It is experimentally shown that measurements of the chemical-potential variation in a magnetic field could give valuable information about the magnetization of thin films. For superconducting single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films two kinds of behavior of μ(H) were observed depending on the substrate used. For films deposited on polished ${\mathrm{NdGaO}}_3$ single-crystal substrates the change of μ was mainly reversible. That means that no magnetic flux was trapped during a magnetic-field sweep. For films deposited on MgO, ${\mathrm{SrTiO}}_3$, and ${\mathrm{LaAlO}}_3$ single-crystal substrates μ changed irreversibly. The estimated critical current density is ${\mathit{j}}_{\mathit{c}}$∼${10}^7$ A/${\mathrm{cm}}^2$ at T=4.2 K in both cases. Oscillations of μ in films evaporated on cleaved MgO substrates were observed. The effect of a magnetic field on μ in a ferromagnetic film, predicted 60 years ago, was confirmed.

Recent experimental studies by Wolfe, Snoke, and Lin have given strong evidence that paraexcitons in optically excited ${\mathrm{Cu}}_2$O crystals form a Bose condensate. In these studies, the observed recombination decay luminescence line shape I(ω) has been analyzed in terms of an ideal Bose-condensed gas. We derive a general expression for I(ω) in terms of the exact single-particle spectral density A(k,ω) of a Bose fluid. This expression is then used to calculate I(ω) for a dilute, weakly interacting Bose gas. Our analytic results, based on the approximate theory due to Popov, are valid in the region just below the Bose-Einstein transition temperature. When formally extended down to T=0, they reproduce the well-known Bogoliubov approximation.

This paper continues our study of vortices in Ginzburg-Landau theories with special attention to applications in superconductivity. In another paper, we derived asymptotic equations governing the dynamics of interacting vortices. Here, we study the hydrodynamic limit of these vortices. For vortices in the solutions of the nonlinear Schrödinger equation, the hydrodynamic equation is the incompressible Euler’s equation in fluid mechanics. For vortices in the time-dependent Ginzburg-Landau equations, the hydrodynamic equations can be thought of as being the complement of the Euler equations. Preliminary results on the numerical studies of the hydrodynamic equations are presented. As applications of the hydrodynamic formalism, we study the pinning of vortex liquids by periodic potentials, and the propagation of magnetic fields into type-II superconductors. The hydrodynamic formalism suggests that to leading order, the vortex liquids are pinned even at small but positive temperature.

Analytic pair potentials proposed by Pettifor and Ward are developed using the Heine-Abarenkov pseudopotential for Li-Na, Na-K, and Na-Cs liquid binary alloys. The corresponding partial structure factors are calculated using the random-phase approximation. The calculated thermodynamic and structural properties using this real-space formalism are in good agreement with experiments.

A Hartree-type of effective potential is derived for an excess electron near the helium liquid-vapor interface. The surface-state eigenvalaue spectrum is thus calculated. Results obtained for both ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ (with an experimental interface profile) and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ (with a theoretical profile) are in reasonable agreement with experimental data.

When a vortex in a two-dimensional Josephson-junction array is driven by a constant external current it may move as a particle in a viscous medium. Here we study the nature of this viscous motion. We model the junctions in a square array as resistively and capacitively shunted Josephson junctions and carry out numerical calculations of the current-voltage characteristics. We find that the current-voltage characteristics in the damped regime are well described by a model with a nonlinear viscous force of the form ${\mathit{F}}_{\mathit{D}}$=η(ẏ)ẏ=[A/(1+Bẏ)]ẏ, where ẏ is the vortex velocity, η(ẏ) is the velocity-dependent viscosity, and A and B are constants for a fixed value of the Stewart-McCumber parameter. This result is found to apply also for triangular lattices in the overdamped regime. Further qualitative understanding of the nature of the nonlinear friction on the vortex motion is obtained from a graphic analysis of the microscopic vortex dynamics in the array. The consequences of having this type of nonlinear friction law are discussed and compared to previous theoretical and experimental studies.

We investigate superconductivity in a grand canonical ensemble with fixed number parity (even or odd). In the low-temperature limit we find small corrections to the BCS gap equation and dispersion E(k). The even-odd free-energy difference in the same limit decreases linearly with temperature, in accordance with the behavior observed experimentally and previously arrived at from a quasiparticle model. The theory yields deviations from the BCS predictions for the specific heat, ultrasound attenuation, nucleon spin-relaxation rate, and electromagnetic absorption.

A criterion for the phase separation in the s-f model is found. It takes place only if the interband exchange W exceeds some critical value ${\mathit{W}}_{\mathit{c}}$. For the two-dimensional square lattice this critical value is ${\mathit{W}}_{\mathit{c}}$= √32πtJ (t is the hopping amplitude in the s band, J is the antiferromagnetic exchange in the f band, and J≪t).

Polarized micro-Raman spectra of a single crystallite of ${\mathrm{HgBa}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ have been measured and used to assign phonon modes of ${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{E}}_{\mathit{g}}$ symmetry intrinsic to the superconducting phase. Predictions of the peak positions, linewidths, and Raman cross sections from compounds with similar bonding, i.e., HgO, Y-124, and Y-123, are closely matched and suggest assignments of the high-frequency ${\mathit{A}}_{1\mathit{g}}$ modes at 592 and 568 ${\mathrm{cm}}^{\mathrm{-}1}$ to apical oxygen stretch vibrations of the ideal structure and of a Cu or O defect (in the Hg plane). The low-frequency 168, 161, and 75 ${\mathrm{cm}}^{\mathrm{-}1}$ modes are assigned to O and Ba vibrations of ${\mathit{E}}_{\mathit{g}}$,${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{E}}_{\mathit{g}}$ symmetry, respectively. An anomaly in the phonon frequency and linewidth of the 568-${\mathrm{cm}}^{\mathrm{-}1}$ ${\mathit{A}}_{1\mathit{g}}$ mode is observed below ${\mathit{T}}_{\mathit{c}}$. If interpreted as a change in the phonon self-energy due to the onset of superconductivity, this behavior is consistent with a gap energy near 568 ${\mathrm{cm}}^{\mathrm{-}1}$, i.e., 8.7${\mathit{kT}}_{\mathit{c}}$ in ${\mathrm{HgBa}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ and constitutes a first estimate of the superconducting gap in this material. The Raman tensor elements for the assigned ${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{E}}_{\mathit{g}}$ modes are given in absolute units.

We report a far-infrared reflectivity study in the temperature range 10–300 K of the double-chain ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ superconductor with Zn and Sr substituting for Cu and Ba, respectively. Superconductivity-induced self-energies of the transverse optical ${\mathit{B}}_{1\mathit{u}}$(z) phonon were used to estimate the position of the superconducting gap. Evidence for a shift of the gap to lower energies upon decreasing the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ was found for Zn- as well as for Pr-doped materials. A comparison of the linewidth changes of the plane-oxygen phonon (at ∼300 ${\mathrm{cm}}^{\mathrm{-}1}$) with the results of a strong-coupling calculation yields the gap-to-${\mathit{T}}_{\mathit{c}}$ ratio 2${\mathrm{\Delta }}_0$/${\mathit{kT}}_{\mathit{c}}$=(8.2±1.0) for ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{3.96}$${\mathrm{Zn}}_{0.04}$)${\mathrm{O}}_8$ which is close to 2${\mathrm{\Delta }}_0$/${\mathit{kT}}_{\mathit{c}}$=(6.6±0.2) reported for (${\mathrm{Pr}}_{0.37}$${\mathrm{Y}}_{0.63}$)${\mathrm{Ba}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$. For Sr in place of Ba in Y(${\mathrm{Ba}}_{2\mathrm{-}\mathit{z}}$${\mathrm{Sr}}_{\mathit{z}}$)${\mathrm{Cu}}_4$${\mathrm{O}}_8$, for which ${\mathit{T}}_{\mathit{c}}$ remains essentially the same within a broad range of compositions (0≤z≤0.6), no change of the temperature behavior of the phonon and thus no variation of the position of the superconducting gap was observed. These experimental findings confirm that the picture of superconductivity-induced changes in the phonon parameters holds in the double-chain compound, similarly to the single-chain material. We also report the effects of these dopants on the phonon anomaly which has been attributed to a spin gap.

We report measurements of the surface impedance ${\mathit{Z}}_{\mathit{s}}$=${\mathit{R}}_{\mathit{s}}$+iωλ’ of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films in an externally applied dc magnetic field B (parallel to the c axis) using a stripline resonator. At T=4.3 K we obtain the surface resistance ${\mathit{R}}_{\mathit{s}}$ and the microwave penetration depth λ’ as a function of applied dc field up to 5 T and as a function of microwave frequency f from 1.2 to 20 GHz. While λ’ increases linearly with B for B>1 T at all frequencies, ${\mathit{R}}_{\mathit{s}}$ is found to be roughly ∝${\mathit{B}}^{\mathrm{\alpha }\left(\mathit{f}\right)}$, where α(f)<1 for f≤10 GHz and α(f)≊1 for f≥10 GHz. In zero dc field, ${\mathit{R}}_{\mathit{s}}$∝${\mathit{f}}^2$. For B>1 T, ${\mathit{R}}_{\mathit{s}}$ shows a much weaker dependence on f. The results for ${\mathit{Z}}_{\mathit{s}}$(f,T,B) have been quantitatively explained using a model developed by Coffey and Clem, based on a self-consistent treatment of vortex dynamics that includes the influence of vortex pinning, viscous drag, and flux creep. The pinning force constant ${\mathrm{\alpha }}_{\mathit{p}}$, the pinning frequency ${\mathrm{\omega }}_{\mathit{p}}$, and the pinning activation energy ${\mathit{U}}_0$(T,B) are obtained through the fitting procedure. We find that the effects of thermally activated flux creep at 4.3 K upon the surface resistance are significant. The low ${\mathit{U}}_0$≊35 K that we determine is interpreted as arising from the interaction of the vortex lattice with a dense random pinning potential as described in the collective-pinning theory.

Pressure-induced structural transformation and the concomitant loss of intermediate-range order (IRO) in high-density ${\mathrm{SiO}}_2$ glass are investigated with the molecular-dynamics (MD) approach. The MD simulations cover a wide range of mass densities—from normal density (2.20 g/${\mathrm{cm}}^3$) to the density corresponding to stishovite (4.28 g/${\mathrm{cm}}^3$). This twofold increase in the density produces significant changes in the short-range order and intermediate-range order. As the density increases from 2.20 to 4.28 g/${\mathrm{cm}}^3$, the Si-O bond length increases from 1.61 to 1.67 Å, the Si-O and O-O coordinations change from 4 to 5.8 and from 6 to 12, respectively, and the O-Si-O bond angle changes from 109° to 90°. These results provide firm evidence of structural transition from a corner-sharing Si(${\mathrm{O}}_{1/2}$${)}_4$ tetrahedral network to a network of Si(${\mathrm{O}}_{1/3}$${)}_6$ octahedra jointed at corners and edges. At normal density, the first sharp diffraction peak (FSDP) in the static structure factor S(q) is at 1.6 A${\mathrm{̊}}^{\mathrm{-}1}$ whereas under pressure the height of the FSDP is considerably diminished and its position shifts to larger q values. At a density of 2.64 g/${\mathrm{cm}}^3$, a peak in S(q) appears at 2.85 A${\mathrm{̊}}^{\mathrm{-}1}$. The height of this peak grows as the density increases. All of these results are in agreement with the recent high-pressure x-ray measurements on ${\mathrm{SiO}}_2$ glass.

The detailed behavior of phonon dispersion curves near momenta which span the electronic Fermi sea in a superconductor is presented. An anomaly, similar to the metallic Kohn anomaly, exists in a superconductor’s dispersion curves when the frequency of the photon spanning the Fermi sea exceeds twice the superconducting energy gap. This anomaly occurs at approximately the same momentum but is stronger than the normal-state Kohn anomaly. It also survives at finite temperature, unlike the metallic anomaly. Determination of Fermi-surface diameters from the location of these anomalies, therefore, may be more successful in the superconducting phase than in the normal state. However, the superconductor’s anomaly fades rapidly with increased phonon frequency and becomes unobservable when the phonon frequency greatly exceeds the gap. This constraint makes these anomalies useful only in high-temperature superconductors such as ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$.

Flux dynamics in a ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_4$ single crystal is investigated in the 20 kHz–3 MHz frequency range by using a mutual-inductance technique. We show that the superconducting ac response is due to thermally activated flux motion; from the dc field, ac amplitude, and frequency dependencies of this ac response, we extract the effective energy barriers U(T,B,J)=${\mathit{U}}_0$(1-T/${\mathit{T}}_{\mathit{c}}$)${\mathit{B}}^{\mathrm{-}0.66}$ln(${\mathit{J}}_{\mathit{c}}$/J). Significantly, these energies account for the time-dependent response of these superconductors over ten decades of time. As a consequence of them, the ac response is found to be strongly nonlinear, i.e., the ac susceptibility depends on the driving-field amplitude for ac fields as small as ${10}^{\mathrm{-}6}$ T superposed to dc fields up to 2 T. The ac transition is well described by a critical-state model with a frequency-dependent effective critical current ${\mathit{J}}_{\mathit{c}}$(T,B,f) stemming from the thermally activated flux motion. Our analysis allows us to obtain the temperature and frequency dependencies of ${\mathit{J}}_{\mathit{c}}$(T,B,f). ${\mathit{J}}_{\mathit{c}}$(f) follows a power law which is shown to be in agreement with the observed logarithmic U(J) dependence. Finally, isothermal cuts carried out for different driving-field amplitudes are used to explore the onset and limits of the nonlinear response.

We have investigated the modifications occurring in the oxygen sublattice and in the phonon spectra of chlorinated ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{\delta }}$ (YBCO) compounds. Susceptibility measurements suggest that the diffusion of chlorine is different in oxygen-deficient and in oxygen-rich materials. We find occurrence of superconductivity after chlorine treatment at 200 °C in an initially nonsuperconducting reduced sample. X-ray photoelectron spectra and x-ray-diffraction patterns of this sample show that the oxidation number of chlorine is between -1 and 0 in the lattice. To explain this result we assume that when an oxygen-deficient sample of approximate composition ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.20}$ is substantially chlorinated, the chlorine enters the structure at vacancies of BaO planes and promotes the diffusion of oxygen towards empty O(1) sites of copper planes. This conclusion is supported by a comparison of vibrational results from chlorinated YBCO with those recently obtained on YBCO pristine thin films presenting oxygen-sublattice disorder and by the different magnetic behavior of chlorinated reduced and oxidized samples. A Raman resonant feature is found at 72 meV (580 ${\mathrm{cm}}^{\mathrm{-}1}$) and is ascribed to defect-induced modes of the oxygen sublattice rather than to electronic excitations. Indeed at 10 K, the line shapes of this band are clearly asymmetric and can be fitted using different Fano functions indicating phononic interaction with an electronic spectrum. The unusual broadening of this line on going from normal to superconducting state suggests the presence of charge-transfer excitons traveling from ${\mathrm{CuO}}_2$ planes to CuO chains.

We have investigated the effect of Ga doping on the structure, oxygen content, and superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ of R${\mathrm{Ba}}_2$${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ (R=Yb, Er, Y, Dy, Gd, Eu, and Nd, and x=0, 0.05, 0.1, 0.15, 0.2, and 0.3). We observed that in these systems, the superconducting ${\mathit{T}}_{\mathit{c}}$, the melting point, and the critical Ga concentration ${\mathit{x}}_{\mathit{O}\mathrm{-}\mathit{T}}$ at which the samples undergo an orthorhombic-tetragonal transition, are all rare-earth-ion size dependent. The results suggest that the decrease of density of states N(${\mathit{E}}_{\mathit{F}}$) and/or the localization of the carriers, which is likely related to a Mott transition due to Ga substitution, are the possible origins for the suppression of superconductivity and the observed metal-semiconductor transition.

Vortex dynamics is studied in coupled multilayer structures, containing N 24-Å-thick ${\mathrm{DyBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ layers, each separated from the next by 96 Å of (${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ (x=0.4, 0.55). When the magnetic field is parallel to the c axis, we find that the activation energy U for flux motion increases linearly with the number of superconducting layers N in the structure for N<3. This linear increase is the result of the coupled motion of pancake vortices belonging to different ${\mathrm{DyBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ layers. For larger N samples the activation energy saturates meaning that the vortex lattice is turning three dimensional. In contrast to samples whose vortex lattice is purely two dimensional and for which we find the activation energy for flux motion proportional to ${\log }_{10}$B, samples in these series show a crossover to a power-law behavior, U≊${\mathit{B}}^{\mathrm{-}0.5}$, at a magnetic field ${\mathit{H}}^{\mathrm{*}}$ which decreases as N increases.

Using transmission electron microscopy, x-ray photoelectron spectroscopy, infrared, positron annihilation, and differential specific-heat measurements, we have studied the properties and structure of the F-doped (Bi,Pb)-Sr-Ca-Cu-O superconductor. The results show that the fluorine atoms are homogeneously distributed in the superconducting phase and substituted for the oxygen atoms at O(2) crystallographic sites; the Pb-F and Cu-F bonds are formed. F-doping results in the variance of the incommensurate modulation structure, the increase of the point defects and chemical pressure, the enhancement of the flux-pinning effect, and the change of the electronic structure and chemical bond, and greatly improves the superconductive properties of the 2223 phase of (Bi,Pb)-Sr-Ca-Cu-O.

Analyzing the normal-state magnetic susceptibility χ for ${\mathrm{TlSr}}_2$(${\mathrm{Lu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$)${\mathrm{Cu}}_2$${\mathrm{O}}_{\mathit{y}}$, whose carrier density covers the entire range from the underdoped to the overdoped regime, as a function of Ca concentration x and oxygen content y, we find significant correlations between χ and the carrier density. (1) The temperature dependence of χ gradually changes from positive to negative on going from the underdoped to the overdoped regime. It is almost entirely temperature independent at the optimum carrier density (maximum ${\mathit{T}}_{\mathit{c}}$). (2) The magnitude of χ gradually increases with the carrier density. For the optimum carrier density, the spin susceptibility at 200 K is estimated to be about 1.2×${10}^{\mathrm{-}4}$ emu/mol Cu. These characteristics are typical of most high-${\mathit{T}}_{\mathit{c}}$ cuprates and seem to be closely related to the high-${\mathit{T}}_{\mathit{c}}$ superconductivity mechanism.

When a bulk ceramic high-temperature superconductor is cooled in a small field and the magnetic moment of the sample is measured as the sample is warmed, an anomalous peak in the magnetic moment is observed. This peak can be as high as 50% of the low-temperature moment for fields less than 1 Oe, but it rapidly decreases as the magnetic field increases. We show that this anomaly is due to the interrelationship between flux trapping by intergranular weak links and the irreversible flux trapping properties of the superconducting grains as recently described by Hao and Clem.