J.C. Soret

Magnetoresistance scaling in the layered cobaltate Ca 3 Co 4 O 9

We investigate the low-temperature magnetic-field dependences of both the resistivity and the magnetization in the misfit cobaltate

Bose-glass behaviour in Bi2Sr2Ca1−xYxCu2O8 crystals with columnar defects: experimental evidence for variable-range hopping

{\text{Ca}}_{3}{\text{Co}}_{4}{\text{O}}_{9}

Current density dependence of the activation energy in Tl-2212 thin films at weak magnetic fields

from 60 K down to 2 K. The measured negative magnetoresistance reveals a scaling behavior with the magnetization which demonstrates a spin dependent scattering mechanism. This scaling is also found to be consistent with a shadowed metalliclike conduction over the whole temperature range. By explaining the observed transport crossover, this result sheds a new light on the nature of the elementary excitations relevant to the transport.

Dimensional crossover vortex fluctuations in irreversibility line of high-quality Tl2Ba2CaCu2O8 thin film

Abstract We report on vortex transport in Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8 crystals irradiated at different doses of heavy ions. We show evidence of a flux-creep resistivity typical of a variable-range vortex hopping mechanism as predicted by Nelson and Vinokur (Phys. Rev. B. 48 1993 13060).

Bose-glass transition in irradiated Bi2Sr2Ca1-xYxCu2O8 crystals

Abstract The flux motion has been investigated in a Tl 2 Ba 2 CaCu 2 O 8 thin film by transport measurements with a weak applied magnetic field perpendicular to the ( a , b ) plane ranging from 20 to 300 mT. We have found a well defined line T ∗ (H) separating ohmic from non ohmic behaviour in the H-T ∗ diagram. This line is well described by 1 - t ∗ ≈ H 2 3 where t ∗ = T ∗ /T c and T c is the zero magnetic field transition temperature. Above T ∗ (H) , a thermally activated resistance is observed. Our experimental results clearly indicate that the activation energy is well approximated by U(T, H) ∼ [l-(T/T c ] 3 2 /H both below and above the T ∗ (H) line in the H-T ∗ diagram. The nonlinear current dependence of the activation energy U ( T , H , I ) is extracted from I – V characteristics, and it is found that U ( T , H , I )= U ( T , H ) ln( I 0 / I ). Satisfactory agreement is found between our results and recent models which take into account a periodic logarithmic-like pinning potential.

Superconducting transition in Tl-2 2 1 2 thin films and polycrystals: Effect of the dimensionality of the sample

Abstract The current-voltage (I–V) characteristics are investigated in a high quality Tl-2212 thin film in magnetic fields (up to 8 Tesla) applied perpendiculary to the layers. We find experimental support for the existence of a crossover from (3D) to (2D) vortex fluctuations occuring at magnetic field B cr . For B ≪ B cr , the scaling behaviour of the vortex glass transition is observed with critical exponents z ≈ 3 and ν ≈ 1.9, which are field independent. At sufficiently large fields B ≫ B cr , we show that the thereshold measurable ohmic resistance can be attributed to melting of a vortex lattice triggered by (2D) vortex fluctuations.

Bose-glass transition in Pb-substituted Bi-2212 single crystals containing parallel tilted columnar defects

Abstract Current-voltage characteristics are investigated in Bi2Sr2Ca1-xYxCu2O8 crystals irradiated parallel to the c-axis with 5.8 GeV Pb ions. Over a wide range of filling fractions 0.026 ≤ f ≤ 1, I–V isotherm curves near the superconducting transition are consistent with the Bose-glass scaling theory. The Bose glass line, derived from scaling analysis, is well described by the Lindemann criterion which accounts for the contribution of correlated disorder. Two characteristic temperatures t0≈0.72 (f≈0.8) and t1≈0.83 (f≈ 1 3 ), beyond which the vortex system is increasingly dominated by vortex-vortex interactions or thermal fluctuations, respectively, are identified.

Bose-Glass Behavior in Bi2Sr2Ca1-xYxCu2O8 Crystals with Columnar Defects: Experimental Evidence for Variable-Range Hopping

A former paper had shown that some Tl-2 2 1 2 thin films can be described as two-dimensional arrays of weakly coupled superconducting grains. We here report on a comparative study of the superconducting transition (R = 0) in a Tl-2 2 1 2 thin film and a Tl-2 2 1 2 polycrystal. We find in this way rather striking evidence for two kinds of voltage-current behaviours respectively which places emphasis on the dimensionality of samples. We show that both these dissipative behaviours can quantatively be explained in terms of phase-coherence transitions predicted in D-dimensional arrays of superconducting grains. In particular, universal scaling properties of the three-dimensional phase-coherence transition and Kosterlitz-Thouless-like phase transition are found for the polycrystal and the film respectively.

Experimental evidence for a dimensional crossover of the vortex fluctuations in Tl2Ba2CaCu2O8 thin film

Abstract We have studied the effect of tilted columnar defects (CD’s) on vortex dynamics in Bi 1.8 Pb 0.2 Sr 2 CaCu 2 O 8+ δ . Single crystals were irradiated by high-energy heavy-ions either parallel or along the direction inclined at θ irr =45° relative to the crystals c -axis. The angular dependence of the resistance R ( ψ ) exhibits an usual directional effect, in the vortex solid state and for T ⩾40 K, when a magnetic field is aligned to the CD’s for both cases. In the case of inclined linear defects, an anomalous clear minimum centred along the perpendicular direction to the CD’s is observed. The melting transition for magnetic fields applied in the direction parallel to CD’s and over a wide range of the filling factor ( f H / B φ ) from f =0.026 to 0.9, is analysed by I – V scaling. Both samples under examination show a 3D Bose-glass (BG) transition with the same critical exponents ( z ′ =5.27±0.05 and ν ′ =1.04±0.06). These results supports the persistence of a BG-to-liquid transition at T BG ( B ) in the case of inclined defects. The R ( ψ ) curves measured at low temperatures in the case of inclined CD’s reveal two main features. first, the dip centred at the tracks at high-temperature shifts away from the CD’s toward the c -axis. Second, the persistence of the directional pinning at low temperatures, in contrast to previous experiments at low temperatures on Bi-2212 single crystals showing isotropic pinning.

A comparative study of the superconducting transition in TI-2212 polycrystas and granular thin films

We report on vortex transport in Bi2Sr2Ca1-xYxCu2O8 crystals irradiated at different doses of heavy ions. We show evidence of a flux-creep resistivity typical of a variable-range vortex hopping mechanism as predicted by Nelson and Vinokur.