A trial shadow wave function is introduced to describe the ground state of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in the solid and liquid phases. We have used Monte Carlo integration to optimize the parameters of this function, and have carried out a thorough analysis of the shadow description of the system. This shows improved pair correlations, an improved condensate fraction, substantially reduced variational energies, and a good equation of state. We have explored the melting and freezing transition, and find the transition densities to be in good agreement with the exact results of Green’s function Monte Carlo (GFMC) calculations. We introduce a second shadow wave function in which a basis set expansion is used to optimize the two-particle correlations. This shadow wave function yields pair-correlation functions in excellent agreement with GFMC, as well as a substantial reduction in the variational energies at all densities.

An exact-diagonalization technique on small clusters is used to study the ground states and single-particle excitations of the Hubbard model with on-site (U) and nearest-neighbor (V) Coulombic repulsive interactions. It is shown that the long-range charge-density-wave state realized in a half-filled two-dimensional square lattice for 4V≳U persists up to quarter filling, and that the doped carriers form a small Fermi surface that encloses an area corresponding to the number of doped carriers rather than the total number of electrons involved in the system. The present result offers a well-defined counterexample in the discussion of the Fermi-surface dimensions of strongly correlated electron systems, and may provide a complementary understanding of the large Fermi surface believed to exist in the ordinary Hubbard model.

We have performed measurements of the surface impedance in the normal and superconducting states of the title compounds in the millimeter wave-frequency range (1–3 ${\mathrm{cm}}^{\mathrm{-}1}$), and have evaluated the complex conductivity for different crystallographic orientations. Above the transition temperature, the materials behave like metals with a scattering rate of approximately 20 ${\mathrm{cm}}^{\mathrm{-}1}$. In the superconducting state the electrodynamics is in good agreement with calculations based on a BCS ground state: while the penetration depth and the coherence length are anisotropic, the superconducting energy gap shows no indications of line nodes.

The classical motion of a single vortex in a proximity-junction array is investigated with the help of the effective-action formalism. The dynamics of the superconducting order parameter of each superconducting grain is assumed to be of the Langevin-type corresponding to local dissipation. The action describing the motion of the vortex in the array is shown to incorporate the propagator, the form of which in terms of the real-time dynamics corresponds to the logarithmical depression of mobility. Since in the array the motion of the vortex takes place in the effective periodic potential we check also that the presence of the periodic potential does not renormalize the anomalous behavior of the mobility in the low frequency limit. The results obtained are in agreement with experimental data on frequency dependence of the vortex contribution to proximity-junction-array response.

We have studied superconductivity and temperature dependence of resistance in thin Ta films covered by a layer of anodic oxide. It was shown that deposition of a thick gold film on top of the oxide dielectric layer enhances the superconductivity of the thin film and changes the sign of the temperature dependence of resistance through changing the dominant quantum correction from electron-electron interaction to antilocalization. The results demonstrate that the proximity of a thick metal film, even without any electrical contact to the thin film, reduces the Coulomb interaction between conduction electrons in the thin film, thereby turning off most of the interaction quantum correction to conductivity. Consequently, the only quantum correction remaining is weak localization. This does not affect the superconductivity and has the antilocalization sign in Ta, as is expected for a metal of large atomic number.

We investigate the behavior of flux lines in the presence of both columnar and point disorder in (1+1) dimensions using renormalization-group (RG) and world-line quantum Monte Carlo (QMC) techniques. In particular, we calculate the transverse wandering correlation function for a single boson and recover known results for point and columnar disorder separately. We then examine the existence of a localization transition of a flux line in the simultaneous presence of both types of disorder. We also performed RG calculations for interacting flux lines. The RG calculation indicate that the vortex glass is unstable with respect to an arbitrary small amount of columnar disorder. Using QMC techniques we find that the Bose glass transition temperature is reduced by the point disorder, in agreement with recent RG calculations. Further we find that the region posited to be an Anderson glass is completely destroyed.

The low-frequency inverse kinetic inductance ${\mathit{L}}^{\mathrm{-}1}$ of an overdamped junction array at temperature T=0 is shown to equal that of an equivalent impedance network. The ijth bond of the network has an inverse inductance (2${\mathit{eE}}_{\mathit{i}\mathit{j}}$/ħ)cos(${\mathrm{\theta }}_{\mathit{i}}^0$-${\mathrm{\theta }}_{\mathit{j}}^0$-${\mathit{A}}_{\mathit{i}\mathit{j}}$), where ${\mathit{E}}_{\mathit{i}\mathit{j}}$ is the Josephson coupling energy of the ijth bond, ${\mathrm{\theta }}_{\mathit{i}}^0$ is the ground-state phase of the grain i, and ${\mathit{A}}_{\mathit{i}\mathit{j}}$ is the usual magnetic phase factor. Using this theorem, we calculate ${\mathit{L}}^{\mathrm{-}1}$ for square lattices as large as 180×180. The calculated ${\mathit{L}}^{\mathrm{-}1}$ agrees well with the T=0 limit of the helicity modulus γ calculated by conventional Monte Carlo techniques. In triangular arrays, the Monte Carlo calculation of γ yields a series of peaks at frustrations f=1/2(1-1/N), where N is an integer ≥2, consistent with experiments.

We compute the density of states ${\mathit{N}}_{\mathit{s}}$(E) for a superconductor having an order parameter Δ(k^) with the following two properties: (i) ‖Δ(k^)‖ is a constant, independent of k; (ii) the average of Δ(k^) over the Fermi surface vanishes: 〈Δ(k^)〉=0. We particularly stress the effect of impurity scattering. We can do the calculation for an arbitrary Fermi surface, and any order parameter with the above two properties. Since our family of order parameters has no nodes, our results pinpoint the effects due to the vanishing Fermi surface average, 〈Δ(k^)〉=0.

Experimental details are given for NMR and NQR measurements of nuclear-spin-lattice relaxation rates for the planar copper and oxygen sites in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. Enrichment of samples with ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$, alignment of powder samples, elimination of artifacts in relaxation-rate measurements, and the unusual aspects of weak magnetic-field experiments used to overcome an observed magnetic-field dependence in the rates are described. The role of fluxoids in the magnetic-field dependence of the relaxation rates is discussed. The relaxation rate data are shown to be inconsistent with a spin-singlet, orbital s-wave pairing state with an isotropic energy gap.

Measurements of ac magnetic susceptibility of bulk ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ and compounds containing noble-metal additives were carried out to investigate the effects of chemical doping and penetration of magnetic field into the intergranular region. Significant enhancement of the intergranular critical current density was demonstrated in Tl-based ceramic superconductors with Au and Ag doping. Methods for convenient determination of the intergranular weak-link phase-locking temperature and average lower critical field in the bulk granular superconductors are discussed.

The electrodynamics of a metallic surface layer in proximity contact to a superconductor are considered. The surface region is modeled as an idealized proximity-coupled superconductor/normal-metal (S/N) bilayer. Analytical expressions for the magnetic-field penetration into the sample are obtained. The behavior of the model and its applicability to magnetic-screening-length measurements on artificial and naturally occurring S/N bilayer superconductors are discussed in various limits. Calculations of the effective penetration depth ${\lambda }_{\mathrm{eff}}$(T) reveal that unconventional penetration depth temperature dependences, such as Δ${\lambda }_{\mathrm{eff}}$(T)∼T found in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ crystals, are qualitatively reproduced with this model.

We report measurements and modeling of magnetic hysteresis in SQUID magnetometers cycled in fields of 0.1 to 10 G peak to peak. We show that the observed hysteresis is caused by the penetration of magnetic vortices around the edges of the superconducting thin-film structures forming the SQUID. A threshold field for the onset of hysteresis is predicted and subsequently observed experimentally. The threshold corresponds to the situation when the Lorentz force driving a vortex into the SQUID washer first exceeds the sum of the surface barrier and pinning forces. The threshold field depends on the pinning strength of the superconducting material forming the edges of the device, and is thus distinguished from the lower critical field, ${\mathit{H}}_{\mathit{c}1}$, for flux entry in type-II superconductors. It is, to our knowledge, the first time a threshold field for flux entry is observed to relate to the local pinning strength of a structure. Comparison of our earlier experimental data with model calculations revealed the existence of a degradation in critical current density at edges of our devices. Subsequent optimization of our high-${\mathit{T}}_{\mathit{c}}$ device fabrication process lead to significant improvement in hysteresis characteristics. The threshold field in the improved devices is clearly observable, near which at 77 K a thousandfold reduction in hysteresis is obtained. We believe the mechanism of magnetic flux entry studied in this work is also relevant to the understanding of the hysteresis loss in high-${\mathit{T}}_{\mathit{c}}$ superconductors at microwave frequencies. Such hysteresis, resulting in a strongly power-dependent loss, is a major factor limiting the high-power applications of thin-film high-${\mathit{T}}_{\mathit{c}}$ microwave devices.

The electronic structure of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_3$ has been studied by photoemission spectroscopy. With increasing x, the spectra are generally shifted towards the Fermi level (${\mathit{E}}_{\mathit{F}}$), indicating a downward shift of ${\mathit{E}}_{\mathit{F}}$ due to hole doping. The magnitudes of the shifts, however, are substantially smaller than those predicted by band-structure calculations. In the cubic metallic phase, although band-structure calculations on the cubic structure predict the density of states (DOS) to show a peak at or close to ${\mathit{E}}_{\mathit{F}}$, the photoemission intensity decreases towards ${\mathit{E}}_{\mathit{F}}$, suggesting that the splitting of the Bi 6s band persists and results in a pseudogap behavior. We propose that the suppression of the DOS peak at ${\mathit{E}}_{\mathit{F}}$ is caused by dynamical lattice distortion: The DOS peak predicted for the cubic structure presumably induces the lattice instability, leading to the dynamical lattice distortion.

The vortex mechanism, leading to very high-frequency random telegraph voltage noise signals switching with MHz frequencies, is discussed. The mechanism assumes that vortices do not flow freely across dc-current-biased superconducting films but undergo subsequent processes of trapping and releasing from pinning centers. Random transitions of vortices between pinned and flow states result in the appearance of a Lorentzian component in a fluctuating-voltage power spectrum. It is shown that fluctuations of randomly distributed Abrikosov vortex density, rigidly moving across the strip, contribute an oscillating component to the noise spectrum. The distance between oscillating peaks corresponds to the time of flight of vortices across the strip.

We report measurements of the surface resistance at 21 GHz as a function of the magnetic field H and of the field orientation ϑ with respect to the (a,b) planes, for different temperatures, on epitaxial ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ films. We find clear experimental evidence for the angular scaling of the microwave surface resistance, at all the temperatures investigated, with no need for specific hypothesis on the origin of the dissipation. The scaling is perfect over four decades of the magnetic field. The angular scaling function is identified with the quasi-two-dimensional, thin-film formula.

Superconducting superlattices composed of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and various "barrier-layer" materials have been studied extensively experimentally and several theoretical attempts to correlate the resistivity $R$ as a function of temperature $T$ with the superlattice structure have appeared in the literature. Theoretical interest in such structures comes about primarily because of the insight they may provide into dimensionality effects, interlayer coupling, and interlayer charge redistribution in the high-$T_c$ superconductors. In this paper, $R\left(T\right)\mathrm{}$ data up to $T=300\mathrm{}$ K are analyzed by least-squares fitting for 17 superlattices consisting of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and barrier layers of Pr${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, ${\mathrm{Pr}}_{0.7}$${\mathrm{Y}}_{0.3}$${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$${\mathrm{O}}_7$, and ${\mathrm{Pr}}_{0.5}$${\mathrm{Ca}}_{0.5}$${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$${\mathrm{O}}_7$. The fitting model incorporates results from Kosterlitz-Thouless (vortex-antivortex unbinding) and Aslamazov-Larkin (fluctuation-enhanced conductivity) theories in the resistive transition region and charge-transfer effects, variable-range hopping, etc., in the normal state. The results show that "interface" and barrier-layer conductivity must be taken into account in order to explain the unusual normal-state $R\left(T\right)\mathrm{}$ behavior in some superlattices. These effects may also contribute in the transition region where vortex-antivortex unbinding and fluctuation effects play the major roles.

Recent high-field (B≤8 T) experiments on single crystals of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{x}}$ (BSCCO-2:2:1:2) have found a significant anisotropy in the specific heat, with an excess contribution for perpendicular field (along the c axis) relative to that for parallel or zero field. One possible explanation for these observations is the undamped vibrational modes of the lattice of pancake vortices, which occur only for the perpendicular orientation. The London model of thin superconducting layers provides a simple theoretical picture that allows a direct evaluation of the elastic energy for small deformations of the vortex lattice. For typical temperatures, the resulting vortex contribution ${\mathit{C}}_{\mathit{v}}$ to the heat capacity has a Debye form for two-dimensional phonons, with a Debye temperature ${\mathrm{\Theta }}_{\mathit{v}}$=1/2ħ${\mathrm{\omega }}_{\mathit{c}}$/${\mathit{k}}_{\mathit{B}}$, where ${\mathrm{\omega }}_{\mathit{c}}$ is the electron cyclotron frequency (for B=10 T and free electrons, ${\mathrm{\Theta }}_{\mathit{v}}$≊6.7 K). For T>${\mathrm{\Theta }}_{\mathit{v}}$, the resulting ${\mathit{C}}_{\mathit{v}}$ approaches ${\mathit{k}}_{\mathit{B}}$ per pancake vortex, as appropriate for vortex dynamics and in qualitative agreement with the experimental observations; for T<${\mathrm{\Theta }}_{\mathit{v}}$, the theory predicts that ${\mathit{C}}_{\mathit{v}}$∝${\mathit{T}}^2$, but this particular power law remains to be verified experimentally. If an inertial mass ${\mathit{m}}_{\mathit{c}}$ is ascribed to each pancake vortex, an additional set of (Newtonian) normal modes yields a further (Einstein) heat capacity, with ${\mathrm{\Theta }}_{\mathit{E}}$∝${\mathit{m}}_{\mathit{c}}^{\mathrm{-}1}$. The presence (absence) of such a contribution to ${\mathit{C}}_{\mathit{v}}$ could help determine (bound) the value of the currently controversial quantity ${\mathit{m}}_{\mathit{c}}$.

The effect of varying oxygen stoichiometry on the spin dynamics and electronic state of a transition-metal ion in ${}_{}{}^{57}{}_{}{}^{}\mathrm{doped}$ CuO has been studied. Oxygen deficiency results in an increase in the spin-lattice relaxation rate. The charge state of the iron shifts towards the ${\mathrm{Fe}}^{2+}$ state with increasing oxygen deficiency. The change in the charge state may be attributed to a dependence of charge-transfer energy and/or the transfer integral on oxygen vacancies. The present studies demonstrate some similarities in the behavior of CuO with that of high-temperature superconductors with respect to the effect of oxygen stoichiometry.

Dynamic electron spin resonance (ESR) measurements compare the paramagnetic and antiferromagnetic (AF) properties of superconducting oxides in the range 4 K to room temperature, at 8 MHz and 9.36 GHz. Two are derivatives of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$: I: Nd(${\mathrm{Nd}}_{0.05}$${\mathrm{Ba}}_{0.95}$${)}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, ${\mathit{T}}_{\mathit{c}0}$=72 K and II: ${\mathrm{Y}}_{0.2}$${\mathrm{Ca}}_{0.8}$${\mathrm{Sr}}_2$[${\mathrm{Cu}}_2$(${\mathrm{Tl}}_{0.5}$${\mathrm{Pb}}_{0.5}$)]${\mathrm{O}}_7$, ${\mathit{T}}_{\mathit{c}0}$=108 K and two are cases where AF ordering dominates the weak superconductivity: III: ${\mathrm{NbO}}_{1.1}$, 1.25≤${\mathit{T}}_{\mathit{c}0}$≤10 K and IV: ${\mathrm{La}}_2$${\mathrm{NiO}}_{4.00}$, 70 K ≥${\mathit{T}}_{\mathit{c}0}$≥40 K. At temperatures 298≥T≥64 K, the ESR absorption by I indicates orthorhombic symmetry. The peaks at ${\mathit{g}}_{\mathit{c}}$=2.06, ${\mathit{g}}_{\mathit{b}}$=2.13, and ${\mathit{g}}_{\mathit{a}}$=2.24 are identified with the presence of 5% ${\mathrm{Nd}}^{3+}$${(}^4$${\mathit{I}}_{9/2}$) in the Ba layer because the characteristic ${\mathrm{Cu}}^{2+}$ impurity hyperfine structure is absent and the ESR signal disappears several degrees below ${\mathit{T}}_{\mathit{c}}$. Near ${\mathit{T}}_{\mathit{c}}$ the ESR absorption is reduced by two orders of magnitude.

Proximity effects give rise to interference fringes with period ${\mathrm{\tau }}_{\mathit{f}}$(T) independent of the field B and the rate of sweep ${\mathit{dB}}_{\mathit{z}}$/dt. ESR is observed below ${\mathit{T}}_{\mathit{c}}$ because flux penetrates the superconductor. The temperature dependence of ${\mathrm{\tau }}_{\mathit{f}}$ leads to an activation energy for the flux motion ${\mathit{E}}_{\mathit{a}}$(I)/R≤16 K and ${\mathit{E}}_{\mathit{a}}$ (III)/R≃3 K ≃${\mathit{T}}_{\mathit{c}}$/4. In the superconducting state a coherent flux expulsion response to a change in ${\mathit{B}}_{\mathit{z}}$ from 500 mT to zero is observed in times ${\mathrm{\tau }}_{\mathit{r}}$=8 to 10 s. The inverse rate of noise spikes due to flux expulsion, when the samples are cooled through ${\mathit{T}}_{\mathit{c}}$ in a magnetic field, varies from ${\mathrm{\tau }}_{\mathrm{noise}}$=3.5 s for III to 21 s for IV. The microwave absorption spectra identify three temperature regimes: (i) For 3.5 K <T<${\mathit{T}}_{\mathit{m}}$≃${\mathit{T}}^{\mathrm{*}}$<${\mathit{T}}_{\mathit{c}}$, superconducting behavior was confirmed by the energy loss near zero magnetic field and the kinetics of high-field noise due to flux expulsion. Near g=2.00 ESR absorption is observed for all materials. A broad absorption near 50 to 100 mT at 9.36 GHz has been attributed to AF resonance. (ii) ${\mathit{T}}_{\mathit{m}}$≃${\mathit{T}}^{\mathrm{*}}$≤T≤${\mathit{T}}_{\mathit{c}}$ identifies the range where flux motion gives rise to interference fringes in the ESR absorption. (iii) ESR and AF resonance are observed immediately after warming above ${\mathit{T}}_{\mathit{c}}$.

Direct transport measurements of the angular dependencies of the critical current density for single-crystalline and melt-textured ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ have been carried out in the temperature range between 77 K and ${\mathit{T}}_{\mathit{c}}$ and applied magnetic fields up to 2.0 T. In contrast to the melt-textured samples, the ${\mathit{J}}_{\mathit{c}}$(θ) dependencies for the twinned single crystals reveal a minimum in the H∥c configuration (θ=0). The nonmonotonic ${\mathit{J}}_{\mathit{c}}$(H) behavior (fishtail effect) appeared to be much more pronounced in tilted magnetic fields. The fishtail as well as the ${\mathit{J}}_{\mathit{c}}$(θ) minimum at H∥c were shown to depend strongly on the ${\mathit{E}}_{\mathit{c}}$ criterion for ${\mathit{J}}_{\mathit{c}}$ definition. An ${\mathit{E}}_{\mathit{c}}$ increase by two orders of magnitude suppresses the fishtail and transforms the ${\mathit{J}}_{\mathit{c}}$(θ) minimum to a maximum. Two alternative approaches for the flux dynamics were used: (i) thermally activated-vortex-hopping theories and (ii) the classical mean-field approximation (MFA). The MFA analyses of the I-V curves for the different θ, H, and T were much more consistent and informative. In the case of the flux-line lattice (FLL) moving in the pinning potential there should exist a region (defined by the level of dissipation) between creep-controlled FLL motion and Bardeen-Stephen flow.

In this region a key role is played by not only the depth of the pinning wells but also the features of the pinning potential, elastic, and viscous properties of the FLL. A scaling behavior predicted by the MFA analysis was observed for the low-field I-V curves. A breakdown of scaling was detected when the ‘‘fishtail effect’’ arose. The fishtail effect is shown to have a dynamic nature. It is controlled by vortex interaction with random pointlike disorder and influenced strongly by the FLL relaxation if the relaxation and pinning times are comparable. The twins are supposed to promote a correlated disorder and inhibit the three-dimensional pointlike random disorder as a dominant state for the pinning of the FLL. This is a reason for the ${\mathit{J}}_{\mathit{c}}$ minimum at H∥c, observed at the low level of dissipation.