The results of a series of measurements of Kosterlitz-Thouless behavior in helium films are presented. Use is made of high-Q piezoelectric crystals so as to probe the phenomena in a single flat substrate geometry at high frequencies. Variations of the superfluid density and dissipation in the vicinity of the transition were measured. Dynamical effects were probed by comparing transitions at two different frequencies. Results are compared with a comprehensive numerical calculation in which finite-size scaling effects are explicitly introduced. A consistent picture for the dependence of critical thickness on temperature, of the scaling of the transition shape with temperature, and of dynamical effects is obtained.

We predict and analyze nontrivial relaxational behavior of magnetically trapped gases near the Bose-condensation temperature ${\mathit{T}}_{\mathit{c}}$. Due to strong compression of the condensate by the inhomogeneous trapping field, particularly at low densities, the relaxation rate shows a strong, almost jumplike, increase below ${\mathit{T}}_{\mathit{c}}$. As a consequence the maximum fraction of condensate particles is limited to a few percent. This phenomenon can be called a ‘‘relaxation explosion.’’ We discuss its implication for the detectability of Bose-Einstein condensation in atomic hydrogen.

Current density, magnetic field, penetrated magnetic flux, and magnetic moment are calculated analytically for a thin strip of a type-II superconductor carrying a transport current I in a perpendicular magnetic field ${\mathit{H}}_{\mathit{a}}$. Constant critical current density ${\mathit{j}}_{\mathit{c}}$ is assumed. The exact solutions reveal interesting features of this often realized perpendicular geometry that qualitatively differs from the widely used Bean critical state model: At the penetrating flux front the field and current profiles have vertical slopes; the initial penetration depth and penetrated flux are quadratic in ${\mathit{H}}_{\mathit{a}}$ and I; the initial deviation from a linear magnetic moment is cubic in ${\mathit{H}}_{\mathit{a}}$; the hysteresis losses are proportional to the fourth power of a small ac amplitude; the current density j is finite over the entire width of the strip even when flux has only partly penetrated; in thin films, as soon as the direction of the temporal change of ${\mathit{H}}_{\mathit{a}}$ or I is reversed, j falls below ${\mathit{j}}_{\mathit{c}}$ everywhere, thus stopping flux creep effectively; the Lorentz force can drive the vortices ‘‘uphill’’ against the flux-density gradient. These analytical results are at variance with the critical-state model for longitudinal geometry and explain numerous experiments in a natural way without the assumption of a surface barrier.

The thermodynamics of a two-dimensional vortex plasma in thin-film superconductors is described in the framework of a cycle approximation. The equation for the density of free vortices is obtained from the virial expansion. This equation provides a unified approach to describe both the Kosterlitz-Thouless-like behavior in the vicinity of the characteristic transition temperature ${\mathit{T}}_{\mathrm{KT}}$ and the resistive tail at low temperatures.

We consider the propagation of electromagnetic waves in a two-dimensional polarizable medium endowed with Chern-Simons terms. The dispersion relation (refractive index) of the waves is computed and the existence of linear birefringence and anomalous dispersion is shown. When absorption is taken into account we find the classic signature of a Voigt effect. In the case where linearly polarized, three-dimensional waves pass through a two-dimensional plane we show that there is optical activity and compute the analog of Verdet’s constant.

We exhibit an additional SU(2) symmetry of the Hubbard model at half-filling, which suggests a simple physical manifestation of this symmetry in the presence of an applied magnetic field. The system governed by a negative-U Hubbard Hamiltonian will lose its superconductivity at an applied magnetic field well below the critical ${\mathit{H}}_{\mathit{c}}$ associated with a conventional superconductor. Though departure from half-filling and the inclusion of a repulsive nearest-neighbor interaction break this symmetry, the above physical effect should still persist strongly and is indicative of a new type of SU(2) superconductor for the negative-U or attractive on-site Hubbard model.

With use of a spin-charge-separation treatment of the t-J model that accurately imposes the constraint of single electron site occupancy, a number of interesting results are obtained for the superconducting state. The most relevant is that superconductivity arises from $p—-wave pairing of holons. This pairing leads to temperature (T) dependences of the electronic properties that are essentially the same as those given by s-wave BCS theory and suggests the possibility that the electrons are correlated in singlet pairs that are symmetric with respect to interchange of spin. On the other hand, the magnetic properties are dominated by the excitations of the spin liquid, leading to a spin susceptibility ${\mathrm{\chi }}_{\mathit{s}}$ that is proportional to T and a nuclear-magnetic-resonance relaxation rate 1/${\mathit{T}}_1$T that is proportional to ${\mathit{T}}^2$.

We have performed zero-field muon-spin-rotation μSr measurements of the ‘‘infinite-layer’’ cuprate compound ${\mathrm{Ca}}_{0.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_2$ and the ‘‘infinite chain’’ system ${\mathrm{Sr}}_2$${\mathrm{CuO}}_3$. A spontaneous magnetic field from the ordered Cu moments is observed below ${\mathit{T}}_{\mathit{N}}$=540±5 K in ${\mathrm{Ca}}_{0.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_2$: below T=360 K, we observe muon-spin precession with a frequency ν(T→0=16 MHz (corresponding to a local field of 1.2 kG). The precession signal is replaced by a rapid depolarization above T=360 K due to the onset of muon diffusion. The hopping rate followed an Arrhenius law, with an activation energy of ${\mathit{E}}_{\mathit{a}}$=0.39(1) eV. The sublattice magnetization ${\mathit{M}}_{\mathit{s}}$, proportional to ν(T), showed a slower decay with increasing temperature in ${\mathrm{Ca}}_{0.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_2$, compared with that observed in ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ and ${\mathrm{Sr}}_2$${\mathrm{CuO}}_2$${\mathrm{Cl}}_2$, indicating that a wider ${\mathrm{CuO}}_2$-layer separation results in a more two-dimensional magnetic behavior. In the infinite-chain system ${\mathrm{Sr}}_2$${\mathrm{CuO}}_3$, the onset of magnetic order was found at T∼5 K with a local field of ∼30 G at the muon site at T→0. The exchange interaction, inferred from susceptibility measurements is on the order of ${10}^3$ K, implying a remarkable suppression of the ordering temperature with ${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{N}}$/J≤0.01 in ${\mathrm{Sr}}_2$${\mathrm{CuO}}_3$. These results demonstarate clear signatures of low-dimensional magnetic behavior in the CuO chains.

We report measurements of surface optical phonons on ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ with high-resolution electron-energy-loss spectroscopy (HREELS). In addition to peaks near 50 and 80 meV (403 and 645 ${\mathrm{cm}}^{\mathrm{-}1}$), which have been previously observed, our loss spectra exhibit a peak at 26 meV (210 ${\mathrm{cm}}^{\mathrm{-}1}$). Loss spectra were measured at temperatures from 45 to 146 K, and the temperature dependence of the peaks was found to be weak. The 50 and 80 meV peaks shift to lower frequency by ∼1.5 meV over this temperature range. All three peaks are attributed to surface optical phonons. The identification of particular bulk modes corresponding to the surface modes observed with HREELS is discussed.

We have studied the relationship between structural defects and superconductivity in ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{Ca}}_{1.2\mathrm{-}\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{6\mathrm{-}\mathrm{\delta }}$ (0≤x≤0.8). The samples were prepared by synthesis under different oxygen pressures, P(${\mathrm{O}}_2$)=50, 250, and 400 atm. Six of the seven samples were found to be superconducting with 22≤${\mathit{T}}_{\mathit{c}}$≤58 K. The structural properties were determined by neutron powder diffraction. The inter-${\mathrm{CuO}}_2$-plane spacing, ${\mathit{d}}_{\mathrm{Cu}\mathrm{-}\mathrm{Cu}}$, increases as Ca on the M(1) site between these planes is replaced by the larger Sr and La ions. The metal-site ordering is influenced by the oxygen pressure during synthesis. We have used the inter-${\mathrm{CuO}}_2$-plane spacing, neutron-diffraction measurements of the scattering from the metal sites, and chemical constraints to determine the occupancies of La, Sr, and Ca at the M(1) site. For the same overall composition, higher oxygen pressure leads to a larger fraction of La on the M(1) site. Oxygen occupancy of the vacant O(3) site in the M(1) plane increases sharply when ${\mathit{d}}_{\mathrm{Cu}\mathrm{-}\mathrm{Cu}}$ exceeds 3.5 Å. The superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ decreases systematically as the occupancy of O(3) increases for samples that would otherwise be expected to be superconducting.

The leading contributions to the c-axis conductivity of layered superconductors arising from superconducting fluctuations of the order parameter are discussed for arbitrary intralayer scattering. The contributions from fluctuations of the normal quasiparticle density of states are shown to be opposite in sign to the Aslamazov-Larkin and Maki-Thompson contributions, leading to a peak in the overall c-axis resistivity ${\mathrm{\rho }}_{\mathit{c}}$(T) above ${\mathit{T}}_{\mathit{c}}$. This peak is enhanced by a magnetic field H∥c^. With increasing H, the relative peak maximum in ${\mathrm{\rho }}_{\mathit{c}}$(T,H) increases in magnitude and is shifted to lower temperatures by an amount proportional to ${\mathit{H}}^2$ for weak fields and to H for strong fields. For comparison, the fluctuation conductivity parallel to the layers has been calculated including the fluctuations of the normal density of states. Our results are discussed in regard to recent experiments with ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$.

A Hamiltonian for two interacting electrons coupled with longitudinal phonons and plasmons is set up. The phonons are treated in the Fröhlich scheme and the plasmons in the single-pole approximation. We are interested in systems with low electron density (n≤${10}^{20}$–${10}^{21}$) such as high-${\mathit{T}}_{\mathit{c}}$ superconductors. It is shown that, in this density range, the electron dynamics do not simply screen the electrostatic interaction but, also, cooperates to the bipolaron formation. Furthermore we find that the binding energies and the effective mass depend on the electronic density. The features of the effective electron-electron potential are discussed, mainly for what concerns the self-energy terms and the long-range tail; the former depends on the pair state and the second shows that the plasmon field tends to screen all the electrostatic interactions, even the electron-phonon one. It is also shown that the Hamiltonian formulation is equivalent to a dielectric formulation where the total dielectric function of the system is the sum of the dielectric function appropriate for an ionic of the system is the sum of the dielectric function appropriate for an ionic material and that appropriate for the electron gas. Within this model we calculate the Bose-condensation critical temperature of a system of correlated pairs and free carriers in a two-fluid model at thermodynamical equilibrium.

The magnetic properties of ${\mathrm{Gd}}_2$${\mathrm{CuO}}_4$ have been investigated by magnetization, electron paramagnetic resonance (EPR) and neutron-diffraction measurements in the temperature range 1.5–50 K to determine the magnetic phase diagram and to search for the origin of the weak ferromagnetism which has been observed in this compound. ${\mathrm{Gd}}_2$${\mathrm{CuO}}_4$ is found to have a rather complex magnetic phase diagram consisting of at least four distinct magnetic phases. The nature of the weak ferromagnetism is discussed and a possible origin of both the weak ferromagnetism and the phase transitions is proposed.

This paper is a report on the results of a study done on the superconductive transitions of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{y}}$/${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CuO}}_{\mathit{x}}$ (BSCCO/BSCO) multilayer thin films in magnetic fields parallel or perpendicular to the films, which were prepared using a multitarget rf magnetron sputtering technique. In the multilayers with BSCCO (a superconductor) of constant nominal layer thickness of 47 Å, the resistive transition showed broadening and reduction of ${\mathit{T}}_{\mathit{c}}$ with an increase of the BSCO (a nonsuperconductor) layer thickness from 36 Å to 144 Å. The resistive transition showed a slight broadening, even under the aligned magnetic field parallel to the Cu-O planes. This result suggests that the superconductive layers are coupled through BSCO layers, and that the interlayer coupling was induced by the proximity effect.

Ceramic samples of the oxide superconductor ${\mathrm{Bi}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ with x=0, 0.2, and 0.3 have been prepared and studied by x-ray-photoelectron spectroscopy. A comparison of the Cu 2p, Bi 4f, and O 1s spectra of undoped and doped samples from ambient temperature down to 96 K near the critical temperature suggests the formation of dimer oxygen species in the superconducting state. A mechanism of charge transfer between ${\mathrm{CuO}}_2$ and BiO planes is proposed to explain this possibility, which is suggested to be related to the change of the conductivity of the ceramics.

The penetration of a quasistatic electric field into epitaxial films of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ was studied around the films’ transition temperature. The electric-field penetration into an ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ electrode was shown to vary by as much as 100 Å in the temperature regime 60<T<110 K. The direction of interfacial movement is reversed at ${\mathit{T}}_{\mathit{c}}$. The data are consistent with the continuous movement, with changing temperature, of an interface between two regions, one penetrated by the electric field and one devoid of electric field. The data indicate stronger delocalization forces along the c axis of the material in both the superconducting and normal state than could be expected from any conventional theory of metals. The effect of charge-carrier concentration on ${\mathit{T}}_{\mathit{c}}$ was studied. The first and second derivatives of ${\mathit{T}}_{\mathit{c}}$ with respect to surface carrier concentration were determined to be, respectively, near zero and negative.

A strongly coupled variational wave function, the doublet spin-projected Néel state (DSPN), is proposed for oxygen holes in three-band models of high-temperature superconductors. This wave function has the three-spin system of the oxygen hole plus the two neighboring copper atoms coupled in a spin-1/2 doublet. The copper spins in the neighborhood of a hole are in an eigenstate of the infinite-range Heisenberg antiferromagnet (SPN state). The doublet three-spin magnetic polaron or hopping polaron (HP) is stabilized by the hopping terms ${\mathit{t}}_{\mathrm{\sigma }}$ and ${\mathit{t}}_{\mathrm{\tau }}$, rather than by the copper-oxygen antiferromagnetic coupling ${\mathit{J}}_{\mathrm{pd}}$. Although, the HP has a large projection onto the Emery (${\mathit{D}}_{\mathit{g}}$) polaron, a non-negligible amount of doublet-u (${\mathit{D}}_{\mathit{u}}$) character is required for optimal hopping stabilization. This is due to ${\mathit{J}}_{\mathit{d}\mathit{d}}$, the copper-copper antiferromagnetic coupling. For the copper spins near an oxygen hole, the copper-copper antiferromagnetic coupling can be considered to be almost infinite ranged, since the copper-spin-correlation length in the superconducting phase (0.06–0.25 holes per in-plane copper) is approximately equal to the mean separation of the holes (between 2 and 4 lattice spacings). The general DSPN wave function is constructed for the motion of a single quasiparticle in an antiferromagnetic background. The SPN state allows simple calculations of various couplings of the oxygen hole with the copper spins. The energy minimum is found at symmetry (π/2,π/2) and the bandwidth scales with ${\mathit{J}}_{\mathit{d}\mathit{d}}$. These results are in agreement with exact computations on a lattice. The coupling of the quasiparticles leads to an attraction of holes and its magnitude is estimated.

In the low-temperature, low-energy, and long-wavelength regime, by considering the hard-core nature of the slave bosons, we can eliminate the infrared divergences induced by the overdamped mode of a gauge field that mediates the interaction between fermions and slave bosons; the stability of the Fermi-liquid ground state of the fermions is thereby retained.

Results on the systematics of the magnetic penetration depth ${\lambda }_{\mathit{a}\mathit{b}}$ in the high-temperature superconductors ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{1\mathrm{-}\mathit{z}}$${\mathrm{Y}}_{\mathit{z}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ (z=0,0.1,0.2,0.3,0.4,0.45) and ${\mathrm{Bi}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ (x=0.15,0.30,0.70) are reported from muon-spin-rotation measurements on polycrystalline samples with known oxygen excess δ. In determining ${\lambda }_{\mathit{a}\mathit{b}}$ various additional sources for inhomogeneous internal field distributions, besides the one arising from the flux-line lattice, have been critically taken into account. The most important one arises from a type of powder broadening due to the anisotropy of the field-cooled magnetization. It is found that ${\lambda }_{\mathit{a}\mathit{b}}$ in the Y-doped compounds correlates in the expected manner with the nominal hole-carrier concentration p (${\lambda }_{\mathit{a}\mathit{b}}^{\mathrm{-}2}$∝p) while the Pb-doped compounds ${\lambda }_{\mathit{a}\mathit{b}}$ is practically independent of p. It is concluded that Pb doping (valence state is +2) does not increase the density of free charge carriers in agreement with results from Hall-effect measurements. No correlation between ${\lambda }_{\mathit{a}\mathit{b}}^{\mathrm{-}2}$ and ${\mathit{T}}_{\mathit{c}}$ in the manner seen in the 1:2:3 family is found in the present case.

ac-susceptibility, dc-magnetization, and ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer spectra of ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{Sr}}_2$${\mathrm{Cu}}_{3\mathrm{-}\mathit{y}}$${\mathrm{Fe}}_{\mathit{y}}$${\mathrm{O}}_{6+\mathit{z}}$ (x=0, 0.1, and 0.2; y=0.3 and 0.4) are presented. Our data indicate bulk superconductivity in these compounds. In the case of y=0.4, the substitution of x=0.1 Ca increases both ${\mathit{T}}_{\mathit{c}}$ and flux pinning. The Mössbauer parameters of the sample containing y=0.4 Fe and no Ca in comparison to those of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_{3\mathrm{-}\mathit{y}}$${\mathrm{Fe}}_{\mathit{y}}$${\mathrm{O}}_{6+\mathit{z}}$ indicate that at least 85% of Fe substitutes the Cu(1) sites and about 15% goes to Cu(2) sites. The existence of superconductivity at such high concentrations of Fe and the revival of superconducting properties by Ca substitution are discussed.