Flux Creep Picture Research Articles

Irreversibility lines in Cu-Nb in-situ composites

The irreversibility lines in Cu-Nb in-situ formed composites have been investigated by DC magnetization measurements. Extended reversible regions are clearly identified in the H-T phase plane. The melting theory of the vortex lattice is found to describe the observed irreversibility lines well for all the three samples, whereas the flux-creep picture is also applicable for the thinnest one, due possibly to the suppression of the irreversibility line to lower temperatures at which pinning and flux creep are more important. The suppression of the irreversibility line with the reduced rolling thickness of the sample has also been discussed within the contexts of several theoretical models.

Low-field diamagnetic response of granular superconductors at finite temperatures.

We study the low-field diamagnetic response of granular superconductors at finite temperatures by means of a simple two-dimensional Josephson-junction array. The temperature effects are taken into account by inserting white-noise current sources in parallel to the resistively shunted junction circuit models of the Josephson junctions of the network. By this analysis we argue that a simplified one-dimensional description of the equivalent circuit, proposed by the authors for cylindrical granular superconductors, is still valid even in the presence of thermally activated flux jumps. A flux-creep picture for intergranular flux motion follows.

Current-density dependence of the vortex pinning energy in YBaCuO

Measurements of the electrical resistivity, using pulsed currents of various magnitudes, in the mixed state of YBaCuO microbridges are reported. The onset of resistance follows a thermally activated behaviour of flux line motion. The dependence of the pinning potential barrier U on the current density j was investigated, revealing two distinctly different regions. At moderate current densities j < 10 4 A/cm 2, no change of U upon current density was observed. We associate this behaviour with thermally assisted flux flow. At high current densities, U declines logarithmically, in accordance with the flux creep picture and a particular pinning potential well proposed by Zeldov et al.

Thin and ultra-thin epitaxial films of YBa 2Cu 3O 7−δ deposited on LiNbO 3 substrates by pulsed excimer laser ablation

We present here detailed transport, magnetic, structural and morphological studies on thin and ultra-thin films of superconducting YBa 2Cu 3O 7−δ (1-2-3) deposited on single crystalline substrates of LiNbO 3. The films were deposited using the technique of excimer laser ablation and have been found to have their c-axis oriented perpendicular to the plane of the film. We report results for films of thickness 500 Å, 1000 Å, 1600 Å and 6000 Å. Superconducting transition temperatures ( T c) have been studied by DC-resistivity, AC- and DC-magnetization. Critical current densities ( J c) from transport measurements are found to be in excess of 10 5 A/cm 2 at 77 K. The temperature dependence of J c is also examined and the flux pinning energy is estimated within the flux creep picture. High field hysteresis loop studies are in agreement with the above results and yield meaningful indications of critical fields H c and irreversibility fields H r. A study of the evolution of film morphology with increase of film thickness is also reported and it reflects on the growth mechanism of 1-2-3 on LiNbO 3.

Size effect in the vortex-glass transition in submicron YBa2Cu3Oy strips: Evidence for softening of vortex matter.

The I-V curves of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$ strips, with widths of 0.54 to 5.6 \ensuremath{\mu}m, were measured in magnetic fields. All the strips showed critical scaling behavior, and a clear decrease in the vortex-glass (VG) transition temperature with decreasing width was observed. This finding proves that the interaction range of vortices is relevant in characterizing the phase transition, and that softening of the vortex-matter elasticity occurs upon approaching this scale. Moreover, the results are not easily explained by the flux-creep picture and give strong support to the VG picture.

Flux pinning and irreversibility in YBa2Cu3O7 superconducting crystal.

Extensive ac-magnetic-permeability studies have been carried out on the irreversible behavior and flux dynamics in superconducting ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ crystals in external magnetic fields up to 7.5 T. From the analysis the irreversibility line ${\mathit{B}}^{\mathrm{*}}$(${\mathit{T}}^{\mathrm{*}}$) is found to scale as (1-${\mathit{T}}^{\mathrm{*}}$/${\mathit{T}}_{\mathit{c}}$${)}^{\mathit{n}}$, where n\ensuremath{\simeq}1.5 for fields higher than 1.5 T, and n\ensuremath{\simeq}1 for lower fields. The exponents are sensitive to the choice of ${\mathit{T}}_{\mathit{c}}$. The frequency dependence of the irreversibility temperature is carefully studied in the low-frequency region from 10 to ${10}^{5}$ Hz and is found to be logarithmic in the whole frequency range. The small logarithmic slope increases with increasing external field. The analysis is done within a flux-creep picture using a single-relaxation-time form for the complex permeability, and a vortex-glass model. The peak in the imaginary part of the permeability in different dc fields is successfully fitted to the predictions of the flux-creep model. A dynamical screening length is needed to account for the observed frequency dependence of the irreversibility temperature.

Magnetic relaxation effects in PbMo6S8 superconducting single crystals: A comparison with high-Tc superconductors

Abstract Magnetic measurements performed on PbMo6S8 single crystals show that this classic isotropic superconductor (Tc = 14.5 K) presents surprisingly similar irreversibility and relaxation properties as those generally attributed to layered high-Tc superconductors. An irreversibility line H ∗ = H ∗ 0 (1-t) m with m∼ 3 2 is observed for H ⪅ 1 kOe. From the magnetic relaxation rates measured below this irreversibility line, activation energies U0 around 15 meV are derived within the flux creep picture. The similarity with the high-Tc oxides is explained by the very short coherence length ξ ≈ 25 A of PbMo6S8 which also makes this compound sensitive to disorder and local defects.

Inter- and intragranular properties of Bi2CaSr2Cu2O2 superconductors

The granular behaviour of sintered bulk Bi2CaSr2Cu2Oy superconductor is investigated by resistivity and χac measurements. The observed temperature and magnetic field dependence is discussed within the framework of a granular model. The frequency dependence of the intragranular losses leads to a flux flow or flux creep picture.

Glassy behavior of high-Tc superconductors

This paper deals with the question of flux creep or glassy behavior in high-Tc superconducting single crystals. It is shown that the flux creep picture is merely a phenomenological approach to the glassy behavior for relatively short times and low temperatures. Glassy effects are predicted for temperatures between 70% and 95% Tc and magnetic fields in the range of 0.03 T to 0.2 T. The glass concept can be understood as a generalization of the traditional flux creep picture. A hierarchy of energy barriers dominates the physical behavior. An important technical aspect is the influence of the glassiness on critical currents.

What is the origin of “glassy” behavior in the high-T c oxides?

The anomalous low-field magnetic properties of the high-T c oxide crystals have been interpreted alternatively in terms of a superconductive glass state or, more recently, giant flux creep. The glass picture as proposed assumes the occurrence of intra granular Josephson junctions on a length scale small compared to the penetration depth, and has been used to explain the properties of both single-crystal and ceramic samples. The flux creep picture deals only with pinning forces and ignores weak links, and thus far has been applied only to single crystals. We show here that neither of these models adequately describes the behavior of ceramic materials. A physically accurate picture of such materials must take into account inter granular Josephson junctions and resultant grain decoupling at low magnetic fields.

Absence of current direction dependence of the resistive state of high temperature superconductors in magnetic fields

Resistive transition of Bi 2 Sr 2 CaCu 2 O 8+ y film in magnetic fields is studied in the light of the currently proposed flux creep model. Measurements with precise angular alignment have clarified that the resistive transition does not depend on the relative direction between magnetic field and transport current. This calls for a significant revision of the Lorentz-force-driven flux creep picture. A fluctuation induced phase-slip phenomenon as an alternative model is also discussed.