The Kosterlitz-Thouless (KT) scaling procedure for the two-dimensional planar spin model is generalized to include an x-axis-applied current density I. Scaling equations for vortex coupling ${\mathit{K}}_{\mathit{l}}$ and vortex pair fugacity ${\mathit{y}}_{\mathit{l}}$ at a general minimum scale a≡${\mathit{e}}^{\mathit{l}}$ are derived, with current density acting as a y-axis ‘‘topological electric field’’ ${\mathit{E}}_{\mathit{l}}$ on the ±1 vortex ‘‘topological charges.’’ A vortex-unbinding onset scale l=${\mathit{l}}_{\mathit{c}}$ is defined by ${\mathit{E}}_{\mathit{l}\mathit{c}}$=${\mathit{K}}_{\mathit{l}\mathit{c}}$, where the current-driven repulsion of the ±1 vortex pairs begins to exceed their attraction. The nonlinear resistance R(T¯,I¯)=2π${\mathit{R}}_{\mathit{l}\mathit{c}}$ is related to the finite-scale phase-slip resistance ${\mathit{R}}_{\mathit{l}}$ that has a minimum at l=${\mathit{l}}_{\mathit{c}}$. Above transition, the zero-current resistance R(T¯,I¯=0) shows KT-like exponential inverse square-root temperature dependence, and is a universal function of dimensionless temperature T¯. The current-voltage exponent α(T¯,I¯) curves (where V∼${\mathit{I}}^{1+\mathrm{\alpha }}$) are universal in T¯ and I¯≡ħI/(2${\mathit{ek}}_{\mathit{B}}$T). Below transition, the α curves are weakly dependent on I¯, with α(T¯,I¯) close to π${\mathit{K}}_{\mathrm{\infty }}$(T¯). Above transition, non-Ohmic behavior α(T¯,I¯)≠0 is predicted for strong current.

Elastic energy-loss experiments were made on a ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ sample, in order to investigate the O mobility and reordering in the ${\mathrm{CuO}}_{\mathit{x}}$ planes at x close to zero. The height of a peak with an activation energy of 0.11 eV, previously attributed to jumps of isolated oxygens (Snoek effect), has been shown to decrease both after severe degassing treatments in vacuum and after several months of room-temperature aging. The discussion is focused on the apparent paradox that the extremely high mobility of the isolated oxygens seems to be in contrast both with the slow reordering kinetics and with the relatively high fraction of isolated oxygens estimated from the height of the dissipation peak below 100 K. An explanation is proposed to solve the above inconsistencies, based on the Coulomb repulsion between the isolated O ions in the semiconducting environment. It is also estimated that the binding energy of an O atom to a chain should be about 0.25 eV in the very diluted tetragonal phase. A brief discussion is made of an alternative interpretation in terms of polaron hopping of the same 0.11-eV relaxation process, recently observed also by NQR measurements.

The penetration and exit of magnetic flux in thin superconductors in a perpendicular applied field is investigated in detail. Flux-density pictures and profiles are obtained by magneto-optics; magnetization curves are measured by torque magnetometry; theoretical space- and time-dependent flux-density and current-density profiles are calculated from Maxwell’s equations in a planar approximation assuming a highly nonlinear current-voltage law E∼(J/${\mathit{J}}_{\mathit{c}}$${)}^{\mathit{n}}$ (n≫1, E=electric field, J=sheet current) with a critical sheet current ${\mathit{J}}_{\mathit{c}}$(B,r) in general depending on the position and on the perpendicular flux density B. Our experiments and calculations show that for inhomogeneous pinning the additional nontrivial condition ${\mathit{J}}_{\mathit{c}}$=∞ for B=0 is appropriate. Our specimens are high-${\mathit{T}}_{\mathit{c}}$ superconductors in the form of platelets, strips, or rings. In two platelets, an inhomogeneous ${\mathit{J}}_{\mathit{c}}$ was produced by heavy-ion irradiation of the edge zone or by thinning down the central part by sputtering. In all cases good qualitative agreement is found between the experimental and theoretical results. In particular, our time-dependent theory reproduces the recently derived static Bean-model profiles in perpendicular geometry, which we also confirm experimentally; field and current profiles in the ring are as predicted for a current-carrying strip in perpendicular field; in the platelet with enhanced edge pinning, when flux starts to leak into the central weak pinning zone the flux lines are driven immediately to the sample center and pile up there; for weaker inhomogeneity of ${\mathit{J}}_{\mathit{c}}$(r), when the flux front arrives from the edges at the central weak-pinning zone the flux lines jump to an intermediate position from where they fill the central zone gradually. Our experiments also confirm the predicted ‘‘uphill motion’’ of flux lines against the flux-density gradient and the occurrence of overcritical current densities in the flux-free regions.

The magnetic-resonance modes for a three-sublattice-coupled paramagnetic–weak-ferromagnetic system are derived in the molecular-field approximation. The model is proposed to describe the microwave-absorption experiments in the Gd cuprates below the ordering temperature of the Cu magnetic moments. Two of the sublattices are associated with the Cu moments and the third describes the ${\mathrm{Gd}}^{3+}$ paramagnetic lattice. An isotropic antiferromagnetic interaction is considered to couple the Cu moments with an antisymmetric exchange being responsible for the weak ferromagnetism. Two separate components are considered for the magnetic anisotropy of the Cu moments: a strong out-of-plane term (coincident with the ${\mathrm{CuO}}_2$ planes), and a smaller in-plane anisotropy. The Cu and Gd magnetic moments are assumed to interact through an isotropic ferromagnetic exchange interaction. Static and dynamic corrections to the paramagnetic and weak-ferromagnetic modes are described. The former are due to internal fields originated in the Cu-Gd interaction. The dynamic corrections are important for some orientations of the applied magnetic field for which the modes become degenerate. Anticrossing and strong mode mixing is observed in this case. The predictions of the model are compared with the available experimental data. It is shown that both static and dynamic corrections are needed in order to account for the anisotropy of the observed microwave absorptions. A consistent description of the complex magnetic resonance behavior of the Gd cuprates is obtained within the proposed model.

We study by means of numerical methods the mechanical properties of model nanostructures. We use molecular dynamics to obtain the zero-temperature size dependences of the Young’s modulus and the Poisson’s ratio for a system of particles interacting with the Stillinger-Weber potential for silicon. Rather simple but interesting dependences on the structure’s sizes and shapes are observed.

It is demonstrated that the Sogami potential, which contains both repulsive and attractive components, can account for the experimental data on photothermal compression of colloidal crystals much more satisfactorily than the purely repulsive DLVO potential. The Sogami theory gives a natural explanation for the non-space-filling nature of the crystals with an experimentally realistic value for the effective surface charge on the particles. A fundamental paradox of the DLVO theory is that in the case of thermal compression, when the whole dispersion is heated, it predicts no contraction at all, in contradiction to the experimental results.

We report polarized micro-Raman spectra of ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C at room temperature. The sample used in the Raman experiment was well characterized by x-ray powder diffraction and ac magnetic susceptibility measurements (${\mathit{T}}_{\mathit{c}}$≊14.5 K). For ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C, there are only four Raman phonons allowed by symmetry and we have identified them at 198 ${\mathrm{cm}}^{\mathrm{-}1}$ (Ni-${\mathit{B}}_{1\mathit{g}}$), 282 ${\mathrm{cm}}^{\mathrm{-}1}$ (Ni-${\mathit{E}}_{\mathit{g}}$), 470 ${\mathrm{cm}}^{\mathrm{-}1}$ (B-${\mathit{E}}_{\mathit{g}}$), and 832 ${\mathrm{cm}}^{\mathrm{-}1}$ (B-${\mathit{A}}_{1\mathit{g}}$).

The induced dipole-dipole interaction can be understood in terms of recurrent elastic scattering of waves. We implement this notion into transport theory of scalar waves in inhomogeneous dielectric media and calculate rigorously the pivotal cooperative effect of pairs of polarizable dipoles.

We have fabricated pillarlike microstructures in Co/Cu multilayers and measured the giant magnetoresistance effect with the current perpendicular to the multilayer plane from 4 K up to 300 K. We analyze the temperature dependence of the magnetoresistance, using a model of Fert and Campbell, originally proposed for magnetic alloys. We find that the temperature-dependent scattering rates are strongly spin dependent, which explains the relatively weak decrease with temperature of the giant magnetoresistance of Co/Cu.

The inductive voltage of a Co-based water-quenched amorphous wire has been measured as a function of applied axial dc field, tensile stress, and twisting, when an ac current flows through it. The results show that the giant ac magnetoresistance recently reported for nonmagnetostrictive amorphous wires is actually a giant magnetoimpedance (GMZ). The origin of GMZ in such wires is their large circular susceptibility at high frequencies, which is related to their small magnetostriction and special domain structure.

Recently it has become possible to fabricate long-term stable point contacts with a constriction as small as a single atom. Here we present experimental investigations on one-atom point contacts in the superconducting state. An applied pressure or tensile force on this atom results in large effects in the superconducting properties. Furthermore, we show that one-atom point contacts can be broken and reestablished in a perfectly reproducible manner.

We update our previous Hartree calculation of magnetic fields induced by a point charge above a plane of anyons, extending the calculation to larger values of the background dielectric constant, which are more appropriate in the context of the muon-spin-relaxation experiments in high-temperature superconductors. We find that the resulting change in magnetic-field estimates is modest, and does not substantially alter the conclusions we have previously reached.

Current-voltage characteristics of intrinsic Josephson junctions of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ were investigated. Although the simple model of stacking junctions indicates a low damping limit, nonzero return current appears in current-biased measurement of the current-voltage characteristics. We measured with biasing voltage and found negative differential resistance. We present a consistent interpretation that the non- equilibrium effect due to very thin electrodes can lead to negative differential resistance and nonzero return current even when the junction is in a low damping limit. A clear gaplike feature was also observed when the field of several tesla was applied.

Measurements of the dielectric permittivity are reported for a solid solution of antiferroelectric (betaine phosphate${)}_{0.15}$ and ferroelectric (betaine phosphite${)}_{0.85}$ at frequencies 20 Hz <ν <1 MHz. The freezing phenomena in ${\mathrm{BP}}_{0.15}$${\mathrm{BPI}}_{0.85}$ reveal the characteristics of a transition into a dipolar glass state. The activation energy was found to be ${\mathit{E}}_{\mathit{b}}$=518 K and the external bias field lowers this value up to 488 K.

We present x-ray, susceptibility, and magnetization (up to 18 T) measurements in the compound ${\mathrm{MgTiOBO}}_3$, which forms in the structure known as warwickite. This structure is characterized by the presence of ribbons along the c axis where the metallic ions are located. This system is magnetic since the Ti ions are in the valence state ${\mathrm{Ti}}^{3+}$ with the electronic configuration 3${\mathit{d}}^1$. Its susceptibility has a temperature dependence, at low temperatures, given by χ(T) ∝ ${\mathit{T}}^{\mathrm{-}\mathrm{\alpha }}$, with the exponent α≊0.83. The low-temperature behavior of the magnetization, as a function of field, is described by m ∝ ${\mathit{H}}^{\gamma }$ with γ≊1-α. These results, together with the x-ray measurements, allow us to identify this material as belonging to a family of inorganic, one-dimensional, disordered magnetic systems.

The magnetic dichroism in the ${\mathit{L}}_{2,3}$ emission spectra of Fe, Co, and Ni, following excitation with circularly polarized x rays, is found to exhibit a dramatic dependence on excitation energy. The observed changes in sign and intensity of the dichroism spectra are explained by a simple model and are shown to negate the use of x-ray fluorescence yield detection for quantitative dichroism absorption measurements. Multielectron effects are found to be minimized by use of monochromatic threshold excitation.

We calculate the dc Josephson current through an unconventional superconducting point contact with various pairing symmetries including s-wave, d-wave, and s+id states. We find that the phase dependence of the dc Josephson current through the contact at low temperatures exhibits a dramatical difference between the conventional s-wave and unconventional d-wave or s+id state. In particular, for the s-wave, the Josephson current is always finite for any value of the phase, while for the d-wave and s+id states, it shows singularity in certain phase values.

It was shown recently that a sizable areal density of midgap states exists on a {110} surface of a ${\mathit{d}}_{\mathit{x}\mathit{a}}^2$-${\mathit{x}}_{\mathit{b}}^2$-wave superconductor [C.-R. Hu, Phys. Rev. Lett. 72, 1526 (1994)]. Here we study how these midgap states are affected if (i) the surface is not exactly {110}; (ii) the crystalline structure is slightly orthorhombic; (iii) and (iv): the order parameter is not a pure d wave, but is s+${\mathit{id}}_{\mathit{x}\mathit{a}}^2$-${\mathit{x}}_{\mathit{b}}^2$ and ${\mathit{d}}_{\mathit{x}\mathit{a}}^2$-${\mathit{x}}_{\mathit{b}}^2$+${\mathit{id}}_{\mathit{x}\mathit{ax}\mathit{b}}$, respectively. We find that for cases (i) and (ii), the midgap surface states still exist but with reduced areal density. For case (iii), all surface-state energies are found to shift away from midgap to ±‖${\mathrm{\Delta }}_{\mathit{s}}$‖. For case (iv), the surface states exist on both {110} and {100} surfaces (and on surfaces of all other orientations) but with a k-dependent energy shift from midgap.

Magnetic and transport phenomena in high-temperature superconductors due to magnetic flux relaxation and transport over the surface barrier are investigated. Vortex dynamics controlled by the penetration both of pancake vortices and vortex lines is discussed. The penetration field ${\mathit{H}}_{\mathit{p}}$ for pancakes decays exponentially with temperature. The size of the magnetization loop is determined by the decay of ${\mathit{H}}_{\mathit{p}}$ during the process of relaxation, but its shape remains unchanged. The irreversibility line associated with the pancake penetration is given by ${\mathit{H}}_{\mathrm{irr}}$ ∝exp(-2T/${\mathit{T}}_0$) and may lie both above and below the melting line.

We have measured and analyzed both the static and dynamic contribution to the anomalous nonmonotonic dependence of the irreversible magnetization, M(B), for underdoped ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{y}}$ and overdoped (${\mathrm{Bi}}_{1.6}$${\mathrm{Pb}}_{0.4}$)${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{y}}$ single crystals. We extracted the ‘‘unrelaxed’’ magnetization ${\mathit{M}}_{\mathit{c}}$(B) and traced the time evolution of M(B,t) from short to long time scales using combined magnetization and flux-creep measurements. Despite their large differences in anisotropy, pinning force, and flux-creep rate s(B)=dln M/dln t, both samples showed the anomaly over a wide range of T, the anomaly being most pronounced in the ‘‘unrelaxed’’ state. This suggests that the primary cause of the anomaly lies in the flux-pinning defect structure of the material. However, M(B,E) also essentially depends on the induced electric field E in the sample, so the shape of the M(B) curve can strongly change with time due to the relaxation of E(t) and the nonmonotonic dependence of s(B).