In this paper, we develop a quasielectrostatic theory describing the coupling of plasmon modes and we discuss its implications for the analysis and the design of the scattering and absorption spectra of complex metal nanostructures. In particular, we show that the interaction of bright plasmon modes determines the onset of zeros in the scattering spectra of nanoscale coupled systems. Under well-defined conditions, these zeros give rise to asymmetric scattering line shapes similar to the spectral signatures described by Ugo Fano in the context of atomic physics. We provide rigorous conditions in which Fano-like resonances occur, and we introduce a method for the direct calculation of their spectral position. In addition, we investigate the role of dark and bright modes in the power absorption near a Fano-like resonance. Our analysis demonstrates the quasielectrostatic origin of Fano-like resonances in subwavelength plasmonic structures.

We study the magnetic properties of Fe${}_{1+y}$Te, a parent compound of the iron-based high-temperature superconductors. Motivated by recent neutron scattering experiments, we show that a spin $S=1$ exchange model, supplemented by a single-ion spin anisotropy, accounts well for the experimentally observed low-temperature magnetic phase diagram, that exhibits a commensurate bicollinear order at low Fe dopings ($y\lesssim 0.12$) and an incommensurate spin-spiral order at high Fe dopings ($y\gtrsim 0.12$). We suggest that the commensurate-incommensurate transition at $y\simeq 0.12$ is due to the competition between the exchange interaction and the local spin anisotropy. At low Fe dopings, the single-ion spin anisotropy is strong and pins the spins along the easy axis, which, together with the spatially anisotropic exchanges, induces a unusual bicollinear commensurate magnetic order. The low-energy spin-wave excitation is gapped due to the explicit breaking of spin-rotational symmetry by the local spin anisotropy. At high Fe dopings, the single-ion anisotropy is weak, and the exchange favors an incommensurate coplanar state. The incommensurate magnetic wave vector averages out the spin anisotropy so that a gapless low-energy spin-wave excitation is obtained. We also analyze the low-energy hydrodynamic model and use it to describe the magnetostructural transition and the static and dynamical spin structure factors across the magnetic ordering transitions.

The electronic structure of Ce${}_{1-x}$Yb${}_x$CoIn${}_5$ has been studied by a combination of photoemission, x-ray absorption, and bulk property measurements. Previous findings of a Ce valence near 3$+$ for all $x$ and of an Yb valence near 2.3$+$ for $x⩾0.3$ were confirmed. One new result of this study is that the Yb valence for $x⩽0.2$ increases rapidly with decreasing $x$ from 2.3 toward 3+, which correlates well with de Haas van Alphen results showing a change of Fermi surface around $x=0.2$. Another new result is the direct observation by angle resolved photoemission Fermi surface maps of $\approx$50$\%$ cross-sectional area reductions of the $\alpha$ and $\beta$ sheets for $x=1$ compared to $x=0$, and a smaller, essentially proportionate, size change of the $\alpha$ sheet for $x=0.2$. These changes are found to be in good general agreement with expectations from simple electron counting. The implications of these results for the unusual robustness of superconductivity and Kondo coherence with increasing $x$ in this alloy system are discussed.

The resonance between the G-band phonon excitation and Landau level optical transitions in graphene has been systematically studied by micromagneto Raman mapping. In purely decoupled graphene regions on a graphite substrate, eight traces of anticrossing spectral features with G-mode peaks are observed as a function of magnetic fields up to 9 T, and these traces correspond to either symmetric or asymmetric Landau level transitions. Three distinct split peaks of the G mode, named $G_{-}$, $G_i$, and $G_{+}$, are observed at the strong magnetophonon resonance condition corresponding to a magnetic field of ∼4.65 T. These three special modes are attributed to (i) the coupling between the G phonon and the magneto-optical transitions, which is responsible for $G_{+}$ and $G_{-}$ and can be well described by the two coupled mode model and (ii) the magnetic field-dependent oscillation of the $G_i$ band, which is currently explained by the G band of graphite modified by the interaction with $G_{+}$ and $G_{-}$. The pronounced interaction between Dirac fermions and phonons demonstrates a dramatically small Landau level width (∼1.3 meV), which is a signature of the ultrahigh quality graphene obtained on the surface of graphite.

The pressure dependence of the Fermi surface of arsenic is examined using the technique of Wannier interpolation, enabling a dense sampling of the Brillouin zone and the ability to capture fine features within it. Focusing primarily on the A7 $\to$ simple-cubic phase transition, we find that this semimetal to metal transition is accompanied by the folding of Fermi surfaces. The pressure dependence of the density of states (DOS) of arsenic indicates that the onset of the Peierls-type cubic to rhombohedral distortion is signified by the appearance of emerging van Hove singularities in the DOS, especially around the Fermi level. As we noted in an earlier study, high levels of convergence are consequently required for an accurate description of this transition.

Monte Carlo simulations of the dynamic magnetic behavior of an assembly of ferromagnetic core/antiferromagnetic shell nanoparticles are reported and compared with the experimental results on a system of Co nanoparticles in Mn matrix. Memory effects on low-field zero-field-cooled magnetization curves have been investigated. Our simulations show that the memory effects increase with the concentration and that both the interface exchange coupling and the dipolar interparticle interactions contribute to the observed dynamic behavior. In particular the interface exchange interaction provides an additive source for the frustration of the system resulting in an enhancement of the memory effect. The numerical data reproduce well the experimental results confirming the glassy behavior of the investigated nanoparticle systems.

We present a combination of analytic calculations and a powerful numerical method for large spin baths in the low-field limit. The hyperfine interaction between the central spin and the bath is fully captured by the density matrix renormalization group. The adoption of the density matrix renormalization group for the central spin model is presented and a proper method for calculating the real-time evolution at infinite temperature is identified. In addition, we study to which extent a semiclassical model, where a quantum spin-1/2 interacts with a bath of classical Gaussian fluctuations, can capture the physics of the central spin model. The model is treated by average Hamiltonian theory and by numerical simulation.

The results of an experimental study of the magnetoresistivity and the Hall and Shubnikov-de Haas effects for a heterostructure with a HgTe quantum well of $20.2$ nm width are reported. The measurements were performed on gated samples over a wide range of electron and hole densities including the vicinity of a charge neutrality point. Analyzing the data, we conclude that the electron and hole energy spectra are in qualitative agreement with those calculated within the framework of $kP$ model. The electron and hole subbands are overlapped due to the nonmonotonic dispersion of the hole subband resulting in a semimetallic state. The main result of the paper, however, is the drastic quantitative difference in the experimental and calculated spectra of the hole subband. So, the hole effective mass found from the analysis of the Shubnikov-de Haas oscillations is positive and equal to approximately $0.2m_0$ and practically independent of the quasimomentum ($k$) starting from $k^2\simeq 0.7\times {10}^{12}$ cm${}^{-2}$, while the theory predicts negative (electronlike) effective mass up to $k^2\simeq 6\times {10}^{12}$ cm${}^{-2}$. The experimental effective mass near $k=0$, where the hole energy spectrum is electronlike, is close to $-0.005m_0$, whereas the theoretical value is about $-0.1m_0$.

We study the entanglement spectra of many-particle systems in states which are closely related to products of Slater determinants or products of permanents, or combinations of the two. Such states notably include the Laughlin and Jain composite fermion states which describe most of the observed conductance plateaus of the fractional quantum Hall effect. We identify a set of entanglement wave functions (EWFs), for subsets of the particles, which completely describe the entanglement spectra of such product wave functions, both in real space and in particle space. A subset of the EWFs for the Laughlin and Jain states can be recognized as composite fermion states. These states provide an exact description of the low angular-momentum sectors of the real-space entanglement spectrum (RSES) of these trial wave functions and a physical explanation of the branches of excitations observed in the RSES of the Jain states.

We report on spin-valve experiments in lateral spin-injection devices with different geometries of (Ga,Mn)As/GaAs spin Esaki diode contacts. We study the influence of the geometry of the contacts, i.e., their widths and the crystallographic orientation, on the magnetization reversal process and the resulting pattern observed in the spin-valve signal. We find that tuning of the magnetic anisotropy of the narrow (Ga,Mn)As stripes by means of lithographically induced anisotropic strain relaxation allows one to realize parallel, antiparallel, and even orthogonal configurations of magnetizations in injector and detector contacts. Understanding of the switching between these configurations during sweeping of the external in-plane magnetic field is crucial for a proper interpretation of the measured nonlocal spin signals.

We present the complete results for the dynamic experiment on measuring the gradient of the Casimir force between magnetic (Ni-coated) surfaces of a plate and a sphere. Special attention is paid to the description of some details of the setup, its calibration, error analysis, and background effects. Computations are performed in the framework of the Lifshitz theory at nonzero temperature with an account of analytic corrections to the proximity force approximation and of surface roughness using both the Drude and the plasma model approaches. The theory of magnetic interaction between a sphere and a plate due to domain structure of their surfaces is developed for both out-of-plane and in-plane magnetizations in the absence and in the presence of spontaneous magnetization. It is shown that in all cases the magnetic contribution to the measured force gradients is much smaller than the total experimental error. The comparison between experiment and theory is done using the rigorous statistical method. It is shown that the theoretical approach taking into account dissipation of free electrons is excluded by the data at a 95% confidence level. The approach neglecting dissipation is confirmed by the data at more than 90% confidence level. We prove that the results of experiments with Ni-Ni, Ni-Au, and Au-Au surfaces taken together cannot be reconciled with the approach including free electrons dissipation by the introduction of any unaccounted background force, either attractive or repulsive.

Indium sesquioxide is the most widely used conductive oxide for optoelectronic applications. The isolation of a novel orthorhombic phase of In${}_2$O${}_3$ at ambient pressures opens a new avenue for tuning the materials properties. Through an explicit comparison of the stable bixbyite and metastable orthorhombic phases of In${}_2$O${}_3$, using a hybrid density functional theory approach, we assess the impact of the loss of inversion symmetry and the distortion of the oxide sublattice on the electronic and defect structure. The band structure of the orthorhombic phase retains the key features of a transparent conductive oxide, with a large separation between the fundamental and optical band gaps. The dominant point defect—the oxygen vacancy—shows similar behavior in both phases with the coexistence of a localized defect wave function in the neutral state and a resonant transition level to the positively charged state. The natural valence band alignment between the cubic and orthorhombic phases is estimated to be 0.22 eV.

Classifying phases of local quantum systems is a general problem that includes special cases such as free fermions, commuting projectors, and others. An important distinction in this classification should be made between classifying periodic and aperiodic systems. A related distinction is that between homotopy invariants (invariants which remain constant so long as certain general properties such as locality, gap, and others hold) and locally computable invariants (properties of the system that can not change from one region to another without producing a gapless edge between them). We attack this problem using a technique inspired by Kirby's “torus trick” in topology. We use this trick to reproduce results for free fermions (in particular, using the trick to reduce the aperiodic classification to the simpler problem of periodic classification). We also show that a similar trick works for interacting phases which are nontrivial but lack anyons; these results include symmetry-protected phases. A key part of this work is an attempt to classify quantum cellular automata.

We discuss the general formalism and validity of weak-coupling perturbation theory as an impurity solver for nonequilibrium dynamical mean-field theory. The method is implemented and tested in the Hubbard model, using expansions up to fourth order for the paramagnetic phase at half filling and third order for the antiferromagnetic and paramagnetic phase away from half filling. We explore various types of weak-coupling expansions and examine the accuracy and applicability of the methods for equilibrium and nonequilibrium problems. We find that in most cases an expansion of local self-energy diagrams including all the tadpole diagrams with respect to the Weiss Green's function (bare-diagram expansion) gives more accurate results than other schemes such as self-consistent perturbation theory using the fully interacting Green's function (bold-diagram expansion). In the paramagnetic phase at half filling, the fourth-order bare expansion improves the result of the second-order expansion in the weak-coupling regime, while both expansions suddenly fail at some intermediate interaction strength. The higher-order bare perturbation is especially advantageous in the antiferromagnetic phase near half filling. We use the third-order bare perturbation expansion within the nonequilibrium dynamical mean-field theory to study dynamical symmetry breaking from the paramagnetic to the antiferromagnetic phase induced by an interaction ramp in the Hubbard model. The results show that the order parameter, after an initial exponential growth, exhibits an amplitude oscillation around a nonthermal value followed by a slow drift toward the thermal value. The transient dynamics seems to be governed by a nonthermal critical point, associated with a nonthermal universality class, which is distinct from the conventional Ginzburg-Landau theory.

We exploit the Mie resonance in dielectric microparticles to design a single-negative metamaterial monolayer with near-unity reflectivity and negligible absorptivity. In contrast to Bragg reflectors and photonic band gap materials, which require multiple layers for high reflection, this metamaterial is both highly reflective and subwavelength in thickness. We identify the underlying physics necessary to design near-perfect all-dielectric reflectors at virtually any wavelength band of interest. Using full-wave, finite-element analysis and realistic optical constants for the constitutive materials, we develop a 0.45-$\mu$m-thick, silicon-based metamaterial monolayer with normal-incidence reflectivity over 99.999% and absorptivity less than 0.001% at a short-wave infrared wavelength of 1.5 $\mu$m.

We discuss a numerical method for the calculation of the spectrum of Lyapunov exponents for spatially extended systems described by coupled Poisson and continuity equations. This approach was applied to the model of collective charge transport in semiconductor superlattices operating in the miniband transport regime. The method is in very good agreement with analytical results obtained for the steady state. As an illustrative example, we consider the collective electron dynamics in the superlattice subjected to an ac voltage and a tilted magnetic field, and conclusively show that, depending on the field parameters, the dynamics can exhibit periodic, quasiperiodic, or chaotic behavior.

Topological superconductors, such as noncentrosymmetric superconductors with strong spin-orbit coupling, exhibit protected zero-energy surface states, which possess an intricate helical spin structure. We show that this nontrival spin character of the surface states can be tested experimentally from the absence of certain backscattering processes in Fourier-transform scanning tunneling measurements. A detailed theoretical analysis is given of the quasiparticle scattering interference on the surface of both nodal and fully gapped topological superconductors with different crystal point-group symmetries. We determine the universal features in the interference patterns resulting from magnetic and nonmagnetic scattering processes of the surface quasiparticles. It is shown that Fourier-transform scanning tunneling spectroscopy allows us to uniquely distinguish among different types of topological surface states, such as zero-energy flat bands, arc surface states, and helical Majorana modes, which in turn provides valuable information on the spin and orbital pairing symmetry of the bulk superconducting state.

In a one-dimensional (1D) superconductor, zero-temperature quantum fluctuations destroy phase coherence. Here we put forward a mechanism which can restore phase coherence: power-law hopping. We study a 1D attractive-$U$ Hubbard model with power-law hopping using Abelian bosonization and density-matrix renormalization group (DMRG) techniques. The parameter that controls the hopping decay acts as the effective, noninteger spatial dimensionality $d_{\mathrm{eff}}$. For real-valued hopping amplitudes we identify analytically a range of parameters for which power-law hopping suppresses fluctuations and restores superconducting long-range order for any $d_{\mathrm{eff}}>1$, at zero temperature. A detailed DMRG analysis fully supports these findings. These results are also of direct relevance to quantum magnetism as our model can be mapped onto an $S=1/2$ XXZ spin chain with power-law decaying couplings, which can be studied experimentally with cold-ion-trap techniques.

In a recent Brief Report, Grünebohm et al. [Phys. Rev. B 84, 132105 (2011)] report that they fail to reproduce the $A_{2u}$ ferroelectric instability of $TiO2$ in the rutile structure calculated with density functional theory within the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) by Montanari et al. [Chem. Phys. Lett. 364, 528 (2002)]. We demonstrate that this disagreement arises from an erroneous treatment of $Ti$ $3s$ and $3p$ semicore electrons as core in their calculations. Fortuitously the effect of the frozen-semicore pseudopotential cancels the phonon instability of the PBE exchange correlation, and the combination yields phonon frequencies similar to the local density approximation harmonic values. Grünebohm et al. also attempted and failed to reproduce the soft acoustic phonon mode instability under (110) strain reported by Mitev et al. [Phys. Rev. B 81, 134303 (2010)]. For this mode the combination of PBE-GGA and frozen semicore yields a small imaginary frequency of $9.8i$. The failure of Grünebohm et al. to find this mode probably arose from numerical limitations of the geometry optimization approach in the presence of a shallow double well potential; the optimization method is not suitable for locating such instabilities.