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Using an array of {mu}m-sized Hall probes to measure local magnetic-field gradients, we have investigated magnetization hysteresis loops in single-crystal Tl{sub 2}Ba{sub 2}CuO{sub 6}. Our technique allows us to delineate bulk and surface contributions to the field profiles across the sample. In particular, we are able to identify the component in the field gradient responsible for the {open_quotes}second peak{close_quotes} in the magnetization loop. We find that this particular component decays with a field-independent relaxation rate, suggesting a static origin of the second peak. {copyright} {ital 1999} {ital The American Physical Society}

We use a combination of relaxation measurements and magnetic hysteresis loops at different field ramp rates to explore the dynamical behavior of the second peak in the magnetization of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} crystals. We find that the second peak is {ital absent} in the short-time limit. It evolves at intermediate time scales due to a different decay rate of the Bean profile at fields above and below the second peak. At long time scales, when the Bean profile for fields below the second peak has decayed, the size of the second peak saturates. Finally, while the Bean profile above the second peak field slowly decays, the second peak decreases and vanishes. {copyright} {ital 1998} {ital The American Physical Society}

Micrometer-sized Hall probes are used to measure the low-temperature magnetic properties of ${\mathrm{HgBa}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ single crystals with and without columnar defects. Introduction of columnar defects by heavy-ion irradiation suppresses the quasiexponential temperature dependence of the critical current, but only when the density of flux lines is less than the density of defects. The current-dependent creep activation energy $U(J)$ is found to be best described by the power law: ${U(J)=U}_{0}[{(J}_{c}{/J)}^{\ensuremath{\mu}}\ensuremath{-}1].$ At high current densities J, and low fields, there is a crossover to a quantum vortex creep process with a temperature-independent magnetic relaxation rate. On the low-J side of this crossover in the unirradiated sample, $\ensuremath{\mu}g~1,$ indicating that the thermally activated vortex creep process is consistent with collective creep. When the flux-line density exceeds the columnar defect density in the irradiated sample, the vortex creep process is characterized by $\ensuremath{\mu}\ensuremath{\sim}1.$ For flux-line densities less than the defect density, an anomalously large exponent is found $(\ensuremath{\mu}g~2),$ which is inconsistent with vortex transport occurring either by the nucleation and expansion of half-loop excitations or by variable-range hopping of vortex lines as described by the Bose-glass theory.

We study the dynamics of superconducting vortices in Nb rings as the system is continuously driven to the depinning threshold by the slow ramping of an external magnetic field. Miniature Hall probes simultaneously detect local and global flux changes arising from vortex motion. With decreasing temperature, the dynamics evolve continuously from smooth flow to several types of avalanche behavior. In particular, we observe a crossover from broad to narrow size distributions of avalanche events which correspond to global decreases in the flux density gradient rather than local redistributions of flux. We show how this evolution can arise from the magnetothermal instability of the Bean state. {copyright} {ital 1997} {ital The American Physical Society}

We investigate the effect of columnar defects on the low-temperature magnetic response of a highly anisotropic superconductor. Using a {ital local} magnetic field sensor, the vortex creep rate in single crystals of Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}} containing columnar defects is found to be a highly nonmonotonic function of vortex density. The creep rate is largest when the vortex density is less than the defect density. A strong suppression of the creep rate is observed near the matching field, {ital B}{sub {phi}}, corresponding to equal numbers of vortices and defects. We tentatively associate this finding with the Mott-insulator line phase proposed by Nelson and Vinokur [Phys. Rev. B {bold 48}, 13060 (1993)]. For higher vortex densities, an additional minimum is found near 3{ital B}{sub {phi}}, emphasizing the importance of vortex interactions. {copyright} {ital 1996 The American Physical Society.}

We have investigated the normal-state resistance and the excess low-frequency (1/{ital f}) noise in 45{degree} tilt [001] grain-boundary junctions in YBa{sub 2}Cu{sub 3}O{sub 7} thin films. A characteristic temperature dependence of the resistance fluctuations and a linear temperature dependence of the grain-boundary resistance (with a negative temperature coefficient) emerge as common features. These observations are quantitatively compared with a model of many, parallel conduction paths across the boundary, taking into account both temperature-independent and thermally activated transport processes.

We report a systematic study of the resistivity and magnetic susceptibility of pure V_2O_3, the original Mott-Hubbard system at half filling, for pressures 0≤P≤25 kbar and temperatures 0.35≤T≤300 K. We also study (V_(0.99)Ti_(0.01)_2O_3 under pressure in order to elucidate the role of disorder on a metal-insulator transition in the highly correlated limit. Despite the low level of doping, we find that the two systems are very different. We observe a conventional collapsing of the Mott-Hubbard gap only for stoichiometric V_2_O3; the Ti disorder stabilizes the long-range antiferromagnetic order and a magnetic Slater gap. Moreover, we discover different P-T phase diagrams for the two systems, with a decoupling of the charge and spin degrees of freedom at the approach to the T=0, pressure-driven metal-insulator transition in pure V_2O_3.

We have investigated the room-temperature oxygen depletion in the near-surface region of YBa{sub 2}Cu{sub 3}O{sub {ital x}} samples kept under high vacuum. Results obtained from secondary-ion-mass-spectroscopy and four-probe-conductivity measurements are presented. The depletion process is limited by oxygen out diffusion in the samples with diffusion coefficients of 10{sup {minus}18}--10{sup {minus}19} cm{sup 2}/s. The reverse oxygen in-diffusion process occurs in oxygen atmosphere. The oxygen out-diffusion process can be accelerated by direct ultraviolet light (UV) irradiation of the samples in vacuum. The in-diffusion process is accelerated if UV light is used in oxygen atmosphere to produce ozone and atomic oxygen.

Magnetic hysteresis loops in single crystals of Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}} for H{parallel}c have been investigated in order to study the peak effect or {open_quotes}fishtail{close_quotes} feature. The peak effect was observed in a series of oxygen-overdoped crystals covering a wide range in T{sub c}. The peak fields H{sub pk} show scaling behavior with reduced temperature T/T{sub c}, and the normalized slope (T{sub c}/H{sub pk}) (dH{sub pk}/dT) is found to assume a nearly constant value for all investigated overdoped Tl-2201 crystals. This suggests that the underlying mechanism for the origin of the peak effect is independent of oxygen defect concentration. {copyright} {ital 1999} {ital The American Physical Society}

We map out the phase boundary separating the Bose-glass and vortex-liquid phases in an irradiated twin-free YBa_2Cu_3O_(7-δ) single crystal. We take the phase boundary to be the temperature T_g and magnetic field H at which the crystal begins to screen a small ac magnetic field, h_(ac). There is a significant change in slope dT_g/dH of the phase boundary at the matching field B_Φ (≃0.5 T) indicating that interstitial vortices significantly weaken pinning in the Bose-glass state. There is also a pronounced peak in the slope dT_g/dH just below B_Φ at higher h_(ac). Both features disappear when the field is tilted away from the columns.

We present a model supported by simulation to explain the effect of temperature on the conduction threshold in disordered systems. Arrays with randomly distributed local thresholds for conduction occur in systems ranging from superconductors to metal nanocrystal arrays. Thermal fluctuations provide the energy to overcome some of the local thresholds, effectively erasing them as far as the global conduction threshold for the array is concerned. We augment this thermal energy reasoning with percolation theory to predict the temperature at which the global threshold reaches zero. We also study the effect of capacitive nearest-neighbor interactions on the effective charging energy. Finally, we present results from Monte Carlo simulations that find the lowest-cost path across an array as a function of temperature. The main result of the paper is the linear decrease of conduction threshold with increasing temperature: $V_t(T) = V_t(0) (1 - 4.8 k_BT P(0)/ p_c) $, where $1/P(0)$ is an effective charging energy that depends on the particle radius and interparticle distance, and $p_c$ is the percolation threshold of the underlying lattice. The predictions of this theory compare well to experiments in one- and two-dimensional systems., 14 pages, 10 figures, submitted to PRB

We map out the phase boundary separating the vortex solid and liquid phases in YBa_2Cu_3O_(7-δ) (YBCO) single crystals with irradiation-induced columnar defects. These randomly distributed, extended defects are expected to localize vortices into a "Bose glass" phase. The transition from the vortex liquid into the Bose glass is predicted to exhibit two fundamental signatures: a vanishing of the linear resistivity and, concomitantly, a screening of de magnetic fields applied perpendicular to the defect axis, the transverse Meissner effect. We have investigated both aspects by systematic measurements on two YBCO single crystals with different defect densities (matching fields of 0.25 and 0.5 T), as well as on an unirradiated control sample. The melting line determined by the temperature, T_m, of vanishing resistance undergoes a 30% decrease in slope as the magnetic field is ramped through the matching field. This is evidence that interstitial vortices are pinned much more weakly than originally thought. If we associate the melting temperature with the Bose glass transition temperature, we obtain static critical exponents of ν⊥=1.7±0.2 and ν⊥=1.9±0.1 for the crystals with matching fields of 0.25 and 0.5 T, respectively. Simultaneously, we use a ten-element, linear array of microfabricated Hall probe magnetometers to observe directly the flux screening associated with the transverse Meissner state. We find the temperature above which the Meissner state breaks down, T_s, to decrease linearly as the magnetic field applied perpendicular to the columnar defect axis increases. This linear trend, found in both irradiated crystals to cover a range of at least 40 K in T_s, is closely in line with the current theoretical expectation ν⊥≃1. However, already for angles as small as one degree, T_s(H⊥) falls below T_m(H⊥) by more than 10 K. Thus, between Ts(H⊥) and T_m(H⊥) we observe a large regime characterized by zero resistivity in the absence of a transverse Meissner effect: vortices remain effectively localized even when rotated off the columnar defects.

We have measured the temperature-dependent resistivities of a series of samples of La_(2-x)Sr_xCuO_4 with 0.02≤x≤0.1 over the temperature range 0.05 K≤T≤300 K. We find the onset of superconductivity as x is increased to be correlated with a substantial drop in the magnitude of the normal-state resistivity. We observe no change, however, in the qualitative shape of the resistivity as the superconducting threshold is crossed. We also find that the low-temperature (T≤8.0 K) resistivities of the least concentrated samples can be described by variable range hopping, with a crossover between Coulomb gap and single-particle behavior occurring as x is increased.

The thickness dependence of superconductivity has been investigated in ultrathin Sn films near the threshold for normal-state conduction. An evolution from insulating to metallic superconducting behavior is observed with increasing thickness and decreasing sheet resistance. A striking feature of the data is the observation of quasireentrant superconductivity which may be a consequence of the existence of local rather than global superconducting order.

Superconductivity has been found in 0.5-..mu..m-thick films of PbTe doped with Tl which were grown on BaF/sub 2/ substrates by molecular-beam epitaxy. The observation that in the T = 0 limit the ratio of the parallel to perpendicular critical magnetic fields is 1.7 suggests that the superconductivity is a bulk property. The highest value of the critical temperature was 0.6 K, which is about a factor of 2 lower than previously reported values for bulk material and polycrystalline films grown on mica substrates.