We show that at the high-temperature limit, the coherent photonic thermal conductance of a multilayer photonic crystal can be significantly below the corresponding thermal conductance of vacuum. Moreover, the thermal conductance at this limit is independent of the thicknesses of the layers but dependent on the refractive indices of the layers only. Such universal features are directly related to the ergodic nature of the distribution of photonic bands in the frequency space.

The optical conductivity of the one-band Hubbard model is calculated using the “dynamical cluster approximation” implementation of dynamical mean-field theory for parameters appropriate to high-temperature copper-oxide superconductors. The calculation includes vertex corrections and the result demonstrates their importance. At densities of one electron per site, an insulating state is found with gap value and above-gap absorption consistent with measurements. As carriers are added the above-gap conductivity rapidly weakens and a three component structure emerges, with a low-frequency “Drude” peak, a mid-infrared absorption, and a remnant of the insulating gap. The mid-infrared feature obtained at intermediate dopings is shown to arise from a pseudogap structure in the density of states. On further doping the conductivity evolves to the Drude peak plus weakly frequency dependent tail structure expected for less strongly correlated metals.

Measurements and modeling of electron-spin transport and dynamics are used to characterize hyperfine interactions in Fe/GaAs devices with $n\text{-GaAs}$ channels. Ga and As nuclei are polarized by electrically injected electron spins, and the nuclear polarization is detected indirectly through the depolarization of electron spins in the hyperfine field. The dependence of the electron-spin signal on injector bias and applied field direction is modeled by a coupled drift-diffusion equation, including effective fields from both the electronic and nuclear polarizations. This approach is used to determine the electron-spin polarization independently of the assumptions made in standard transport measurements. The extreme sensitivity of the electron-spin dynamics to the nuclear-spin polarization also facilitates the electrical detection of nuclear magnetic resonance.

We present an experimental study of the evolution of InAs/GaAs quantum dots partially capped with GaAs, as an annealing process transforms them first into quantum rings and later into holes penetrating the whole cap layer. Shape, composition, and optical emission were monitored as a function of annealing time by means of atomic force microscopy, x-ray photoemission microscopy, and photoluminescence, respectively. Our results show a progressive dissolution of the original dot, with the outdiffused material forming a second wetting layer on the planar region surrounding the dot. For the longest annealing time, in a situation close to thermodynamic equilibrium, no residual dot material is left in the holes, and an In-rich layer covers uniformly the surface. Our findings were examined by taking into account the surface chemical potential previously written for Stranski-Krastanov islands partially covered with a cap layer. We show that the energy gain due to the formation of the second wetting layer is the driving force for the shape and composition evolution, but its analytical expression should be modified, with respect to previous formulations, by taking into account the observed material interdiffusion.

We have studied the superconducting and magnetic properties of ${\text{Fe}}_{1+y}{\text{Se}}_x{\text{Te}}_{1-x}$ single crystals $\left(0\le x\le 0.5\right)$ by magnetic susceptibility, muon-spin rotation, and neutron diffraction. We find three regimes of behavior: (i) commensurate magnetic order for $x\lesssim 0.1$, (ii) bulk superconductivity for $x\sim 0.5$, and (iii) a range $x\approx 0.25–0.45$ in which superconductivity coexists with incommensurate magnetic order. The results are qualitatively consistent with two-band mean-field models in which itinerant magnetism and extended $s$-wave superconductivity are competing order parameters.

Calculations of the pairing interaction in multiband models of the Fe superconductors show that it is attractive in both the $A_{1g}$ ($s$-wave) and $B_{1g}$ ($d$-wave) channels. This raises the possibility that these materials may have collective excitonic modes. Here, assuming an $s$-wave ground state, we investigate the $d$-wave collective excitonic mode and its coupling to the Raman scattering.

Low-temperature $\left(T\right)$ heat-capacity measurements under hydrostatic pressure of up to $p\approx 2.1\text{GPa}$ have been performed on single-crystalline ${\text{CeCu}}_2{\text{Si}}_2$. A broad superconducting (SC) region exists in the $T\text{−}p$ phase diagram. In the low-pressure region antiferromagnetic spin fluctuations and in the high-pressure region valence fluctuations had previously been proposed to mediate Cooper pairing. We could identify these two distinct SC regions. We found different thermodynamic properties of the SC phase in both regions, supporting the proposal that different mechanisms might be implied in the formation of superconductivity. We suggest that different SC order parameters are characterizing the two distinct SC regions.

Bulk specimens of the hetærolite solid solution ${\text{Zn}}_x{\text{Mn}}_{3-x}{\text{O}}_4$ with $x=0$, 0.25, 0.5, 0.75, and 1 have been prepared as homogeneous, phase-pure polycrystalline samples as ascertained by neutron-diffraction measurements. Samples with $x=0.25$, 0.5, and 0.75 exhibit shifted magnetic hysteresis loops at low temperature, characteristic of exchange bias typically seen in magnetic composites. We propose that the unusual magnetic behavior arises as a result of a nanoscale mixture of ferrimagnetic and antiferromagnetic regions that are distinct but lack long-range order. While some glassy behavior is seen in ac magnetic measurements, its magnitude is not sufficient to account for the observed dramatic exchange bias. Furthermore, isothermal and thermoremanent magnetization measurements distinguish this material from a pure spin glass. The title system offers insights into the alloying of a ferrimagnet ${\text{Mn}}_3{\text{O}}_4$ with an antiferromagnet ${\text{ZnMn}}_2{\text{O}}_4$ wherein distinct magnetic clusters grow and percolate to produce a smooth transition between competing orders.

We study the renormalization group flow of the Lagrangian for statistical and quantum systems by representing their path integral in terms of a tensor network. Using a tensor-entanglement-filtering renormalization approach that removes local entanglement and produces a coarse-grained lattice, we show that the resulting renormalization flow of the tensors in the tensor network has a nice fixed-point structure. The isolated fixed-point tensors $T_{\text{inv}}$ plus the symmetry group $G_{\text{sym}}$ of the tensors (i.e., the symmetry group of the Lagrangian) characterize various phases of the system. Such a characterization can describe both the symmetry breaking phases and topological phases, as illustrated by two-dimensional (2D) statistical Ising model, 2D statistical loop-gas model, and $1+1\text{D}$ quantum spin-1/2 and spin-1 models. In particular, using such a $\left(G_{\text{sym}},T_{\text{inv}}\right)$ characterization, we show that the Haldane phase for a spin-1 chain is a phase protected by the time-reversal, parity, and translation symmetries. Thus the Haldane phase is a symmetry-protected topological phase. The $\left(G_{\text{sym}},T_{\text{inv}}\right)$ characterization is more general than the characterizations based on the boundary spins and string order parameters. The tensor renormalization approach also allows us to study continuous phase transitions between symmetry breaking phases and/or topological phases. The scaling dimensions and the central charges for the critical points that describe those continuous phase transitions can be calculated from the fixed-point tensors at those critical points.

Using a Fermi-liquid approach, we provide a comprehensive treatment of the current and current noise through a quantum dot whose low-energy behavior corresponds to an $\text{SU}\left(N\right)$ Kondo model, focusing on the case $N=4$ relevant to carbon nanotube dots. We show that for general $N$, one needs to consider the effects of higher-order Fermi-liquid corrections even to describe low-voltage current and noise. We also show that the noise exhibits complex behavior due to the interplay between coherent shot noise, and noise arising from interaction-induced scattering events. We also treat various imperfections relevant to experiments, such as the effects of asymmetric dot-lead couplings.

We use low-temperature magnetic force microscopy (MFM) to study the hopping motion of vortices in an array of artificial pinning centers (APCs). The array consists of nanoscale holes etched in a niobium thin film by Ar-ion sputtering through an anodic aluminum-oxide template. Variable-temperature magnetometry shows a transition temperature of 7.1 K and an enhancement of the magnetization up to the third matching field at 5 K. Using MFM with attractive and repulsive tip-vortex interaction, we measure the vortex-pinning strength and investigate the motion of individual vortices in the APC array. The depinning force for individual vortices at low field ranged from 0.7 to 1.2 pN. The motion of individual vortices was found to be reproducible and consistent with movement between adjacent holes in the film. The movements are repeatable but the sequence of hops depends on the scan direction. This asymmetry in the motion indicates nonuniform local pinning, a consequence of array disorder and hole-size variation.

Self-organized In(Ga)As/GaAs quantum dots (QDs) grown on (111) substrate are proposed as ideal sources for the generation of entangled photon pairs. Due to the threefold rotational symmetry of the (111) surface, QDs with $C_{3v}$ symmetry or higher are expected to develop during growth. In contrast to QDs on (001)-oriented substrates, the symmetry of the confinement potential of (111) QDs is not lowered by piezoelectric effects. As a result the excitonic bright splitting vanishes and the $\text{biexciton}\to \text{exciton}\to 0$ recombination cascade can be used for the generation of entangled photons. We evaluate the spectroscopic separability of excitonic and biexcitonic emissions as a function of QD size, shape, and composition using the configuration-interaction model in conjunction with eight-band $\mathbf{k}\cdot \mathbf{p}$ theory. The piezoelectric field in (111) QDs predominantly aligns along the growth direction and gives rise to vertical charge separation. First- and second-order piezoelectric fields are oriented in opposite directions. The In/Ga ratio inside the QD determines the leading contribution and can be employed to balance both terms in order to achieve a field-free situation with maximal electron-hole overlap. The biexciton binding energy depends on the net piezoelectric potential drop across the QD vertical extension and becomes maximal if the first- and second-order fields outweigh each other within the QD interior.

By removing one empty site between two occupied sites, we map the ground states of chains of hardcore bosons and spinless fermions with infinite nearest-neighbor repulsion to ground states of chains of hardcore bosons and spinless fermions without nearest-neighbor repulsion, respectively, and ultimately in terms of the one-dimensional Fermi sea. We then introduce the intervening-particle expansion, where we write correlation functions in such ground states as a systematic sum over conditional expectations, each of which can be ultimately mapped to a one-dimensional Fermi-sea expectation. Various ground-state correlation functions are calculated for the bosonic and fermionic chains with infinite nearest-neighbor repulsion, as well as for a ladder model of spinless fermions with infinite nearest-neighbor repulsion and correlated hopping in three limiting cases. We find that the decays of these correlation functions are governed by surprising power-law exponents.

We study the phase diagram of a frustrated spin-$\frac{1}{2}$ ferromagnetic chain with anisotropic exchange interactions in an external magnetic field using the density matrix renormalization group method. We show that an easy-axis anisotropy enhances the tendency toward multimagnon bound states while an easy-plane anisotropy favors chirally ordered phases. In particular, a moderate easy-plane anisotropy gives rise to a quantum phase transition at intermediate magnetization. We argue that this transition is related to the finite-field phase transition experimentally observed in the spin-$\frac{1}{2}$ compound ${\text{LiCuVO}}_4$.

We calculate the density-density response function and electron self-energy for undoped bilayer graphene within the random phase approximation. We show that the quasiparticle decay rate scales linearly with the quasiparticle energy, and quasiparticle weight vanishes logarithmically in the low-energy limit, indicating non-Fermi-liquid behavior. This is a consequence of the absence of a Fermi surface for neutral bilayer graphene and corresponding larger phase space available for scattering processes. Experimental consequences of our results as well as their differences from those of single-layer graphene are discussed.

The cluster expansion (CE) method has been used to evaluate configurational properties in multicomponent systems based on the density-functional theory (DFT) calculations. Appropriate selections of not only clusters but also structures for DFT calculations (DFT structures) are crucial for the accuracy and the efficiency of the CE. In a conventional procedure to construct the CE, the CE error is reduced mainly through an appropriate selection of clusters. In the present paper, we propose an improved procedure that systematically leads to optimal selections of both clusters and DFT structures. DFT structures are chosen to cover as much of the configurational space as possible. During the iterative process, the predictive power of the out-of-sample structures can be increased up to the accuracy that is required to describe alloy thermodynamics. We apply the procedure to configurational behaviors in a simple MgO-ZnO pseudobinary system and in a complex ${\text{MgAl}}_2{\text{O}}_4$ system. The CE error is reduced in both systems, in particular, in the complex system, thereby significantly improving configurational properties at high temperatures compared with the conventional CE procedure.

We study nonequilibrium properties of an electronic Mach-Zehnder interferometer built from integer quantum Hall edge states at filling fraction $\nu =1$. For a model in which electrons interact only when they are inside the interferometer, we calculate exactly the visibility and phase of Aharonov-Bohm fringes at finite source-drain bias. When interactions are strong, we show that a lobe structure develops in visibility as a function of bias, while the phase of fringes is independent of bias, except near zeros of visibility. Both features match the results of recent experiments [Neder et al., Phys. Rev. Lett. 96, 016804 (2006)].

I derive a general set of boundary conditions for quasiclassical transport theory of metals and superconductors that is valid for equilibrium and nonequilibrium situations and includes multiband systems, weakly and strongly spin-polarized systems, and disordered systems. The formulation is in terms of the normal state scattering matrix. Various special cases for boundary conditions are known in the literature, which are, however, limited to either equilibrium situations or single band systems. The present formulation unifies and extends all these results. In this paper I will present the general theory in terms of coherence functions and distribution functions and demonstrate its use by applying it to the problem of spin-active interfaces in superconducting devices and the case of superconductor/half-metal interface scattering.

The manganite $\left(\text{La},\text{Pr},\text{Ca}\right){\text{MnO}}_3$ is well known for its micrometer-scale phase separation into coexisting ferromagnetic metallic (FMM) and insulating regions. Fabrication of bridges with widths smaller than the phase-separation length scale has allowed us to probe the magnetic properties of individual phase-separated regions. At the onset of phase separation, a magnetic field induced insulator-to-metal transition among a discrete number of domains within the narrow bridges gives rise to abrupt, low-field colossal magnetoresistance steps at well-defined switching fields. At lower temperatures when the FMM phase becomes energetically favorable, the insulating regions shrink to form thin insulating strips separating adjacent FMM regions with different coercive fields. Tunneling magnetoresistance is observed across the naturally occurring intrinsic insulating strips (tunnel barriers) spanning the width of the bridges. The presence of such intrinsic tunnel barriers introduces an alternative approach to fabricating novel nanoscale magnetic tunnel junctions.

Selection rules of ultrasound attenuation and sound-velocity renormalization are analyzed in view of their potential application to identify Pomeranchuk instabilities (electronic nematic phase). It is shown that the transverse sound attenuation along [110] direction is enhanced by the Fermi-surface fluctuations near a $d_{x^2-y^2}$-wave Pomeranchuk instability, while the attenuation along [100] direction remains unaffected. Moreover the fluctuation regime above the instability is analyzed by means of a self-consistent renormalization scheme. The results could be applied directly to ${\text{Sr}}_3{\text{Ru}}_2{\text{O}}_7$ which is a potential candidate for a Pomeranchuk instability at its metamagnetic transition in strong magnetic fields.