Layered type-II superconductors (say, ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ and superconductor/insulator superlattices) possess fascinating mixed states. A flux line at an angle with the superconducting layers may be cut into a series of pancake vortices. In the existing literature, the interactions between these vortices have been attributed to a combination of magnetic and Josephson effects. Both these effects lead to an attraction between pancake vortices in neighboring superconducting layers. We show that there exists a third mechanism of interaction, namely the indirect effect of the Coulomb interaction, which leads to a repulsion between such vortices. The relative strength of this coupling is estimated. We consider the competition between different mechanisms and solve the effective total Hamiltonian asymptotically for small in-plane separation between neighboring pancake vortices. The third coupling mechanism is an observable consequence of vacuum fluctuations.

The microstructure of Sr-free bismuth cuprates, i.e., ${\mathrm{Bi}}_2$(${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{1+\mathit{x}}$)${\mathrm{CuO}}_{6.5+\mathrm{\delta }}$ and (${\mathrm{Bi}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$)${\mathrm{LaCaCuO}}_{6.5+\mathrm{\delta }}$, was determined by means of x-ray diffraction and electron diffraction (ED). ED analysis revealed that the two types of Sr-free compounds possess a monoclinic superstructural modulation, which is similar to ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CuO}}_{6+\mathrm{\delta }}$. In the ${\mathrm{Bi}}_2$(${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{1+\mathit{x}}$)${\mathrm{CuO}}_{6.5+\mathrm{\delta }}$ system, the modulation wavelength along the b and c axes increased markedly with the Ca substitution for La. At the same time this substitution facilitated the commensurate-to-incommensurate transition. For the (${\mathrm{Bi}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$)${\mathrm{LaCaCuO}}_{6.5+\mathrm{\delta }}$ system, a similar change in the modulation wave vector was also observed. The dependence of the modulation vector on chemical composition can be best interpreted by the crystal-misfit model. In addition, Raman scattering analysis of the ${\mathrm{Bi}}_2$(${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{1+\mathit{x}}$)${\mathrm{CuO}}_{6.5+\mathrm{\delta }}$ system showed that the vibrational properties of the oxygen atoms in the Bi-O bonding also changed with the increase in the modulation wavelength, and this behavior can be regarded as a consequence of the structural relaxation caused by the enhancement of the degree of crystal fit.

Using magnetic and magnetotransport measurements, we show that a ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ thin-film sample has a single irreversibility line (IRL), which is the same as the vortex-glass-transition phase boundary. Results suggest that the effects of pinning persist above the glass-transition temperature ${\mathit{T}}_{\mathit{g}}$, and that it is the loss of critical-current density ${\mathit{J}}_{\mathit{c}}$ rather than an onset of reversible magnetic behavior that characterizes the vortex-glass transition. Measurements of dc magnetization are shown to confirm a theoretical model that explains why the determination of ${\mathit{T}}_{\mathit{g}}$ requires that field-cooled data be collected on cooling (FCC) rather than on warming (FCW), as is frequently done. The use of the ac susceptibility response for measuring ${\mathit{T}}_{\mathit{g}}$ is shown to be a valid measure only at low frequency. This is because both the fundamental-frequency and third-harmonic ac susceptibilities measure the onset of ac flux penetration rather than the onset of irreversibility. The frequency dependence of the onset temperature of ac flux penetration (${\mathit{T}}_{\mathrm{on}}$) is shown to follow the vortex-glass-model dependence ${\mathit{T}}_{\mathrm{on}}$=C(2πf${)}^{\mathrm{\{}1/\left[\right(\mathit{z}\mathrm{-}1)\nu ]\mathrm{\}}}$+${\mathit{T}}_{\mathit{g}}$, with values of z and ν consistent with those determined from scaling of the magnetotransport data.

We report a set of experimental data on third-harmonic generation at microwave frequencies in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ single crystals near ${\mathit{T}}_{\mathit{c}}$ at zero applied field. A theory is also developed which fully accounts for the experimental data. The theory is based on the two-fluid model and the hypothesis that the microwave magnetic field modulates the partial concentrations of both normal and superconducting electrons.

We present a model for layered superconductors, where the order parameter between the layers is nonzero, due to proximity effects. In the limit of decoupled layers, this model gives all the terms of the usual Lawrence-Doniach free energy, as well as new terms arising from the existence of an order parameter between the layers. The corresponding Josephson coupling is found to be field dependent. The upper critical field normal to the layers has positive curvature as a function of T, and decreases with increasing interlayer distance, contrary to the predictions of the Lawrence-Doniach model. In addition, the anisotropy deduced from the critical fields ${\mathit{H}}_{\mathit{c}2}$ is markedly different from the low-field anisotropy.

The electron-doped ${\mathit{L}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$ (L=lanthanide) superconductors have intergrowth structures in which ${\mathrm{CuO}}_2$ sheets alternate with (L,Ce${)}_2$${\mathrm{O}}_2$ fluorite layers along the c axis. Stabilization of such intergrowth structures requires bond-length matching between Cu-O and (L,Ce)-O bonds. Any bond-length mismatch will result in the buildup of compressive or tensile stresses in the Cu-O and (L,Ce)-O bonds. The consequences of such internal stresses in ${\mathit{L}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$ are investigated by a systematic variation through ${\mathit{L}}^{3+}$ size of the lattice parameter a. A decrease in the degree of bond-length mismatch or internal stresses with decreasing ${\mathit{L}}^{3+}$ size causes a systematic decrease in the Ce solubility limit and in the ease with which oxygen vacancies can be created. The concentration of oxygen vacancies decreases—or the oxygen content increases—with decreasing ${\mathit{L}}^{3+}$ size for a given ${\mathrm{N}}_2$-annealing temperature and Ce content; it also decreases with increasing Ce content for a given ${\mathit{L}}^{3+}$ ion. The decreasing oxygen-vacancy concentration with decreasing size of ${\mathit{L}}^{3+}$ causes an apparent increase in the critical Ce concentration ${\mathit{x}}_{\mathit{c}}$ required to induce the antiferromagnetic semiconductor to superconductor transition as the size of ${\mathit{L}}^{3+}$ decreases, although the transition seems to occur at a fixed critical electron concentration ${\mathit{n}}_{\mathit{c}}$=0.175±0.005 irrespective of the ${\mathit{L}}^{3+}$ size.

The structural and superconducting properties of (${\mathrm{Y}}_{1\mathrm{-}\mathit{x}\mathrm{-}\mathit{y}}$${\mathrm{Ca}}_{\mathit{y}}$${\mathrm{Hf}}_{\mathit{x}}$)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{z}}$ samples are investigated using resistivity, ac susceptibility, and dc magnetization measurements. The effect of increasing the Hf concentration in ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Hf}}_{\mathit{x}}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{z}}$ lowers the oxygen content and decreases ${\mathit{T}}_{\mathit{c}}$. The substitution of Ca for Y in (${\mathrm{Y}}_{0.85\mathrm{-}\mathit{y}}$${\mathrm{Ca}}_{\mathit{y}}$${\mathrm{Hf}}_{0.15}$)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{z}}$ provides proper matching between the ionic radius and valence of ${\mathrm{Y}}^{3+}$ (0.89 Å) and the average ionic radius and valence of ${\mathrm{Hf}}^{4+}$ (0.78 Å) and ${\mathrm{Ca}}^{2+}$ (0.99 Å). As the Ca content increases, the ${\mathit{T}}_{\mathit{c}}$ increases from y=0 (74 K) to y=0.15–0.20 (compensated oxides) having ${\mathit{T}}_{\mathit{c}}$ of 84.5–85.5 K. Magnetization measurements on powdered Hf-doped samples exhibit pronounced M(H) hysteresis loops indicative of extensive flux pinning and enhanced critical currents.

Within the framework of the ladder-diagram approximation, the manifestly gauge-invariant expression for the static electromagnetic response kernel of the anisotropic narrow-band superconductor consisting of Bloch electrons is derived, on the basis of the charge-conserving current expression for them under the magnetic field. For the two-dimensional square lattice, the magnetic penetration depth ${\lambda }_{\mathit{L}}$(T) is calculated as a function of hole density ${\mathit{n}}_{\mathit{h}}$ and temperature T (<${\mathit{T}}_{\mathit{c}}$). The resultant ${\lambda }_{\mathit{L}}$(0) has a symmetrical dependence on the hole and electron densities, and is almost unaffected by the anisotropy of the order parameter. The behavior of ${\lambda }_{\mathit{L}}$(T)/${\lambda }_{\mathit{L}}$(0) as a function of T/${\mathit{T}}_{\mathit{c}}$ for the d-wave state substantially deviates from those for the extended and usual s-wave states for all T<${\mathit{T}}_{\mathit{c}}$. The obtained results are compared with those of the effective hole- and electron-mass approximations.

We measured Mössbauer spectra from bcc alloys of compositions near ${\mathrm{Fe}}_3$Al, ${\mathrm{Fe}}_3$Si, and ${\mathrm{Fe}}_3$Ge, and reanalyzed older data from ${\mathrm{Fe}}_3$Ga. All four alloys developed some D$0_3$ chemical order, and their Mössbauer spectra were qualitatively similar. The local (first-nearest-neighbor) could be distinguished from the nonlocal (beyond the first-nearest-neighbor) contributions to the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ isomer shifts from the solute atoms owing to differences in their hyperfine magnetic fields. The nonlocal isomer shift contributions correlated best to the valence of the solute atom, and not to the atomic volume. The local isomer shift contributions were best attributed to a loss of Fe 3d electrons at the solute sites.

The intensity of electrons scattered from atoms in the keV range is peaked in the forward direction. This effect is the basis of the projection-imaging technique of near-surface atomic geometry in solids, using secondary electrons. By changing the angle of incidence of primary electrons that are used for the excitation, the probing depth can continuously be varied. Thus, one can investigate the degree of electron focusing as a function of the number of atoms in a chain along which the scattering occurs. It is found that, for a Pt (110) surface, the angular size of the observed spots decreases as the number of atoms along the path of emission increases. In contrast to previous conclusions, this observation shows an enhancement of electron focusing in a chain of atoms.

It is found that substitutions of Sc by Fe in ${\mathrm{Li}}_3$${\mathrm{Sc}}_2$(${\mathrm{PO}}_4$${)}_3$ and of Fe by Sc in the isostructural compound ${\mathrm{Li}}_3$${\mathrm{Fe}}_2$(${\mathrm{PO}}_4$${)}_3$ result in suppression of an order-disorder phase transition in the lithium subsystem of the ${\mathrm{Li}}_3$${\mathrm{Sc}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$(${\mathrm{PO}}_4$${)}_3$ mixed crystals. Thus a lithium disorder, dominating at T≥570 K, is preserved at lower temperatures. This leads to a glassylike behavior of the crystals, in particular, Vogel-Fulcher-Tammann temperature dependence of the ionic conductivity.

Fast nonlinear optical signals on picosecond and nanosecond time scales have been studied in the mixed crystal system ${\mathrm{KTa}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Nb}}_{\mathit{x}}$${\mathrm{O}}_3$ using transient degenerate four-wave mixing. The two distinct signal components, denoted as an instantaneous response and slow response in previous studies, show the dependence of the polarization configuration of the optical pulses. The results were consistent with a lattice-relaxation model based on the laser-induced dynamic displacement of the Nb ion. In this model the signal variations result from the competition between the dynamic displacement and the intrinsic distortion associated with the spontaneous polarization along the optic axis.

The effect of lattice vibrations on the ordering tendencies and phase stability of substitutional binary alloys is analyzed. The alloy Hamiltonian is cast onto an Ising-like Hamiltonian through a cluster expansion of the vibrational free energy computed in the harmonic approximation. The temperature dependence of the vibrational effective cluster interactions (VECI’s) is found for a general d-dimensional alloy. Analytic results for the VECI’s for d=1 are derived. It is argued that the vibrations can have pronounced effects on the phase diagram and on the short-range order at high temperatures.

We study numerically the out-of-equilibrium dynamics of the hypercubic cell spin glass in high dimensionalities. We obtain evidence of aging effects qualitatively similar both to experiments and to simulations of low-dimensional models. This suggests that the Sherrington-Kirkpatrick model as well as other mean-field finite connectivity lattices can be used to study these effects analytically.

Using a reduced Lanczos scheme, we evaluate the ground-state energy and all two-particle correlation functions of a one-dimensional Heisenberg spin-1/2 model with antiferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions. The effects of anisotropy are also considered.

We have cooled a high-purity specimen of copper to a lattice temperature of 7±1 μK in a field of 6.8 mT. We have followed the time evolution of the lattice temperature as the specimen subsequently warms in order to investigate to lower temperatures the apparent anomaly in the spin-lattice relaxation time recently observed in this metal. We find the behavior of the lattice temperature to be entirely consistent with the conventional Korringa relation.

The magnetic and electrical properties of the layered misfit compounds (LX${)}_{1.15}$Nb${\mathit{X}}_2$ have been studied for the rare earths L=Ce, Sm, Gd, and Yb and the chalcogens X=S and Se, on single crystals grown by vapor transport. The compounds consist of double layers of LX, which are 111 rocksalt planes, inserted between layers of Nb${\mathit{X}}_2$. In spite of the fact that the LX rocksalt compounds SmS, SmSe, and YbS contain divalent lanthanides at atmospheric pressure, in the misfit compounds the lanthanides are trivalent. This may be due to a preference for a 1 electron per rare earth charge transfer to the Nb${\mathit{X}}_2$ layer.

Weak-field-induced magnetization M and temporal relaxation of its remanence μ are investigated in ${\mathrm{Fe}}_{0.47}$${\mathrm{Zn}}_{0.53}$${\mathrm{F}}_2$ by using Faraday rotation and superconducting quantum interference device (SQUID) magnetometry. Stretched logarithmic decay as predicted by Nattermann and Vilfan is confirmed at T=2.8 K for freezing fields 3≤B≤4.9 T. At very low fields, 0.001≤B≤1.5 T, virtually constant remanence, μ∼44 A/m, is observed. It is due to immobile domains, which form via piezomagnetic coupling to built-in random stress fields.

Experiments over a long range of frequencies reveal that the critical dynamics of random-field Ising models is dominated by activated hopping but the relevant exponent is quite different from the theoretical expectations based on the zero-temperature fixed point. We show that a careful application of dynamic scaling, and zero-temperature fixed point leads to an ever present ‘‘correction to scaling,’’ which causes the measured exponent to differ strongly from the expected value.

Partial substitution of ${\mathrm{Ca}}^{2+}$ at the ${\mathrm{Y}}^{3+}$ site in the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ system shows several interesting features, namely (1) creation of oxygen vacancies without any change in the lattice parameters, (2) no change in the effective copper valence, and (3) decrease in the critical temperature ${\mathit{T}}_{\mathit{c}}$ with substitution, accompanied by a decrease in the normal-state conductivity. A host of measurements such as low-temperature resistivity, magnetic susceptibility, x-ray diffraction, oxygen stoichiometry, and the effective copper valence, reported here, suggest that ${\mathit{T}}_{\mathit{c}}$ depression with Ca substitution is unlikely to be due to an overdoping effect. An alternate explanation, which takes stock of the above observations, and involves the creation of disorder in ${\mathrm{CuO}}_2$ planes, is invoked.