The surface phase $\mathrm{Sn}∕\mathrm{Cu}\left(100\right)\text{−}(3\sqrt{2}\times \sqrt{2})R45°$ exhibits a temperature induced phase transition to a $(\sqrt{2}\times \sqrt{2})R45°$ phase above $360\mathrm{K}$. We report an angle-resolved photoemission study of the two-dimensional Fermi surface of the low-temperature phase. The Fermi surface is formed by folding of a quasi-two-dimensional surface band and presents three contours. It is characterized by gaps appearing in optimally nested regions of one of the contours, whereas other sections remain ungapped. We address the origin of the folding and of the surface phase transition, which is attributed to the formation of a surface charge density wave.

We study a coplanar system of electrostatically coupled semiconductor quantum rings each charged by a single electron using a configuration-interaction approach. We find that the electron densities confined in separate rings are significantly deformed by the inter-ring coupling which results in reduction of the persistent currents and the magnetic dipole moments that they generate, particularly at half flux quanta for which the persistent current loops are broken by appearance of zeros of angular single-ring wave functions. For three and more rings, the magnetic properties of the system strongly depend on the geometry of the ring configuration. We demonstrate a frustration mechanism which prevents the charge density deformation and reduction of the magnetization that occurs for rings near symmetry centers of the system. Due to this mechanism, the average magnetization produced by a linear array of rings exceeds several times the one generated by a square array. We discuss the nature of the electron-electron spatial inter-ring correlation. In the ground state, the correlation is semiclassical and can be reproduced by a separable wave function. Quantum correlation is weak and short range in the ground state. Excited states with longer range zigzag correlated structures formed in the internal degrees of freedom are also discussed.

We present an experimental realization of the transmon qubit, which is an improved superconducting charge qubit derived from the Cooper pair box. We experimentally verify the predicted exponential suppression of sensitivity to $1∕f$ charge noise. This removes the leading source of dephasing in charge qubits which results in homogeneously broadened transitions with relaxation and dephasing times in the microsecond range. Our systematic characterization of the qubit spectrum, anharmonicity, and charge dispersion shows excellent agreement with theory.

We consider the elastic scattering of a fast incident electron both prior and subsequent to its involvement in an atomic inner-shell ionization (core-loss) event in a crystal. By using numerical simulations, it is shown that elastic scattering subsequent to ionization can strongly affect the qualitative features of atomic resolution core-loss images of crystals recorded in the scanning transmission electron microscope (STEM). This conclusion holds even for a thin crystal foil and for a relatively large detector, which is matched to the probe-forming aperture in an aberration-corrected STEM. Such a conclusion is potentially very important for the interpretation of experimental core-loss images. We also introduce an approximate model that incorporates the effects of elastic scattering subsequent to ionization in the case of a small detector.

The Josephson effect and superconducting proximity effect were observed in superconductor-graphene-superconductor (SGS) Josephson junctions with coherence lengths comparable to the distance between the superconducting leads. By comparing the measured gate dependence of the proximity induced subgap features (multiple Andreev reflections) and of the supercurrent to theoretical predictions, we find that the diffusive junction model yields close quantitative agreement with the results. By contrast, predictions of the ballistic SGS model are inconsistent with the data. We show that all SGS devices reported so far, our own as well as those of other groups, fall in the diffusive junction category. This is attributed to substrate induced potential fluctuations due to trapped charges and to the invasiveness of the metallic leads.

In this paper, we show how the two-particle Green function can be obtained within the framework of the dual fermion approach. This facilitates the calculation of the susceptibility in strongly correlated systems where long-ranged nonlocal correlations cannot be neglected. We formulate the Bethe–Salpeter equations for the full vertex in the particle-particle and particle-hole channels and introduce an approximation for practical calculations. The scheme is applied to the two-dimensional Hubbard model at half-filling. The spin-spin susceptibility is found to strongly increase for the wave vector $\mathbf{q}=(\pi ,\pi )$, which indicates the antiferromagnetic instability. We find a suppression of the critical temperature compared to the mean-field result due to the incorporation of the nonlocal spin fluctuations.

We theoretically calculate and compare the single particle relaxation time $\left({\tau }_s\right)$ defining the quantum level broadening and the transport scattering time $\left({\tau }_t\right)$ defining the Drude conductivity in two-dimensional (2D) graphene layers in the presence of screened charged impurity scattering and short-range defect scattering. We find that the ratio ${\tau }_t∕{\tau }_s$ strongly increases with increasing $k_F{z}_i$ and $\kappa$, where $k_F$, $z_i$, and $\kappa$ are, respectively, the Fermi wave vector, the separation of the substrate charged impurities from the graphene layer, and the background lattice dielectric constant. A critical quantitative comparison of the ${\tau }_t∕{\tau }_s$ results for graphene with those for the corresponding modulation-doped semiconductor structures is provided, showing significant differences between these two 2D carrier systems.

We analyze the competing effects of moderate to strong Coulomb electron-electron interactions and weak quenched disorder in graphene. By using a one-loop renormalization group calculation controlled within the large-$N$ approximation, we demonstrate that at successively lower energy (temperature or chemical potential) scales, a type of non-Abelian vector potential disorder always asserts itself as the dominant elastic scattering mechanism for generic short-ranged microscopic defect distributions. Vector potential disorder is tied to both elastic lattice deformations (“ripples”) and topological lattice defects. We identify several well-defined scaling regimes, for which we provide scaling predictions for the electrical conductivity and thermopower, valid when the inelastic lifetime due to interactions exceeds the elastic lifetime due to disorder. Coulomb interaction effects should strongly figure into the physics of suspended graphene films, where $r_s>1$; we expect the vector potential disorder to play an important role in the description of transport in such films.

We derive the first two moment sum rules of the conduction electron retarded self-energy for both the Falicov–Kimball model and the Hubbard model coupled to an external spatially uniform and time-dependent electric field (this derivation also extends the known nonequilibrium moment sum rules for the Green functions to the third moment). These sum rules are used to further test the accuracy of nonequilibrium solutions to the many-body problem; for example, we illustrate how well the self-energy sum rules are satisfied for the Falicov–Kimball model in infinite dimensions and placed in a uniform electric field turned on at time $t=0$. In general, the self-energy sum rules are satisfied to a significantly higher accuracy than the Green function sum rules.

We investigate the two-magnon Raman scattering from the $S=\frac{1}{2}$ Heisenberg antiferromagnet on the triangular lattice by considering both the effect of the renormalization of the one-magnon spectrum by $1/S$ corrections and the final-state magnon-magnon interactions. The bare Raman intensity displays two peaks related to one-magnon Van Hove singularities. We find that $1/S$ self-energy corrections to the one-magnon spectrum strongly modify this intensity profile. The central Raman peak is significantly enhanced due to plateaus in the magnon dispersion, the high frequency peak is suppressed due to magnon damping, and the overall spectral support narrows considerably. Additionally, we investigate final-state interactions by solving the Bethe-Salpeter equation to $O\left(1/S\right)$. In contrast to collinear antiferromagnets, the noncollinear nature of the magnetic ground state leads to an irreducible magnon scattering that is retarded and nonseparable already for the lowest order. We show that final-state interactions lead to a rather broad Raman continuum centered on approximately twice the “roton” energy. We also discuss the dependence on the scattering geometry.

The electronic contribution to friction at semiconductor surfaces was investigated by using a Pt-coated tip with 50 nm radius in an atomic force microscope sliding against an $n$-type GaAs(100) substrate. The GaAs surface was covered by an approximately 1 nm thick oxide layer. Charge accumulation or depletion was induced by the application of forward or reverse bias voltages. We observed a substantial increase in friction force in accumulation (forward bias) with respect to depletion (reverse bias). We propose a model based on the force exerted by the trapped charges that quantitatively explains the experimental observations of excess friction.

We study quasiparticle and electron-hole pair excitations in the presence of spin polarization within the framework of many-body perturbation theory. The influence of magnetic ordering on linear optical response is examined in detail for collinear spins. Whereas Hedin’s expression for the exchange-correlation self-energy in the $GW$ approximation remains diagonal with respect to the one-particle spin quantum number, the Bethe-Salpeter equation becomes more complex: The separation between triplet and singlet excitons no longer holds. The submatrices of the two-particle Hamiltonian are coupled by the matrix elements of the bare Coulomb potential which account for local-field effects. Therefore, the rank of the corresponding eigenvalue problem doubles. Numerical results are computed from first principles for the antiferromagnetic insulator MnO starting from electronic states obtained with nonlocal exchange-correlation potentials. The calculated electronic structure and optical absorption spectra including excitonic and local-field effects are compared with experimental data. The dominance of interatomic transitions between Mn atoms in the interband region of the optical absorption is revealed.

We present an analysis of recent [M. Yogi et al., J. Phys. Soc. Jpn. 75, 124702 (2006)] Sb-NQR measurements of the heavy-fermion superconductor ${\text{PrOs}}_4{\text{Sb}}_{12}$ and the related La-doped series of superconductors ${\text{Pr}}_{1-x}{\text{La}}_x{\text{Os}}_4{\text{Sb}}_{12}$. We find that this data, along with previous temperature-dependent superfluid density results and are well described by a strong-coupling order parameter containing a doping-dependent proportion of the first cubic harmonic $\left(k_x^4+k_y^4+k_z^4\right)$ component along with an $s$-wave component, when inelastic scattering due to Coulomb effects are taken into account. The resulting gap function has deep minima along the cubic axis [100], etc. but no nodes. We use the order parameter fits to generate, within the Ginzburg–Landau theory, the effective thermal mass enhancement relative to the ${\text{LaOs}}_4{\text{Sb}}_{12}$ material and find that it rapidly varies for small $x$.

Scanning tunneling spectroscopy (STS) measurements find that the surface of Bi-2212 is characterized by nanoscale sized regions, “gap patches,” which have different magnitudes for the $d$-wave energy gap ${\Delta }_0\left(\mathbf{r}\right)$. Recent studies have shown that the tunneling conductance can be fitted using a BCS-type density of states for a $d$-wave superconductor with a local quasiparticle scattering rate. The fit is made with a scattering rate which varies linearly with energy and has a slope $\alpha \left(\mathbf{r}\right)$ that is positively correlated with the local value of the gap. We consider first the question of what is actually measured in such an experiment. To this end, we revisit a model of quasiparticle scattering by impurities and spin fluctuations which was previously used to describe the lifetimes of nodal quasiparticles measured by angle-resolved photoelectron spectroscopy (ARPES). We argue that the broadening of the local density of states is determined, except in the case of localized impurity bound states, by the imaginary part of the self-energy of the system averaged over a small region. The size of this region is set by a mean free path which depends on the energy. At low energies, this region is found to be significantly larger than a gap “patch,” so that the density of states measured by STS is homogeneous in this energy range. At higher energies where the mean free path is comparable with the patch size, the density of states is inhomogeneous. We show that a local self-energy in the impurity-plus-spin fluctuation model, while not strictly linear, yields a local density of states nearly identical to the full theory, and we argue that it is consistent with the STS data as well as the phenomenological linear scattering rate extracted from experiment. We also explore the qualitative consequences of this phenomenology for the spectral widths observed in ARPES and predict the existence of Fermi arcs in the superconducting state.

We report on a magneto-optical Kerr effect (MOKE) investigation of microstructured rectangular islands, which have been arranged on kagome lattices. The magnetization reversal was studied by regular longitudinal vector MOKE in specular geometry as well as in Bragg MOKE geometry, using the diffraction spots from the grating for hysteresis measurements. The magnetization at remanence is imaged by using magnetic force microscopy and x-ray photoemission electron microscopy. The measurements have been compared to the results of micromagnetic simulations, which allow a detailed interpretation of the experimental data. It is demonstrated that the magnetization reversal process in an external magnetic field strongly depends on the shape and the interparticle spacing between the islands. For the thorough understanding of the nontrivial dependence of magnetization reversal on the array geometry, the magnetic moments of the islands and the energy barriers for magnetization switching in individual elements have been theoretically analyzed. The performed analysis provides insight into the magnetostatic interactions and probability of switching paths taken by the particles arranged on lattices with kagome symmetry during the magnetization reversal process.

The crystal structure and numerous normal and superconducting state properties of layered tetragonal $(P4∕nmm)$ LaFeAsO, with F doping of $\approx 11\%$, are reported. Resistivity measurements give an onset transition temperature $T_c=28.2\mathrm{K}$, and low field magnetic susceptibility data indicate bulk superconductivity. In applied magnetic field, analysis of the resistive transition results in a critical field $H_{c2}\approx 30\mathrm{T}$ and a coherence length ${\xi }_{\mathrm{GL}}\approx 35\mathrm{\AA }$. An upper limit for the electron carrier concentration of $1\times {10}^{21}{\mathrm{cm}}^{-3}$ is inferred from the Hall data just above $T_c$. Strong electron-electron correlations are suggested from temperature-dependent resistivity, Seebeck coefficient, and thermal conductivity data. Anomalies near $T_c$ are observed in both Seebeck coefficient and thermal conductivity data.

Quantum electrodynamics in $2+1$ dimensions is an effective gauge theory for the so-called algebraic quantum liquids. A new type of such a liquid, the algebraic charge liquid, has been proposed recently in the context of deconfined quantum critical points [R. K. Kaul et al., Nat. Phys. 4, 28 (2008)]. In this context, by using the renormalization group in $d=4-\epsilon$ space-time dimensions, we show that a deconfined quantum critical point occurs in a SU(2) system provided the number of Dirac fermion species $N_f\ge 4$. The calculations are done in a representation where the Dirac fermions are given by four-component spinors. The critical exponents are calculated for several values of $N_f$. In particular, for $N_f=4$ and $\epsilon =1$ $\left(d=2+1\right)$, the anomalous dimension of the Néel field is given by ${\eta }_N=1/3$, with a correlation length exponent $\nu =1/2$. These values considerably change for $N_f>4$. For instance, for $N_f=6$, we find ${\eta }_N\approx 0.75191$ and $\nu \approx 0.66009$. We also investigate the effect of chiral symmetry breaking and analyze the scaling behavior of the chiral holon susceptibility, $G_{\chi }\left(x\right)\equiv ⟨\overline{\psi }\left(x\right)\psi \left(x\right)\overline{\psi }\left(0\right)\psi \left(0\right)⟩$.

We describe the thermal $\left(\kappa \right)$ and electrical $\left(\sigma \right)$ conductivities of quasi-one-dimensional wires, across a quantum phase transition from a superconductor to a metal induced by pair-breaking perturbations. Fluctuation corrections to BCS theory motivate a field theory for quantum criticality. We describe deviations in the Wiedemann-Franz ratio $\kappa ∕\sigma T$ (where $T$ is the temperature) from the Lorenz number $({\pi }^2∕3){(k_B∕e)}^2$, which can act as sensitive tests of the theory. We also describe the crossovers out of the quantum-critical region into the metallic and superconducting phases.

We theoretically show how constant-energy maps of the angle-resolved photoemission intensity can be used to test wave function symmetry in graphene. For monolayer graphene, we demonstrate that the observed anisotropy of angle-resolved photoelectron spectroscopy spectra is a manifestation of what has been recently branded as an electronic chirality. For bilayer graphene, we show that the anisotropy of the constant-energy maps may be used to extract information about the magnitude and sign of interlayer coupling parameters and about symmetry breaking inflicted on a bilayer by the underlying substrate.

The concept of a charge density wave (CDW), which is induced by Fermi-surface nesting, originated from the Peierls idea of electronic instabilities in purely one-dimensional metals and is now often applied to charge ordering in real low-dimensional materials. The idea is that if Fermi surface contours coincide when shifted along the observed CDW wave vector, then the CDW is considered to be nesting derived. We show that, in most cases, this procedure has no predictive power, since Fermi surfaces either do not nest at the right wave vector or nest more strongly at the wrong vector. We argue that only a tiny fraction, if any, of the observed charge ordering phase transitions are true analogs of the Peierls instability because electronic instabilities are easily destroyed by even small deviations from perfect nesting conditions. By using prototypical CDW materials ${\text{NbSe}}_2$, ${\text{TaSe}}_2$, and ${\text{CeTe}}_3$, we show that such conditions are hardly ever fulfilled and that the CDW phases are actually structural phase transitions driven by the concerted action of electronic and ionic subsystems, i.e., a $\mathbf{q}$-dependent electron-phonon coupling plays an indispensable part. We also mathematically show that the original Peierls construction is so fragile that it is unlikely to apply to real materials. We argue that no meaningful distinction between a CDW and an incommensurate lattice transition exists.