Small-angle neutron scattering has been used to study the magnetic ordering of ${\mathrm{Ni}}_{80}$${\mathrm{Co}}_{20}$/Cu multilayers, a system which exhibits giant magnetoresistance. This system was selected for study because of its near-zero magnetic anisotropy, so that the exchange coupling may be determined with minimal interference from anisotropic effects. For a Cu thickness around 20 Å, clear antiferromagnetic order is seen, which may be suppressed by an external field of only 160 Oe. Combining the neutron data with magnetization data yields a value of 5.8×${10}^{\mathrm{-}3}$ erg/${\mathrm{cm}}^2$ for the exchange coupling, considerably less than values quoted hitherto for Cu/transition-metal multilayers.

We use the effective-action approach to quantum-field theory to study some of the thermodynamic properties of a charged ideal nonrelativistic gas of spin-0 bosons in a weak external magnetic field. It is shown how Bose-Einstein condensation and the Meissner-Oschenfeld effect arise. By using generalized ζ functions and dimensional regularization a very complete analysis for a space of arbitrary dimension can be given.

Two-dimensional electrons interacting with the ripplon field on the surface of a thin film of liquid helium is investigated by introducing a modified Lee-Low-Pines transformation that depends explicitly on the coupling constant. The theory yields correct results in the weak- and strong-coupling limits. Ground-state properties of the ripplonic polaron are calculated as functions of the coupling constant. It is found that the ground-state energy varies smoothly with the coupling constant throughout the whole range, and there is no indication of any phase-transition-like abrupt change. However, there exists a transition region when a quasi-free-polaron changes to the self-trapped state.

The nonlinear dependence of the surface energy of liquid hydrogen as a function of the density of helium gas in equilibrium with the liquid surface has been used to determine the second virial coefficient of the two-dimensional gas of helium atoms adsorbed on the surface. The surface energy of both liquid hydrogen and liquid deuterium has been measured in the presence of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. The experimental results are in rough agreement with theoretical prediction.

A second-quantization description is given for Andreev reflection. We introduce an effective interaction Hamiltonian for the interface between the superconductor and the normal metal, which enables us to treat transmission and reflection consistently. The interaction Hamiltonian couples quasiparticles in the normal region (N) and Bogoliubov quasiparticles in the superconducting region (S). It facilitates the description of the elementary excitations of the SN-coupled system, that are generated by Andreev reflections. Moreover, it naturally brings us to the description where the effect of macroscopic quantum fluctuations of the superconducting state on Andreev reflections is considered. This provides a powerful tool for investigating of the mesoscopic aspects of Andreev reflection.

We carry out driven-diffusion Monte Carlo simulations of the two-dimensional classical lattice Coulomb gas in an applied uniform electric field, as a model for vortex motion due to an applied dc current in a periodic superconducting network. A finite-size version of dynamic scaling is used to extract the dynamic critical exponent z, and infer the nonlinear response at the transition temperature. We consider the Coulomb gases f=0 and f=1/2, corresponding to a superconducting network with an applied transverse magnetic field of zero, and one-half flux quantum per unit cell, respectively.

Spatially resolved images of the dynamic states of curent-biased two-dimensional arrays of Nb/${\mathrm{AlO}}_{\mathit{x}}$/Nb Josephson junctions were obtained using low-temperature scanning electron microscopy. The arrays were square or rectangular and the maximum size was 20 junctions ×20 junctions. In overdamped arrays our images at zero or small applied perpendicular magnetic field together with model calculations confirm the nucleation of a vortex at one sample edge (or an antivortex at the opposite edge) and its subsequent motion into the array interior. Vortex annihilation due to vortex-antivortex collision was observed to take place in the middle of the array or at the edge opposite to the nucleation edge. These dynamics and the underlying model considerations are similar to that for Abrikosov vortices in the current-induced resistive state of thin film type-II superconductors. The phenomenon of ‘‘row switching’’ is directly confirmed in images of underdamped arrays. The specific rows experiencing this process change randomly when the same bias point on the current-voltage characteristic is established many times, starting each time from zero current.

The analysis of Blonder, Tinkham, and Klapwijk describing the crossover between tunnel junction and metallic contact between a normal and a superconductive electrode is applied to the case of a superconductor with an energy-dependent gap function. Such energy dependence arises in the presence of electron-hole asymmetry and is predicted within the theory of hole superconductivity. We study a tight-binding model where the interface is described by a reduced value of the intersite hopping amplitude and the superconducting gap function has on-site and nearest-neighbor components. The tunneling conductance as function of barrier strength is found to exhibit certain differences with the electron-hole symmetric case: the reflection and transmission coefficients are different for incident electrons and holes, and Andreev reflection processes for both electrons and holes are suppressed even in the limit of vanishing barrier strength. A temperature gradient across the barrier gives rise to a thermoelectric effect of universal sign, whose magnitude depends on the degree of electron-hole asymmetry in the superconductor as well as on the barrier strength. For temperatures close to ${\mathit{T}}_{\mathit{c}}$ an analytic form for the thermoelectric voltage for arbitrary barrier strength is found. These results give information on the effect of nonideal tunnel barriers on the predicted thermoelectric effect. The possibility of observing these effects in high-temperature and conventional superconductors, and interpretation of existing experimental findings in light of these results is discussed.

Tight-binding calculatoins are performed which include both Cu-O and O-O interactions in the ${\mathrm{CuO}}_2$ plane. These calculations reconcile inconsistencies in observed behaviors of the thermopower S and the Hall coefficient ${\mathit{R}}_{\mathit{H}}$: the sign of S of high-${\mathit{T}}_{\mathit{c}}$ cuprates at room temperature becomes negative in the overdoped regime, while ${\mathit{R}}_{\mathit{H}}$ remains positive. A striking feature of the ${\mathrm{CuO}}_2$ antibonding band is that a holelike Fermi surface is formed even when the band is less than half-filled. This brings about an unusual electron state in which the Hall (cyclotron) mass parallel to the Fermi surface is holelike (<0) but the transport mass perpendicular to it is electronlike (>0). This electronlike transport mass contributes to negative S, while the holelike Hall mass results in positive ${\mathit{R}}_{\mathit{H}}$. In such a state, the electron on the Fermi surface has complete duality: it is holelike in one direction, but electronlike in another. In the overdoped regime, where ${\mathit{R}}_{\mathit{H}}$>0 and S<0, the hole doping increases the carrier concentration defined as ∝ ${\mathit{R}}_{\mathit{H}}^{\mathrm{-}1}$, but it decreases the carrier concentration defined as (n/${\mathit{m}}^{\mathrm{*}}$${)}_{\mathit{D}}$ in Drude’s formula. This qualitatively explains the recent muon-spin-rotation (μSR) results that the superconducting carrier concentration ${\mathit{n}}_{\mathit{s}}$/${\mathit{m}}^{\mathrm{*}}$∼(n/${\mathit{m}}^{\mathrm{*}}$${)}_{\mathit{D}}$ decreases with hole doping in the overdoped regime.

Measurements of fundamental and higher harmonics of ac susceptibility of polycrystalline low-${\mathit{T}}_{\mathit{c}}$ and high-${\mathit{T}}_{\mathit{c}}$ superconductors show a failure of the critical-state model in describing the higher harmonics dependence on temperature at fixed dc and ac magnetic fields. The results of measurements at different frequencies suggest that an effect of the fluxon dynamics could take place. Indeed simple considerations lead to the conclusion that thermally activated flux flow, flux creep, and flux flow (FF) should have an important role in determining the higher harmonics of ac susceptibility. A simplified critical-state model, only including pinning and FF, seems to explain part of the experimental results.

Results of local-density linear muffin-tin orbital (LMTO) electronic-structure calculations for both known and recently obtained niobium oxide phases with different crystal lattices and different coordination of Nb atoms are presented. Perovskite-like compounds with one- or two-dimensional Nb-O condensed clusters, as well as hexagonal layered structures with trigonal-prismatic coordination of Nb atoms, are used. For ${\mathrm{SrNbO}}_3$, ${\mathrm{BaNb}}_5$${\mathrm{O}}_8$, ${\mathrm{BaNb}}_4$${\mathrm{O}}_6$, and ${\mathrm{Sr}}_2$${\mathrm{Nb}}_5$${\mathrm{O}}_9$, the Fermi level is located at the shoulder of the Nb d conduction band with a low density of oxygen states. For hexagonal ${\mathit{M}}_{\mathit{x}}$${\mathrm{NbO}}_2$ (M=Li,Na) compounds, oxygen contributions definitely appear at ${\mathit{E}}_{\mathit{F}}$, but remain small compared with those of the high-${\mathit{T}}_{\mathit{c}}$ superconducting cuprates. Pair-interaction energies and superexchange-interaction parameters were calculated using the LMTO Green-function method. Typical for high-${\mathit{T}}_{\mathit{c}}$ superconductors, antiferromagnetic coupling between metal atoms in the Nb-O planes is shown to exist, but not the ferromagnetic exchange between metal d and oxygen p states. The calculations show that perovskite niobium oxide systems are not good candidates in the search for new high-${\mathit{T}}_{\mathit{c}}$ superconducting materials, but that hexagonal layered niobium oxide phases of the type considered in this paper might be of interest in studies of low ${\mathit{T}}_{\mathit{c}}$ superconducting oxides.

Positron annihilation lifetime spectra were measured on F-doped Bi(Pb) 2:2:2:3 superconductors as a function of F content and analyzed by using a numerical Laplace inversion method. While the nominal fluorine content increases, it was clearly observed that (1) the average positron annihilation rate is decreased continuously, (2) the width of the positron-annihilation-rate distribution (ARD) is increased, (3) the region of the ARD becomes wider and exends down towards a lower annihilation rate. The increase in ${\mathit{T}}_{\mathit{c}}$ with the increase in F content has a correlation with the changes of the ARD. Our positron results indicate that the F doping can produce a number of anion vacancies (or oxygen deficiency) and raise the concentration of Bi-O vacancy complexes in the Bi-O planes, which are substantiated by an anneal experiment in vacuum and responsible for the increases in ${\mathit{T}}_{\mathit{c}}$ and ${\mathit{J}}_{\mathit{c}}$.

A multiband CuO Hubbard model is studied which incorporates long-range (LR) repulsive Coulomb interactions. In the atomic limit, it is shown that a charge transfer from copper to oxygen ions occurs as the strength of the LR interaction is increased. The regime of phase separation is replaced by a uniform state with doubly occupied oxygens. As the holes become mobile a superfluid condensate is formed, as suggested by a numerical analysis of pairing correlations and flux quantization on one-dimensional chains. The competition between charge-density wave and superconductivity is also analyzed. It is argued that the results are also applicable to two dimensions.

We have investigated the effect of hydrostatic pressure on the structure of orthorhombic ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ between 1 atm and 0.6 GPa, using neutron powder diffraction. Increasing pressure causes the tilt angle of ${\mathrm{CuO}}_6$ octahedra to decrease, leading to a transition to tetragonal symmetry. There is a strong compressibility anisotropy for the orthorhombic structure. Order parameters for the transition versus pressure are consistent with Landau theory for a second-order transition. It is shown that this transition can be universally scaled over (x,T,P) space in the context of Landau theory. We conclude that ${\mathit{T}}_{\mathit{c}}$ varies inversely with the tilt angle and is maximum for the tetragonal structure, i.e., for flat and square ${\mathrm{CuO}}_2$ planes, for this compound.

Neutron powder-diffraction measurements have been performed to determine the magnetic phase diagram of the ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$${)}_3$${\mathrm{O}}_{\mathit{y}}$ system for an oxygen concentration y close to 6.2. The temperature and concentration dependence of the magnetic moments on the ${\mathrm{Cu}}_1$ and ${\mathrm{Cu}}_2$ sites have been deduced from Rietveld refinements. At low temperature, the results show the occurrence of a noncollinear phase for x<0.03. At higher iron content, a collinear phase is observed, with a doubling of the magnetic unit cell along the c axis, and a moment μ${\mathrm{Cu}}_1$ on the ${\mathrm{Cu}}_1$ sites whose absolute value increases with iron content. The succession of magnetic phases is well understood by a simple energy calculation. In the collinear phase, the sign of μ${\mathrm{Cu}}_1$ with respect to the moment on the neighboring ${\mathrm{Cu}}_2$ sites is correlated to the local iron environment and can be modified through an appropriate heat treatment.

Local-density-calculations of the electronic structure of ${\mathrm{HgBa}}_2$${\mathrm{CuO}}_4$ have been performed with the self-consistent linear muffin-tin orbital method. The positron-density distribution and its sensitivity due to different potentials are calculated. The annihilation rates are computed in order to study the chemical bonding and to predict the Fermi-surface signal. Comparisons are made with previous calculations on other high-${\mathit{T}}_{\mathit{c}}$ copper oxides concerning the Fermi-surface properties and electron-positron overlap. We discuss the possibility of observing the Fermi surface associated with the Cu-O planes in positron-annihilation experiments.

The resistivity ρ and Hall coefficient ${\mathit{R}}_{\mathit{H}}$ have been measured for samples of the two-plane superconductor ${\mathrm{HgBa}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{6+\mathrm{\delta }}$ (${\mathit{T}}_{\mathit{c}}$∼124 K) with different annealing histories. The observed temperature dependence ${\mathit{R}}_{\mathit{H}}$∼1/T (up to 470 K) matches the steep dependence previously seen in 90-K ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and indicates low-impurity scattering. We find T-linear behavior for ρ in the lowest-resistivity samples and confirm that cot${\mathrm{\theta }}_{\mathit{H}}$∼${\mathit{T}}^2$. These findings add support to Anderson’s proposal of distinct longitudinal and transverse scattering times.

We have calculated the effect of impurity scattering in the Born (weak) and unitary (strong) limits on the absolute value of the zero-temperature penetration depth λ(0). The gap is taken to exhibit ${\mathit{d}}_{\mathit{x}}^{2\mathrm{-}}$${\mathit{y}}^2$ symmetry in the copper-oxygen planes and a model two-dimensional tight-binding band structure is used with nearest-neighbor hopping only. Impurities depress the value of the critical temperature and, at the same time, increase the value of λ(0). Weak and strong scattering are compared. The predicted increase in λ(0) is smaller than indicated in recent experiments.

High-resolution measurements of low-field dc magnetization are used to obtain flux-penetration fields ${\mathit{H}}_{\mathit{p}}$(T) along the two main crystallographic directions of several ${\mathit{L}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ (L=Pr,Nd,Sm) single crystals. The departure from the linearity of the virgin M(H) curve and the appearance of remanence and magnetic relaxation are used to determine ${\mathit{H}}_{\mathit{p}}$. It is shown that ${\mathit{H}}_{\mathit{p}}$(T) may not be safely identified with ${\mathit{H}}_{\mathit{c}1}$(T) when H∥c; the implications of these results are discussed in order to estimate the values of the fundamental superconducting quantities as the lower critical field, the penetration depth, the Ginzburg-Landau parameter, and the anisotropy in these electron-doped copper oxide superconductors.

We have performed tunneling measurements on single crystals of the superconducting cuprate ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. With increasing concentration of Pr, the structure seen in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ in the energy region where a gap is expected, becomes even less pronounced (more ‘‘gapless’’) as the ${\mathit{T}}_{\mathit{c}}$ is reduced. In the vicinity of the superconductor-insulator transition, in tunnel junctions of demonstrably high quality, this structure becomes very weak. We draw upon a strong analogy between this system and ion beam damaged ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ to discuss the results.