Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that ${\text{CaFe}}_2{\text{As}}_2$ undergoes a pressure-induced transition to a nonmagnetic volume “collapsed” tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.

We show that the fundamental time-reversal invariant (TRI) insulator exists in $4+1$ dimensions, where the effective-field theory is described by the $(4+1)$-dimensional Chern-Simons theory and the topological properties of the electronic structure are classified by the second Chern number. These topological properties are the natural generalizations of the time reversal-breaking quantum Hall insulator in $2+1$ dimensions. The TRI quantum spin Hall insulator in $2+1$ dimensions and the topological insulator in $3+1$ dimensions can be obtained as descendants from the fundamental TRI insulator in $4+1$ dimensions through a dimensional reduction procedure. The effective topological field theory and the $Z_2$ topological classification for the TRI insulators in $2+1$ and $3+1$ dimensions are naturally obtained from this procedure. All physically measurable topological response functions of the TRI insulators are completely described by the effective topological field theory. Our effective topological field theory predicts a number of measurable phenomena, the most striking of which is the topological magnetoelectric effect, where an electric field generates a topological contribution to the magnetization in the same direction, with a universal constant of proportionality quantized in odd multiples of the fine-structure constant $\alpha =e^2∕\hslash c$. Finally, we present a general classification of all topological insulators in various dimensions and describe them in terms of a unified topological Chern-Simons field theory in phase space.

A finite flux of the (time-reversal-symmetric) pseudomagnetic field, which would represent the effect of a bulge in the graphene sheet, for example, is shown to be a catalyst for spontaneous breaking of the time-reversal symmetry of Dirac fermions in two dimensions. Possible experimental consequences of this effect for graphene are discussed.

The modulation of charge density and spin order in ${\left({\text{LaMnO}}_3\right)}_{2n}/{\left({\text{SrMnO}}_3\right)}_n$ $\left(n=1–4\right)$ superlattices is studied via Monte Carlo simulations of the double-exchange model. $G$-type antiferromagnetic barriers in the ${\text{SrMnO}}_3$ regions with low charge density are found to separate ferromagnetic ${\text{LaMnO}}_3$ layers with high charge density. A metal-insulator transition with increasing $n$ is observed in the direction perpendicular to the interfaces. Our simulations provide insight into how disorder-induced localization may cause the metal-insulator transition occurring at $n=3$ in experiments.

We introduce a diagrammatic quantum field formalism for the evaluation of normalized expectation values of operators, and suitable for systems with localized electrons. It is used to develop a convergent series expansion for the energy in powers of overlap integrals of single-particle orbitals. This method gives intuitive and practical rules for writing down the expansion to arbitrary order of overlap and can be applied to any spin configuration and to any dimension. Its applicability for systems with well-localized electrons is illustrated with examples, including the two-dimensional Wigner crystal and spin singlets in the low-density electron gas.

Hard x-ray photoemission and optical spectroscopies have been performed on YbS and Yb metal to determine the precise $f$-electron occupation. A comparison of the photoemission spectra with the energy-loss functions in bulk and surface, obtained from optical reflectivity, enables us to distinguish between the energy-loss satellite of ${\text{Yb}}^{2+}$ peak and the ${\text{Yb}}^{3+}$ multiplet. The results clearly indicate a purely divalent Yb state except for the surface of YbS which is altered by oxidation. We demonstrate that the present method is highly reliable in identifying the electronic structure and the mean valence in $f$-electron systems.

We find evidence for decaying magnons at strong magnetic field in the square lattice spin-1/2 Heisenberg antiferromagnet. The results are obtained using quantum Monte Carlo simulations combined with a Bayesian inference technique to obtain dynamics and are consistent with predictions from spin-wave theory.

Through the first-principles local-density-$\text{approximation}+\text{Hubbard}\text{−}U$ (LDA+$U$) electronic-structure calculation method, the microscopic properties of the Nb antisite $\left({\text{Nb}}_{\text{Li}}\right)$ and the electron-lattice interaction are investigated. The atomic structure is found to depend on the capture of electrons at the defect level, and especially when the defect level is occupied by two electrons, the ${\text{Nb}}_{\text{Li}}$ undergoes a large-lattice-relaxation (LLR), accompanied with the formation of the deep level. The main driving force toward the LLR is suggested to be the orbital hybridization of the defect level state and the conduction-band state. As a result, the ${\text{Nb}}_{\text{Li}}$ defect exhibits a negative-$U$ property. Based on the computational results, several well-known light-induced phenomena in ${\text{LiNbO}}_3$ and the polaron model are discussed.

A procedure is outlined for the determination of magnetic exchange constants in anisotropic perovskite anitferromagnets using powder inelastic neutron scattering. Spin-wave densities of states are measured using time-of-flight inelastic neutron scattering for ${\text{LaMnO}}_3$ ($A$-type antiferromagnet), ${\text{LaVO}}_3$ ($C$ type), and ${\text{LaFeO}}_3$ ($G$ type) and compared to Heisenberg model calculations. The anisotropy of in-plane $\left(J_{ab}\right)$ and out-of-plane $\left(J_c\right)$ exchange constants can be obtained from the data. The procedure quickly determines the magnetic exchange interactions without the need for single-crystal dispersion measurements and allows for rapid systematic studies of the evolution of magnetism in perovskite systems.

Using perturbative expansions and the contractor renormalization (CORE) algorithm, we obtain effective hard-core bosonic Hamiltonians describing the low-energy physics of $S=1/2$ spin-dimer antiferromagnets known to display supersolid phases under an applied magnetic field. The resulting effective models are investigated by means of mean-field analysis and quantum Monte Carlo simulations. A “leapfrog mechanism,” through means of which extra singlets delocalize in a checkerboard-solid environment via correlated hoppings, is unveiled that accounts for the supersolid behavior.

We calculate the dynamical fluctuation spectrum of electronic spins in a semiconductor under a steady-state illumination by light containing polarization squeezing correlations. Taking into account quasiparticle lifetime and spin relaxation for this nonequilibrium situation we consider up to fourth order optical effects which are sensitive to the squeezing phases. We demonstrate the possibility to control the spin fluctuations by optically modulating these phases as a function of frequency, leading to a non-Lorentzian spectrum which is very different from the thermal equilibrium fluctuations in $n$-doped semiconductors. Specifically, in the time-domain spin-spin correlation can exhibit time delays and sign flips originating from the phase modulations and correlations of polarizations, respectively. For higher light intensity we expect a regime where the squeezing correlations will dominate the spectrum.

Spin transport currents and the spin-transfer torques in voltage-biased superconducting-ferromagnetic nanopillars (SFNFS point contacts) are computed. We develop and implement an algorithm based on the Ricatti formulation of the quasiclassical theory of superconductivity to solve the time-dependent boundary conditions for the nonequilibrium Green’s functions for spin transport through the ferromagnetic interfaces. A signature of the nonequilibrium torque is a component perpendicular to the plane spanned by the two ferromagnetic moments. The perpendicular component is absent in normal-metal-ferromagnetic nanopillar contacts but is shown to have the same order of magnitude as the in-plane torque for nonequilibrium SFNFS contacts. The out-of-plane torque is due to the rotation of quasiparticle spin by the exchange fields of the ferromagnetic layers. In the ballistic limit the equilibrium torque is related to the spectrum of spin-polarized Andreev bound states, while the ac component, for small bias voltages, is determined by the nearly adiabatic dynamics of the Andreev bound states. The nonlinear voltage dependence of the nonequilibrium torque, including the subharmonic gap structure and the high-voltage asymptotics, is attributed to the interplay between multiple Andreev reflections, spin filtering, and spin mixing. These properties of spin angular momentum transport may be exploited to control the state of nanomagnets.

A monolayer of ${\text{C}}_{60}$ molecules on a flat surface provides an unusual substrate for the adsorption of simple gases. Both the lattice constant and the corrugation are larger than is typical of most traditional surfaces. These differences give rise to different phenomena, such as unusual commensurate phases. This paper discusses the ordering transition of various gases corresponding to the filling of the honeycomb array of threefold coordinated hollow sites located between ${\text{C}}_{60}$ molecules. That transition is investigated with Monte Carlo simulations and analytical (lattice-gas) models. The value of the resulting transition temperature $\left(T_c\right)$ depends on the form assumed for the long-range interaction and the role of many-body effects, i.e., screening, due to gas molecules’ close proximity to the ${\text{C}}_{60}$ layer. The effect of three-body interactions is found to be large in all cases considered.

We have successfully synthesized the fluoroarsenide SrFeAsF, which is another parent phase with the ZrCuSiAs structure. The temperature dependence of resistivity and dc magnetization both reveal an anomaly at about $T_{\text{an}}=173\text{K}$, which may correspond to the structural and/or spin-density-wave (SDW) transition. Strong Hall effect and moderate magnetoresistance were observed below $T_{\text{an}}$. Interestingly, the Hall coefficient $R_H$ is positive below $T_{\text{an}}$, which is opposite to the cases of the two parent phases of FeAs-based systems known so far, i.e., LnFeAsO ($\text{Ln}=\text{rare}$-earth elements) and $\left(\text{Ba},\text{Sr}\right){\text{Fe}}_2{\text{As}}_2$, where the Hall coefficient $R_H$ is negative. This strongly suggests that the gapping of the Fermi surface induced by the SDW order leaves one of the hole pockets fully or partially ungapped in SrFeAsF. Our data show that it is possible for the parent phases of the arsenide superconductors to display dominant carriers that are either electronlike or holelike.

We have measured the conductivity of high-mobility (001) Si metal-oxide-semiconductor field effect transistors over wide ranges of electron densities $n=\left(1.8–15\right)\times {10}^{11}{\text{cm}}^{-2}$, temperatures $T=30\text{mK}-4.2\text{K}$, and in-plane magnetic fields $B_{\parallel }=0–5\text{T}$. The experimental data have been analyzed using the theory of interaction effects in the conductivity $\sigma$ of disordered two-dimensional (2D) systems. The parameters essential for comparison with the theory, such as the intervalley scattering time and valley splitting, have been measured or evaluated in independent experiments. The observed behavior of $\sigma$, including its quasilinear increase with decreasing $T$ down to $\sim 0.4\text{K}$ and its downturn at lower temperatures, is in agreement with the theory. The values of the Fermi-liquid parameter obtained from the comparison agree with the corresponding values extracted from the analysis of Shubnikov–de Haas oscillations based on the theory of magneto-oscillations in interacting 2D systems.

We found that a single molecule in a vibrationally excited state on a surface can be delivered precisely to a desired position with the aid of a local electric field in a scanning tunneling microscope (STM) junction. An individual ${\text{CH}}_3\text{S}$ molecule on a Cu(111) surface hops laterally to move away from a negatively charged STM tip and to come toward a positively charged tip when the lateral hopping motion is induced by inelastically tunneled electrons and the molecule is positioned in an inhomogeneous electric field. These experimental findings reveal that a vibrationally excited molecule on a surface responds with high sensitivity to a local electric field in an STM junction.

The results of the single-crystal studies of the quasi-two-dimensional (2D) copper-octacyanotungstenate coordination polymer ${\left\{{\left({\text{tetrenH}}_5\right)}_{0.8}{\text{Cu}}_4^{\text{II}}{\left[{\text{W}}^{\text{V}}{\left(\text{CN}\right)}_8\right]}_4\cdot 7.2{\text{H}}_2\text{O}\right\}}_n$ are presented. It was found that the three-dimensional (3D) magnetic ordering at $T_c\approx 33\text{K}$ gives rise to antiferromagnetic structure which under relatively small magnetic field changes to ferromagnetic. There is strong easy-plane anisotropy confining the magnetic moments to the $ac$ crystallographic plane. Detailed analysis of the scaling behavior of the dc susceptibility above $T_c$ is performed. For the direction of the external magnetic field parallel to the $ac$ crystallographic plane the ordering process involves one stage only, whereas for the direction parallel to the $b$ crystallographic axis a two-stage process is revealed. The corresponding crossover at about 39 K is from the 2D short-range order state to the 3D long-range one. The well-established short-range order above the transition is consistent with the low value of the entropy amounting to only 15% of the maximal expected value. The scaling behavior of the Berezinski-Kosterlitz-Thouless type has been checked for the temperature dependence of magnetization in the $ac$ plane yielding the value of $\nu =0.56$. The spin-flip field is explained by assuming that below the transition temperature there is a domain structure coupled through the dipole-dipole interactions. The ratio of the interbilayer to the intrabilayer exchange interaction was estimated to amount to $1\times {10}^{-4}$.

Motivated by recent Monte Carlo simulations of Höglund and Sandvik (arXiv:0808.0408), we study edge response in square lattice quantum antiferromagnets. We use the O(3) nonlinear $\sigma$ model to compute the decay asymptotics of the staggered magnetization, energy density, and local magnetic susceptibility away from the edge. We find that the total edge susceptibility is negative and diverges logarithmically as the temperature $T\to 0$. We confirm the predictions of the continuum theory by performing a $1/S$ expansion of the microscopic Heisenberg model with the edge. We propose a qualitative explanation of the edge dimerization seen in Monte Carlo simulations by a theory of valence-bond-solid correlations in the Néel state. We also discuss the extension of the latter theory to the response of a single nonmagnetic impurity, and its connection to the theory of the deconfined critical point.

We have experimentally studied a superconducting flux qubit strongly coupled to a single mode of a high-quality transmission-type coplanar-waveguide resonator. The qubit-resonator coupling is revealed in the resonator transmission spectrum as a vacuum Rabi splitting. In the dispersive regime the qubit energy levels are spectroscopically probed. We observe a shift in the qubit transition frequency that linearly depends on the number of photons in the resonator, which is attributed to the ac-Zeeman shift. The observations are in agreement with the theoretical predictions.

We present a systematic angle-resolved photoemission spectroscopic study of the high-$\mathrm{T}c$ superconductor class ${\left(\text{Sr}/\text{Ba}\right)}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$. By utilizing a photon-energy-modulation contrast and scattering geometry we report the Fermi surface and the momentum dependence of the superconducting gap, $\Delta \left(\vec{k}\right)$. A prominent quasiparticle dispersion kink reflecting strong scattering processes is observed in a binding-energy range of 25–55 meV in the superconducting state, and the coherence length or the extent of the Cooper pair wave function is found to be about $20\text{Å}$, which is uncharacteristic of a superconducting phase realized by the BCS-phonon-retardation mechanism. The observed $40\pm 15\text{meV}$ kink likely reflects contributions from the frustrated spin excitations in a $J_1\text{−}{J}_2$ magnetic background and scattering from the soft phonons. Results taken collectively provide direct clues to the nature of the pairing potential including an internal phase-shift factor in the superconducting order parameter which leads to a Brillouin zone node in a strong-coupling setting.