We report on dc magnetic susceptibility measurements in polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ as a function of the oxygen content in the limit of small δ. We focus mainly on the detailed behavior of the zero-field-cooled–field-cooled irreversibility line at fields below 7 kOe. We find that this line shows a crossover from a ${\mathit{H}}^2$ dependence at higher fields to the more commonly observed ${\mathit{H}}^{2/3}$ behavior at lower fields. This feature is analogous to the anisotropy induced crossover from the high-field Gabay-Toulouse line to the low-field de Almeida-Thouless behavior, characteristic of the spin-glass transition. Our results are discussed in the light of a dirty-limit vortex-glass model where the grain charging energy plays a major role. Data concerning the behavior of the resistive transition as a function of the distribution of the oxygen vacancies are also presented.

The transition temperatures (${\mathit{T}}_{\mathit{c}}$) of the homologous series ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_{\mathit{n}\mathrm{-}1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+2+\mathrm{\delta }}$ of high-temperature superconductors show very large (∼30 K) pressure enhancements, saturating at 118, 154, and 164 K for Hg1201, Hg1212, and Hg1223, respectively. We have measured the compressibility of the first three mercury superconductors in a diamond-anvil cell using energy-dispersive synchrotron x-ray diffraction. We report the tetragonal-cell parameters a and c up to 30 GPa. We find ambient-pressure bulk moduli, ${\mathit{K}}_{\mathit{V}0}$==-${\mathit{V}}_0$(∂P/∂V${)}_0$, of 65.4(3.0), 91.6(4.7), and 83.8(10.5) GPa for Hg1201, Hg1212, and Hg1223, respectively. The Hg superconductors are highly compressible compared to other high-temperature superconductors which generally show bulk moduli in the range of 100–200 GPa. The relation c=A+B(n-1) holds for the c-axis cell parameter at ambient pressure, however, the relation breaks down at high pressures.

The dynamic spin-correlation function in the superconducting states (i.e., D wave and S wave) in the vicinity of the commensurate point Q=(π,π) is obtained within mean-field theory. As a model we take the generalized t-J model as analyzed by Tanamoto, Kohno, and Fukuyama though we interpret the fermion loop as associated with a renormalized hole. For T≥${\mathit{T}}_{\mathit{c}}$ where ${\mathit{T}}_{\mathit{c}}$ is the superconducting transition temperature our result is essentially the same as Tanamoto, Kohno, and Fukuyama, but the superconducting correlation is easily incorporated within the present scheme. We find recent neutron-scattering data from both ${\mathrm{La}}_{1.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_4$ by Mason et al. and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ by Rossat-Mignod et al. are consistent with the D-wave model but incompatible with the S-wave model.

The magnetoresistance and the V-I curves (I>${\mathit{I}}_{\mathit{c}}$) of a well-characterized Ag-clad (Bi,Pb${)}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10+\mathit{y}}$ tape (BPSCCO) with ${\mathit{J}}_{\mathit{c}}$(77 K, B=0)=10 kA/${\mathrm{cm}}^2$ have been analyzed in the temperature (T) range 80–110 K and magnetic field (B) up to 60 mT. The results obtained after the correction for the parallel conduction through the Ag sheathing and BPSCCO core are similar to those for corresponding single crystals and the epitaxial thin films. The effective activation energy for the flux creep ${\mathit{U}}_0$ is larger than those for the single crystals and thin films of Bi-Sr-Ca-Cu-O (BSCCO) compound. The variations of the differential resistance ${\mathit{R}}_{\mathit{f}}$ (deduced from V-I curves at I≫${\mathit{I}}_{\mathit{c}}$) with T and B are consistent with those for the conventional type-II superconductors at elevated temperatures and low fields. Hence the unusual features deduced from the total dissipation in similar tapes are probably due to improper data analysis. The analysis presented here relies on the homogeneity of dissipation within the core which has to be checked for each tape.

We report on the hysteretic field-dependent critical currents of large inductance arrays with up to ten ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ grain-boundary junctions in parallel. A peak in the critical current, roughly equal to its initial zero-field value, is observed after each reversal of the field sweep. This behavior indicates that the junction phases become equal modulo 2π (self-organized phase coherence) as the system relaxes away from the critical state. The primary features can be modeled using a Frenkel-Kontorova model generalized to include global interactions due to mutual inductance.

The defect structure of oxygenated and reduced ${\mathrm{Nd}}_2$${\mathrm{CuO}}_{\mathit{y}}$ (y≊4) was investigated by single-crystal neutron diffraction. Structural refinements indicate the presence of interstitial oxygen atoms in the ‘‘apical’’ O(3) position, directly above or below the copper atoms. The occupancy of the apical oxygen site is ∼0.10 (per formula unit) for the oxygenated sample and ∼0.04 for the reduced one. Both the in-plane O(1) and the out-of-plane O(2) oxygen sites were found to be deficient, but their occupancy did not vary, within the error bars, between the two crystals. The difference in oxygen content between the two crystals, as calculated from the structural refinements, is consistent, within the error bars, with the thermogravimetric analysis values reported in the literature.

The planar-oxygen Knight shift ${}_{}{}^{17}{}_{}{}^{}$K and nuclear spin-lattice relaxation rate ${(}^{17}$${\mathit{T}}_1$${)}^{\mathrm{-}1}$ have been determined in the superconductor ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$. The present oxygen results have been analyzed in connection with the planar-copper relaxation rate ${(}^{63}$${\mathit{T}}_1$${)}^{\mathrm{-}1}$ data. In the normal state, both ${}_{}{}^{17}{}_{}{}^{}$K and ${(}^{17}$${\mathit{T}}_1$T${)}^{\mathrm{-}1}$ decrease as the temperature is lowered towards ${\mathit{T}}_{\mathit{c}}$=81 K, while ${(}^{63}$${\mathit{T}}_1$T${)}^{\mathrm{-}1}$ shows a maximum around 160 K. Comparison with ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ data strongly suggests that both the q and the temperature dependences of the magnetic fluctuations in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ present a similarity to those for the 60-K phase of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ in the normal state above 110 K. In the superconducting state, ${(}^{63}$${\mathit{T}}_1$T${)}^{\mathrm{-}1}$ decreases significantly with decreasing temperature, while the slope for ${(}^{17}$${\mathit{T}}_1$T${)}^{\mathrm{-}1}$ shows a slight change near ${\mathit{T}}_{\mathit{c}}$. The considerable decrease in ${}_{}{}^{17}{}_{}{}^{}$${\mathit{T}}_1$${/}^{63}$${\mathit{T}}_1$ below ${\mathit{T}}_{\mathit{c}}$ suggests that the loss of antiferromagnetic fluctuations is dominant in the superconducting state.

Measurements of field-induced changes of the a-b plane surface resistance of high-quality (>${10}^{10}$ A/${\mathrm{m}}^2$ at 77 K and B=0) ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films at 12.92 GHz reveal flux-flow resistivity at temperatures several degrees below the superconducting onset temperature. Flux-flow resistivity is identified by its temperature dependence and dependence on the Lorentz force. The results are compared to the surface resistance theory of Coffey and Clem, which includes a model for pinning.

The crystalline structure of the heavy alkaline earth chalcogenide SrSe was studied at high pressure by x-ray diffraction using a synchrotron source. The NaCl-type (B1) structure of SrSe was found to be stable on loading to 14.2 GPa where the CsCl-type (B2) structure was evidenced with a 10.7% decrease in relative atomic volume. The B2 phase of SrSe persists to 40 GPa, the highest pressure attempted in the experiments. The B1-B2 phase transition was found to be reversible with small hysteresis. The bulk modulus and its pressure derivative were determined for both phases. Systematic behavior among the strontium chalcogenides is discussed.

The nonlinear response is studied in a two-component composite with concentration p of perfect conductor (S) and concentration (1-p) of normal metal (N) with nonlinear response of the form J=${\mathrm{\sigma }}_{\mathit{A}}$E+${\mathrm{\chi }}_{\mathit{A}}$‖E${\mathrm{\Vert }}^2$E. Below the percolation threshold ${\mathit{p}}_{\mathit{c}}$ of the perfect conductor, the response of the composite can be represented by 〈J〉=${\mathrm{\sigma }}_{\mathit{e}}$〈E〉+${\mathrm{\chi }}_{\mathit{e}}$‖〈E〉${\mathrm{\Vert }}^2$〈E〉, where 〈 . . .〉 represents spatial averages. The magnitude of the crossover field ‖${\mathbf{E}}_{\mathit{c}}$‖, defined as the electric field at which the linear and nonlinear response of the composite become comparable, is found to have a power-law dependence ‖${\mathbf{E}}_{\mathit{c}}$‖∼(${\mathit{p}}_{\mathit{c}}$-p${)}^{\mathit{M}}$ as the percolation threshold is approached from below. Within the effective-medium approximation, the exponent M is found to be 1/2 for all spatial dimensions. By relating the cubic nonlinear composite problem to the noise problem in linear composites, the exponent M is found to be M=(κ’+s)/2, where κ’ and s are the noise and conductivity exponents in a S/N composite, respectively. Previously derived bounds on the noise exponent κ’ thus lead to bounds on the exponent M.

Conductivities of the N×N percolation systems with resistors and capacitors mixed near the percolation threshold ${\mathit{p}}_{\mathit{c}}$ were calculated. The statistical distribution of these conductivities were obtained after more than ${10}^4$ configurations considered. We have shown that the argument of Rammal et al., which is only for the case of dc response, can be properly extended to the case of nonzero capacitances (ac response). Some other interesting features, including the functional form, of the distribution were discussed as a function of the ratio ω/${\mathrm{\omega }}_0$ with ${\mathrm{\omega }}_0$≡1/RC.

Hole burning in the proton NMR line in water was observed in well-defined noise fields. The hole shape was measured in two-state and Gaussian noise fields. The jumping-time dependences of the hole shape were well explained by the theory of motional narrowing and the conventional stochastic theory of relaxation. This approach is very useful for the experimental test of relaxation theories.

We look into the nature of Lebed’s states [i.e., the particular field-induced spin-density-wave state (FISDW) which appears at Lebed’s resonance angle] and find that they are characterized by a set of integers (N,M) different from the surrounding FISDW. This pair of integers implies the quantized Hall conductivity at low temperatures; ${\mathrm{\sigma }}_{\mathit{a}\mathit{b}}$=2${\mathit{e}}^2$N/h and ${\mathrm{\sigma }}_{\mathit{a}\mathit{c}}$=2${\mathit{e}}^2$M/h where c and b are the corresponding lattice constants and h is the Planck constant. We predict the double quantum Hall effect, if both M and N are different from zero. We show also that the dips observed in the magnetoresistance can be interpreted in terms of the FISDW fluctuation which produces a sharp peak in ${\mathrm{\sigma }}_{\mathit{a}\mathit{a}}$ at Lebed’s resonances.

Spin-wave theory is used to study the effects of interchain coupling on the properties of quasi-one-dimensional Heisenberg ferromagnets. The asymptotic expressions of magnetization, internal energy, and specific heat with temperature and interchain coupling strength are given at low temperatures. It is found that, for a weak interchain interaction, with the increase of temperature from zero, the magnetization and specific heat change from exhibiting three-dimensional behavior (Δm∝-${\mathit{T}}^{3/2}$,${\mathit{C}}_{\mathit{m}}$∝${\mathit{T}}^{3/2}$) to quasi-one-dimensional behavior (Δm∝-T,${\mathit{C}}_{\mathit{m}}$∝${\mathit{T}}^{1/2}$).

A cluster of four rhodium or silver spins interacting with their nearest and next-nearest neighbors is investigated. Randomly generated spin configurations are employed to calculate the density of states N(E) and the entropy S(E)∝lnN(E). The second law of thermodynamics, T=dE/dS, allows the determination of the entropy as a function of temperature. As in recent experimental data, the calculated S(T) is bigger for positive than for negative spin temperatures.

The electron-paramagnetic-resonance technique has been used to obtain an unambiguous identification of resonances between spin-cluster states from the distinctive satellite spectra observed in the ranges 10–36 GHz and 2.6–4.2 K from a large single crystal of the one-dimensional Ising ferromagnet [(${\mathrm{CH}}_3$${)}_3$NH]${\mathrm{FeCl}}_3$⋅${2\mathrm{H}}_2$O, known as FeTAC. The frequency and temperature dependencies of the spectra are explained by the addition of the term Δ${\mathit{S}}_{\mathit{x}}$ to the Zeeman term in the effective S=1/2 spin Hamiltonian describing the first set of excited cluster states. The spin-cluster levels are the eigenvalues of the resulting tridiagonal matrix, and lead to a satellite structure in FeTAC which is independent of the chain length for chains of more than about 30 ${\mathrm{Fe}}^{2+}$ ions.

Using soft-x-ray magnetic circular dichroism we have observed a change in the easy axis of magnetization in Ni ultrathin films on Cu(001) from parallel to the surface for 5–9 monolayers (ML) to perpendicular for 10–75 ML. This is an unusual effect which is counter to the predictions of magnetic surface anisotropy arguments. We conclude that the effect is due to a shape anisotropy, introduced either by a change in the Ni lattice structure or by three-dimensional morphological changes in the film.

Recent experiments on the magnetic properties of frozen ferrofluids indicate clear deviations from canonical spin glass behavior. To elucidate these differences we present simulations based on a local-mean-field theory approach. This theoretical scheme was previously found to be quite successful for traditional spin glass and random field systems. In a parallel fashion we study frustration effects associated with a magnetic dipole-dipole interaction for a random spatial arrangement of Heisenberg spins in a quenched ferrofluid. Some of the experimental features can be reproduced if the spins are assumed to be inhomogeneously distributed in space.

Anderson’s model for localized impurity states hybridized with the conduction band is extended to include transfers of pairs of electrons between the band states and the localized levels. The interplay between the hybridization and the pair-transfer coupling is analyzed. It is found that when the effect of the hybridization is small and the localized states are not resonant, the temperature range over which the system is superconducting is increased by the pair-transfer mechanism. In the opposite case, the pair-transfer coupling combines with the hybridization to reduce that range. Thus, pair-transfer interaction can be detrimental to superconductivity.

The superconducting correlation function of finite two-dimensional systems is calculated within the framework of the Ginzburg-Landau theory in the Gaussian approximation. A crossover is found from zero-dimensional behavior at low temperatures to two-dimensional behavior at higher temperatures. The relation between this approach and the Kosterlitz-Thouless theory is briefly discussed.