We report on the conductance of a short (<100 nm) silicon-based two-dimensional electron gas with superconducting source and drain electrodes. Below 1 K a zero-bias enhanced conductance is observed which depends on the carrier concentration in the electron gas. The data can qualitatively be understood as due to reflectionless tunneling; i.e., quantum-coherent-enhanced Andreev scattering.

Band properties of the quaternary superconductor ${\mathrm{LuNi}}_2$${\mathrm{B}}_2$C (${\mathit{T}}_{\mathit{c}}$≊17 K) and related intermetallic phases (including, for example, LuNiBC and ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C) have been calculated in the local-density approximation via the linear augmented-plane-wave method. The results feature a density-of-states peak near the top of nearly filled Ni(3d) bands, with only modest B and C orbital admixture. The calculated ${\mathrm{LuNi}}_2$${\mathrm{B}}_2$C bands suggest that these interesting boride carbide phases represent a family of conventional (rather than high-${\mathit{T}}_{\mathit{c}}$) superconductors.

During systematic low-field magnetization measurements on high-quality single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ we discovered several crystals with a paramagnetic contribution to the field-cooled magnetization. This magnetization saturates at very low fields. It is attributed to spontaneous current loops caused by π junctions which are aligned in small external fields (Wohlleben effect). The effect is only seen with the field parallel to the c axis, demonstrating that the spontaneous currents are confined to the ${\mathrm{CuO}}_2$ planes.

Flux-creep studies have quantified the time decay of supercurrents in a proton-irradiated ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ crystal. We determine the dependence of the characteristic exponent μ (which governs current decay) on current density and magnetic field. The results are compared with collective-creep theory for the case of random pointlike disorder. Observed are predicted regimes of creep with small and large bundles of vortices, indications of single vortex motion, crossovers in current density, and other qualitative consistencies.

The reentrant behavior of the sublattice magnetization in the doped cuprates ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathit{x}}$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ is explained by a model which assumes that the localized holes reduce the sublattice magnetization more strongly than the mobile holes. Good agreement with the experiments is obtained, using the measured temperature dependence of the localized hole density, which is characterized by the (measurable) hole excitation energy E(x), and the difference between mobile and localized holes, described by a single x-dependence parameter. The theory also accounts for the absence of the reentrant behavior in highly doped samples. At higher temperatures we find additional corrections, due to a renormalization of the spin waves by the impurities.

We report observation and resonance Raman studies of spin-pair excitations in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ (0.37<x<0.7) and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ superconductors. These excitations at energy ∼3J (2700±150 ${\mathrm{cm}}^{\mathrm{-}1}$), similar to two-magnon excitations in insulators, are evidence that antiferromagnetic fluctuations with spatial extent of at least three lattice constants are not overdamped in the underdoped superconductors. With optimal hole dopings, where ${\mathit{T}}_{\mathit{c}}$>80 K, the two-magnon peak can no longer be seen above the flat Raman continuum. Photoluminescence and the origin of the broad-spectrum Raman continuum are also discussed.

The isotope effect on the superconducting transition temperature is reexamined in the model proposed by Chakravarty et al. where phonons modulate the electron-electron attraction energy on ${\mathrm{C}}_{60}$ molecules. We find additional contributions to the effective interaction that increase the magnitude of the isotope shift. One of these contributions, which is nonadiabatic, has a stronger dependence on the intermolecular motion of electrons, which could result in an enhanced isotope effect in disordered samples.

The full elastic and photoelastic tensors of quartz-homeotypic ${\mathrm{GaPO}}_4$ were measured at room temperature by Brillouin scattering. The phonon spectra were taken in 90° scattering with a computer-controlled triple-pass Fabry-Perot interferometer. The six independent elastic constants show remarkable differences with quartz and berlinite; the value of ${\mathit{c}}_{14}$ is only about one-fourth, while ${\mathit{c}}_{12}$ and ${\mathit{c}}_{13}$ are much larger than in those two homeotypic materials. The photoelastic constants are mostly higher than in quartz.

It is shown that visible light incident perpendicular to the surface of a normal metal and partly reflected at the surface can excite in the metal a dc bulk current that extends over a distance of the electron mean free path. For the case where the current is short circuited by another conductor (for instance, a superconductor) the magnetic flux built up in such a loop is estimated. It is expected to be much larger than the photoinduced magnetic flux associated with the dc surface current that was recently observed in normal metals illuminated by light falling obliquely on a metal surface. A microscopic theory of such a photomagnetic dc current is worked out for the case where the current is due to light-excited interband transitions of the electrons and their interaction with the metal surface.

An algorithm for minimization of the density-functional energy is described that replaces the diagonalization of the Kohn-Sham Hamiltonian with block diagonalization into explicit occupied and partially occupied (in metals) subspaces and an implicit unoccupied subspace. The progress reported here represents an important step toward the simultaneous goals of linear scaling, controlled accuracy, efficiency, and transferability. The method is specifically designed to deal with localized, nonorthogonal basis sets to maximize transferability and state-by-state iteration to minimize any charge-sloshing instabilities. It allows the treatment of metals, which is important in itself, and also because the dynamics of ‘‘semiconducting’’ systems can result in metallic phases. The computational demands of the algorithm scale as the particle number, permitting applications to problems involving many inequivalent atoms.

Interest in the lowest even-parity two-photon states in linear-chain π-conjugated polymers has been largely limited to their energies relative to the lowest odd-parity one-photon state. We show that the intensities of the two-photon absorptions to even-parity states that occur below the lowest one-photon states should be tiny in the infinite chain, because of cancellations. This has important implications for the interpretation of third-harmonic generation spectra of polydiacetylenes. A resonance seen in third-harmonic generation of polydiacetylenes that has commonly been assigned as a two-photon resonance is interpreted here as a three-photon resonance.

We estimate the widths of the harmonic mode-locked steps observed on the dc I-V characteristic of charge-density-wave (CDW) conductors when ac and dc voltages are applied together using the classical deformable medium model. For weakly pinned CDW’s in the high-velocity limit, δ${\mathit{E}}_{\mathit{p}/1\mathrm{}}$/${\mathit{E}}_{\mathit{T}}$ =2‖${\mathit{J}}_{\mathit{p}}$(${\mathrm{\omega }}_{\mathit{c}0}$${\mathit{E}}_{\mathit{a}\mathit{c}}$/${\mathrm{\omega }}_{\mathit{a}\mathit{c}}$${\mathit{E}}_{\mathit{T}}$)${\mathrm{\Vert }}^{4/\left(4\mathrm{-}\mathit{D}\right)}$ where δ${\mathit{E}}_{\mathit{p}/1\mathrm{}}$ is the width in electric field of the p/1 step and D is the effective dimension of the pinning. An analytic argument suggests that the phase deformations are much larger in the mode-locked state than in the normal sliding state. They are largest for ac amplitudes that yield maxima in the step width. On the p/1 step the time-averaged phase-phase correlation length is predicted to vary as δ${\mathit{E}}_{\mathit{p}/1\mathrm{}}^{\mathrm{-}1/2}$. These analytic estimates are supported by numerical simulations. Measurements of the step width variation with ac amplitude and frequency for ${\mathrm{NbSe}}_3$ crystals whose static pinning is two dimensional (2D) are consistent with the 2D step width prediction.

The anisotropy of the magnetoresistance has been measured systematically by changing the current and field direction with respect to the crystallographic axes in the quasi-two-dimensional organic conductor α-(BEDT-TTF${)}_2$KHg(SCN${)}_4$, where BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene, which shows an antiferromagneticlike ordering as a spin-density wave below ${\mathit{T}}_{\mathit{A}}$=10 K. The open direction of the Fermi surface changes from θ=0° in the normal-metal phase to 30° in the antiferromagneticlike phase (θ is the tilt angle from the c axis in the conductive a-c plane). The open direction in the normal-metal phase is in agreement with a tight-binding band-structure calculation based on the crystal structure at room temperature. Angle-dependent magnetoresistance oscillations (ADMRO’s) have been measured in both phases using a double-axis sample rotation system in fields up to 26 T. In the normal-metal phase, the angle-dependent magnetoresistance oscillations show two-dimensional behavior resulting from the closed Fermi surface with cross-sectional area 14% of the Brillouin zone. On the other hand, in the antiferromagneticlike phase, the angle-dependent magnetoresistance oscillations show one-dimensional behavior, the open direction of which is along θ=30° as observed in the current-direction dependence. At 26 T higher than ${\mathit{H}}_{\mathit{A}}$ (T=1.5 K)=23 T, the phase boundary between the antiferromagneticlike phase and the normal-metal phase, we have observed exactly the same two-dimensional ADMRO effect as in the normal-metal phase at higher temperatures. In order to explain these anomalous temperature and field dependences of the Fermi surface, we propose a superlattice model which reconstructs the Fermi surface at the phase transition. This model shows that a pair of open Fermi surfaces in the normal-metal phase is nested to each other by the vector Q(=1/6${\mathbf{k}}_{\mathit{a}}$+1/2${\mathbf{k}}_{\mathit{b}}$+ 1) / 3 ${\mathbf{k}}_{\mathit{c}}$) and the new open Fermi surface is reconstructed in antiferromagneticlike phase by the multiconnected closed Fermi surface.

Relations between transient charge transport and the glass-transition temperature, ${\mathit{T}}_{\mathit{g}}$, have been investigated in amorphous Se-based chalcogenides. The phenomenological relationship between the extrapolated zero-field drift-mobility activation energy, ${\mathit{E}}_0$, and the glass-transition temperature, ${\mathit{T}}_{\mathit{g}}$, is found to be ${\mathit{E}}_0$=1.2×${10}^{\mathrm{-}3}$${\mathit{T}}_{\mathit{g}}$ in these materials, where the units of ${\mathit{E}}_0$ and ${\mathit{T}}_{\mathit{g}}$ are eV and K, respectively. This relation can be understood by considering that the transient transport of photoinjected carriers is controlled by multiple trapping in an exponential-tail state, whose width is solely determined by ${\mathit{T}}_{\mathit{g}}$.

Resonant Auger measurements for CO adsorbed on six different d-metal surfaces—Ni(100), Pd(100), Pt(111), Cu(100), Ag(110), and Au(110)—have been performed at the C 1s and O 1s x-ray-absorption maxima. Peaks appearing at the high-kinetic-energy side of the spectra are identified and interpreted as being due to decay processes involving occupied 2${\mathrm{\pi }}^{\mathrm{*}}$-metal d orbitals (2${\mathrm{\pi }}_{\mathit{o}}$). These features provide valuable information on the adsorbate-substrate interaction, and a qualitative understanding of their behavior was obtained based on the properties of the core excited intermediate states. A feature close to the Fermi level in the photoelectron spectrum was found to resonate around the C 1s edge for the CO/Ag(110) system. This effect was interpreted as resonant photoemission of the 2${\mathrm{\pi }}^{\mathrm{*}}$-derived hybrid orbitals. A feature is observed in the C 1s excited decay spectrum for CO/Ni(100), that is interpreted as the result of a shake up in the resonant Auger decay. Differences in the relative intensities of the resonant Auger peaks due to different adsorption sites are discussed, with the CO/H/Ni(100) system as a model. Photon-energy-dependent resonant Auger results for CO/Ni(100) c(2×2) are presented. The degree of localization of the excited electron, and its influence on the decay spectra is discussed.

The moments method is used to calculate the density of states and optical-absorption spectra of large quantum systems. This method uses random wave functions to calculate 500 Chebyshev moments of the density of states (${500}^2$ for the optical-absorption spectra), and transforms these moments back to energy space. The results compare well with direct calculations on a large, 2048 Si-atom bulklike supercell system. To demonstrate its utility, the spectra of a realistic quantum dot with 1035 Si and 452 H atoms are calculated using an empirical pseudopotential Hamiltonian and a plane-wave basis of wave functions.

A joint uv-photoemission (hν=21.2 and 40.8 eV) and inverse-photoemission (hν=11–17 eV) spectroscopy investigation is reported on Lu and some Lu–transition-metal (M) compounds, namely ${\mathrm{LuCo}}_2$, ${\mathrm{LuRh}}_2$, and ${\mathrm{Lu}}_7$${\mathrm{Rh}}_3$ focused on the Lu and the M d-derived filled and empty electron states. The analysis is performed by considering the substitution effects of a 3d with a 4d M partner (${\mathrm{LuCo}}_2$ vs ${\mathrm{LuRh}}_2$) and by changing the stoichiometry and structure within the Lu-Rh phase diagram (${\mathrm{LuRh}}_2$ vs ${\mathrm{Lu}}_7$${\mathrm{Rh}}_3$ vs pure Lu). In ${\mathrm{LuRh}}_2$ the electron states, both below and above the Fermi level (${\mathit{E}}_{\mathit{F}}$), appear at lower energy (i.e., more bounded) as compared to ${\mathrm{LuCo}}_2$. ${\mathrm{Lu}}_7$${\mathrm{Rh}}_3$ shows a strong narrowing of the Rh d band below ${\mathit{E}}_{\mathit{F}}$, due to a strongly decreased Rh-Rh interaction whereas above ${\mathit{E}}_{\mathit{F}}$ a similarity with pure Lu at high-electron energies and a significant redistribution of the Rh 4d states are observed. A critical discussion of the results considering (i) the crystal structures and the atomic coordinations, and (ii) thermochemical estimations is performed. A comparison of the ${\mathrm{LuCo}}_2$ data with published theoretical results indicates that the d-derived empty electron states are appropriately described within a single-particle picture at variance with the d occupied states for which remarkable discrepancies between theory and experiment are found probably due to the larger radial extent and more pronounced delocalization of the antibonding empty states with respect to the filled states. Some general consequences of these effects will be discussed.

Degenerate-four-wave-mixing experiments were performed on ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ samples in toluene using 20-ps pulses at 532 nm for several pump-beam crossing angles. In the ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ samples, an anomalously large periodic scattering was observed with a peak at least an order of magnitude larger than the ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ peak. Compared to toluene, the probe scattering due to ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ at zero delay was lowered in the presence of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$. The behaviors were interpreted using an index grating model and the periodic signal was attributed to laser-induced phonons.

The local-spin-density approximation yields a metallic ground state for the antiferromagnetic insulators CoO, FeO, ${\mathrm{FeF}}_2$, and ${\mathrm{CoF}}_2$. By using the generalized gradient approximation (GGA) we find the difluorides to be insulators, while in CoO the GGA splits the bands near ${\mathit{E}}_{\mathit{F}}$ and opens a direct gap, but a small indirect band overlap remains. For FeO a significant improvement is found but not quite enough to make it an insulator. The magnetic moments and gaps increase. The GGA improves angular (and also in-out) correlations and thus affects the energy bands, a feature in addition to the total-energy improvement.

A one-dimensional lattice system of electrons with on-site (U) and bond-site (X) interactions is considered. Using the Green-function formalism in the Bloch representation, the occurrence of the possible instabilities in the ground state is analyzed within a mean-field-type approximation. At half filling, the X term does not modify the ground-state phase diagram of the Hubbard model; at lower (higher) band fillings a rather small bond-site attraction (repulsion) can favor the pairing of electrons.