidates for topological quantum computation. In this paper, we analyze the ground-state properties of disordered non-Abelian anyonic chains. The resemblance of fusion rules of non-Abelian anyons and real-space decimation strongly suggests that disordered chains of such anyons generically exhibit infinite-randomness phases. Concentrating on the disordered golden chain model with nearest-neighbor coupling, we show that Fibonacci anyons with the fusion rule $\tau \otimes \tau =1\oplus \tau$ exhibit two infinite-randomness phases: a random-singlet phase when all bonds prefer the trivial fusion channel and a mixed phase which occurs whenever a finite density of bonds prefers the $\tau$ fusion channel. Real-space renormalization-group (RG) analysis shows that the random-singlet fixed point is unstable to the mixed fixed point. By analyzing the entanglement entropy of the mixed phase, we find its effective central charge and find that it increases along the RG flow from the random-singlet point, thus ruling out a $c$ theorem for the effective central charge.

The magnetism of 1-ML-thick films of ${\text{Fe}}_x{\text{Co}}_{1-x}$ on Pt(111) was investigated both experimentally, by x-ray magnetic circular dichroism and magneto-optical Kerr effect measurements, and theoretically, by first-principles electronic structure calculations, as a function of the film chemical composition. The calculated Fe and Co spin moments are only weakly dependent on the composition and close to $3{\mu }_B/\text{atom}$ and $2{\mu }_B/\text{atom}$, respectively. This trend is also seen in the experimental data, except for pure Fe, where an effective spin moment of only $S_{\text{eff}}=\left(1.2\pm 0.2\right){\mu }_B/\text{atom}$ was measured. On the other hand, both the orbital moment and the magnetic anisotropy energy show a strong composition dependence with maxima close to the ${\text{Fe}}_{0.5}{\text{Co}}_{0.5}$ stoichiometry. The experiment, in agreement with theory, gives a maximum magnetic anisotropy energy of 0.5 meV/atom, which is more than 2 orders of magnitude larger than the value observed in bulk bcc FeCo and close to that observed for the $L{1}_0$ phase of FePt. The calculations clearly demonstrate that this composition dependence is the result of a fine tuning in the occupation number of the $d_{x^2-y^2}$ and $d_{xy}$ orbitals due to the Fe-Co electronic hybridization.

We consider a nanomechanical resonator coupled to a double quantum dot. We demonstrate how the finite-frequency current-noise spectrum through the double quantum dot can be used to distinguish classical and quantum behavior in the nearby nanoelectromechanical resonator. We also show how the full-frequency current-noise spectrum gives important information on the combined double quantum dot-resonator energy spectrum. Finally, we point out regimes where the quantum state of the resonator becomes squeezed and also examine the cross-correlated electron-phonon current noise.

One of the earliest proposed phase transitions beyond the Landau-Ginzburg-Wilson paradigm is the quantum critical point separating an antiferromagnet and a valence-bond solid on a square lattice. The low-energy description of this transition is believed to be given by the $2+1$ dimensional ${\text{CP}}^1$ model—a theory of bosonic spinons coupled to an Abelian gauge field. Monopole defects of the gauge field play a prominent role in the physics of this phase transition. In the present paper, we use the state-operator correspondence of conformal field theory in conjunction with the $1/N$ expansion to study monopole operators at the critical fixed point of the ${\text{CP}}^{N-1}$ model. This elegant method reproduces the result for monopole scaling dimension obtained through a direct calculation by Murthy and Sachdev. The technical simplicity of our approach makes it the method of choice when dealing with monopole operators in a conformal field theory.

We describe quantum entanglement inherent to the polaron ground states of coupled electron-phonon (or, more generally, particle-phonon) systems based on a model comprising both local (Holstein-type) and nonlocal (Peierls-type) couplings. We study this model using a variational method supplemented by the exact numerical diagonalization on a system of finite size. By way of subsequent numerical diagonalization of the reduced density matrix, we determine the particle-phonon entanglement as given by the von Neumann and linear entropies. Our results are strongly indicative of the intimate relationship between the particle localization/delocalization and the particle-phonon entanglement. In particular, we find a compelling evidence for the existence of a nonanalyticity in the entanglement entropies with respect to the Peierls-coupling strength. The occurrence of such nonanalyticity—not accompanied by an actual quantum phase transition—reinforces analogous conclusion drawn in several recent studies of entanglement in the realm of quantum-dissipative systems. In addition, we demonstrate that the entanglement entropies saturate inside the self-trapped region where the small-polaron states are nearly maximally mixed.

A superglass is a phase of matter which is characterized at the same time by superfluidity and a frozen amorphous structure. We introduce a model of interacting bosons in three dimensions that displays this phase unambiguously and that can be analyzed exactly or using controlled approximations. Employing a mapping between quantum Hamiltonians and classical Fokker-Planck operators, we show that the ground-state wave function of the quantum model is proportional to the Boltzmann measure of classical hard spheres. This connection allows us to obtain quantitative results on static and dynamic quantum correlation functions. In particular, by translating known results on the glassy dynamics of Brownian hard spheres we work out the properties of the superglass phase and of the quantum phase transition between the superfluid and the superglass phase.

We generalize the spectral sum rule preserving density matrix numerical renormalization group (DM-NRG) method in such a way that it can make use of an arbitrary number of not necessarily Abelian local symmetries present in the quantum impurity system. We illustrate the benefits of using non-Abelian symmetries by the example of calculations for the $T$ matrix of the two-channel Kondo model in the presence of magnetic field, for which conventional NRG methods produce large errors and/or take a long run-time.

Measurements of dc conductivity and ac magnetic susceptibility under pressure to 12 GPa as well as of the crystal structure under pressure were made on antiferromagnetic ${\text{LaMnO}}_3$ and ferromagnetic ${\text{LaMn}}_{0.5}{\text{Ga}}_{0.5}{\text{O}}_3$ crystals. Collapse of magnetic ordering in both crystals has been found at a phase where the static Jahn-Teller distortion is suppressed. These results resolve the question whether a spin-ordered phase will survive if the orbital degree of freedom is released in an $e$ orbital system, a topic of broad current interest.

We report our study of the evolution of superconductivity and the phase diagram of the ternary $\text{Fe}{\left({\text{Se}}_{1-x}{\text{Te}}_x\right)}_{0.82}$ $\left(0\le x\le 1.0\right)$ system. We discovered a superconducting phase with $T_{c,\text{max}}=14\text{K}$ in the $0.3<x<1.0$ range. This superconducting phase is suppressed when the sample composition approaches the end member ${\text{FeTe}}_{0.82}$, which exhibits an incommensurate antiferromagnetic order. We discuss the relationship between the superconductivity and magnetism of this material system in terms of recent results from neutron-scattering measurements. Our results and analyses suggest that superconductivity in this class of Fe-based compounds is associated with magnetic fluctuations and therefore may be unconventional in nature.

We present an experimental study of the infrared conductivity, transmission, and reflection of a gated bilayer graphene and their theoretical analysis within the Slonczewski-Weiss-McClure (SWMc) model. The infrared response is shown to be governed by the interplay of the interband and the intraband transitions among the four bands of the bilayer. The position of the main conductivity peak at the charge-neutrality point is determined by the interlayer tunneling frequency. The shift of this peak as a function of the gate voltage gives information about less known parameters of the SWMc model such as those responsible for the electron-hole and sublattice asymmetries. These parameter values are shown to be consistent with recent electronic structure calculations for the bilayer graphene and the SWMc parameters commonly used for the bulk graphite.

A rapid acquisition method for two-dimensional (2D) magnetic resonance force microscopy images is described. Spatial resolution is based on gradients in the static magnetic field $B_0$ and in the applied radiofrequency field $B_1$. Hadamard- and Fourier-encoding schemes, respectively, are employed to obtain high sensitivity by taking advantage of a maximum multiplex effect. 2D images of a patterned ${\left({\text{NH}}_4\right)}_2{\text{SO}}_4$ crystal sample are reconstructed from the known field distributions with a resolution of $1\mu \text{m}$.

We present new x-ray and neutron-scattering measurements of stripe order in ${\text{La}}_{1.875}{\text{Ba}}_{0.125}{\text{CuO}}_4$, along with low-field susceptibility, thermal conductivity, and specific-heat data. We compare these with previously reported results for resistivity and thermopower. Temperature-dependent features indicating transitions (or crossovers) are correlated among the various experimental quantities. Taking into account recent spectroscopic studies, we argue that the most likely interpretation of the complete collection of results is that an unusual form of two-dimensional superconducting correlations appears together with the onset of spin-stripe order. Recent theoretical proposals for a sinusoidally modulated superconducting state compatible with stripe order provide an intriguing explanation of our results and motivate further experimental tests. We also discuss evidence for one-dimensional pairing correlations that appear together with the charge order. With regard to the overall phenomenology, we consider the degree to which similar behavior may have been observed in other cuprates and describe possible connections to various puzzling phenomena in cuprate superconductors.

We analyze the crossover from an $hc/e$ periodicity of the persistent current in flux-threaded clean metallic rings toward an $hc/2e$-flux periodicity of the supercurrent upon entering the superconducting state. On the basis of a model calculation for a one-dimensional ring we identify the underlying mechanism, which balances the $hc/e$ versus the $hc/2e$ periodic components of the current density. When the ring circumference exceeds the coherence length of the superconductor, the flux dependence is strictly $hc/2e$ periodic. Further, we develop a multichannel model which reduces the Bogoliubov–de Gennes equations to a one-dimensional differential equation for the radial component of the wave function. The discretization of this differential equation introduces transverse channels whose number scales with the thickness of the ring. The periodicity crossover is analyzed close the critical temperature.

The systematic pressure shifts of $T_c$ were investigated in the whole phase diagram of the FeAs-based superconducting compound ${\text{K}}_x{\text{Sr}}_{1-x}{\text{Fe}}_2{\text{As}}_2$. Different regions, arising from corresponding responses of $T_c$ to pressure ($d{T}_c/dp>0$, $\simeq 0$, or $<0$), can be clearly distinguished. This reveals an interesting similarity of the FeAs superconductors and the high-$T_c$ cuprates. This behavior is a manifestation of the layered structure of the FeAs compounds and the pressure-induced charge transfer between the $\left({\text{Fe}}_2{\text{As}}_2\right)$ and (K/Sr) layers. The coexistence of superconductivity and spin-density wave behavior were also observed, and the pressure effects on the latter is explored.

The real-time dynamics of interacting electrons out of equilibrium contains detailed microscopic information about electronically correlated materials, which can be read out with time-resolved optical spectroscopy. The reflectivity that is typically measured in pump-probe experiments is related to the nonequilibrium optical conductivity. We show how to express this quantity in terms of real-time Green’s functions using dynamical mean-field theory. As an application we study the electrical response of the Falicov-Kimball model during the ultrafast buildup of the gapped phase at large interaction.