1PO2-100 Hall Effect in a Hg(Re)-1223 Superconductor

J. Roa-Rojas 1,2, P. Pureur 3, M.T.D. Orlando 4,5, and E. Baggio-Saitovich 4,5. 1 Instituto de Física, Universidade Federal do Rio Grande do Sul, P.O. Box 15051, 91501-970 Porto Alegre, Brazil. 2 Escuela Colombiana de Ingeniería, Bogotá DC, Colombia. 3 Universidade Federal do Rio Grande do Sul, P.O. Box 15051, 91501-970 Porto Alegre, Brazil. 4 Departamento de Física, Universidade Federal do Espírito Santo, 29060-900 Vitória, Brazil. 5 Centro Brasileiro de Pesquisas Físicas, Rua Dr. Xavier Sigaud 150, 22290-180 Rio de Janeiro, Brazil.

Presenting Author: J. Roa-Rojas

We measure the Hall and longitudinal resistivities as functions of the temperature of a Hg(Re)BaCaCuO polycrystalline sample, in fields H = 1, 2, 4, 5 T . In the normal phase, the Hall coefficient is positive and may be accurately fitted to RH =1/(aT + b). The Hall angle varies approximately as predicted by the Anderson's formula. When the temperature approaches the mean-field T_c from above, the Hall resistivity decreases abruptly in consequence of thermal fluctuations. Below T_c and in H = 1 and 2 T, the Hall response changes signal, goes through a minimum and becomes again positive before the zero resistance state is attained. In higher applied fields, the Hall resistivity remains positive in the mixed phase, though its qualitative shape, presenting a local minimum below T_c, is preserved. We ascribe the double sign reversal feature to combined effects of thermal fluctuations and vortex motion. Close to the zero resistance state, the Hall resistivity varies as a power law of the longitudinal resistivity, with a field-independent exponent. This scaling behavior may be interpreted either as evidence of a vortex-glass transition or a pinning-dependent flux-flow effect.

1PO2-101 Flux-Charge Analogy and Nonlinear Response Function in HTS and Two-Dimension Charge Systems

Yu Wang, Wei Wang, Handong Chen, Daole Yin, Kaixuan Chen, Ke Wu, Guo Lu, and Chuanyi Li, Department of Physics, Peking University, Beijing 100871, China

Presenting Author: D. Yin

The physics of flux lines in HT_c pinned by strongly correlated defects can be mapped onto charges in two dimensions (2D). Considering the viscous dissipation of moving vortices during hopping, we find a nonlinear response function. This function is compatible with so far suggested different model barriers U(J) and able to make a consistent description of the vortex system near transition. A comparison with the scaling behavior of the measured isothermal current-voltage curves with YBCO samples shows fair agreement. This nonlinear response function also shows an empirical fit with I~V behavior of some 2D charge systems.

1PO2-104 Possible Electrical Current-Driven Superconductor-Insulator Transition

C.A.M. dos Santos 1, Y. Kopelevich 2, S. Moehlecke 2, and A.J.S. Machado 1. 1 Departamento de Engenharia de Materiais – FAENQUIL, 12600-000 Lorena SP, Brazil. 2 Instituto de Física "Gleb Wataghin", Universidade Estadual de Campinas – UNICAMP, 13083-970 Campinas SP, Brazil.

Presenting Author: C.A.M. dos Santos

The occurrence of either superconducting or insulating zero-temperature state under variation of system parameters is one of the fundamental problems in the condensed matter physics. Thus, the superconductor-insulator transition (SIT) in two-dimensional superconductors tuned by applied magnetic field [1] has recently been the subject of intense research interest.

In this work we have studied the effect of applied electrical current on the resistive behavior of single phase Y_1-xPr_xBa₂Cu₃O_7-d polycrystalline samples with x close to the critical Pr concentration Xc » 0.57 above which the superconductivity vanishes. The obtained results suggest the occurrence of SIT driven by the electrical current, and provide evidence that the dynamics of Josephson intergranular vortices plays a crucial role in this phenomenon.

In particular, a crossing of current-voltage (I-V) isotherms at magnetic-field-dependent current Ic(H) is found, and the scaling behavior of nonlinear resistance R(I,T) = V(T)/I similar to that predicted in the scaling theory of SIT [1] is obtained.

[1] M.P.A. Fisher, Phys. Rev. Lett. 65, 923 (1990).

1PO2-105 Size Effects in the Critical Currents of Superconducting Films

Pablo A. Venegas, Denise F. de Mello, and Jose D. Reis, Jr., Departamento de Fisica, Universidade Estadual Paulista, CP 473, 17033-360, Bauru, S.P., Brazil

Presenting Author: P.A. Venegas

The critical currents for infinitely long film with finite cross section under a parallel magnetic field and perpendicular transport current are investigated. Using the London theory with boundary conditions appropriate for the present geometry, an expression for the Gibbs free energy is obtained. Different from previous calculations where a pre-determined configuration for vortex chains is supposed, here using Monte Carlo simulated annealing technique, a complete minimization of the free energy, including surface effects, is done. The film cross section is supposed to be of size less than or equal to the penetration depth. The system is analyzed for several values of current and field in two cases, when the field is increased at zero transport current and when the transport current is increased at constant field. The analysis shows a matching effect on critical currents and critical fields as found in experiments with finite size films.

1PO2-106 Flux Field Profiles and Magnetization Curves for Bent Thin Strips in Transverse Field

Leonardo R.E. Cabral and J. Albino Aguiar, Departamento de Fisica, Universidade Federal de Pernambuco, Recife 50670-901, Brasil

Presenting Author: J. Albino Aguiar

The understanding of the superconducting mixed state is a fundamental task, because of scientific and technological applications of superconducting materials. Although, the electromagnetic behavior of a type-II superconductor in the mixed state has been found to be a very intricated state, due to the vortex-vortex and vortex-defects interactions, and thermal fluctuations.

Phenomenological London and Ginzburg-Landau theories may be employed in order to study the mixed state, but these theories are not suitable to take into account the sample geometry, considering its actual dimensions. To mimic superconductor's electromagnetic behavior, one could use the critical state models. These models can explain the flux field profiles and hysteresis loops usually observed in superconducting samples. Recently it was shown that the sample geometry plays a fundamental role in the behavior of these field profiles and hysteresis loops. In this work, we calculate the flux field profiles and hysteresis loops of bent strips subjected to transverse applied fields and compare this result with those calculated for a flat strip. Our aim is to verify whether small strip distortions could give rise to significantly different measured magnetization curves.

1PO2-107 Superconducting Fluctuation Probed by c-axis Conductivity in YBa₂Cu₂O_7-d Single Crystal

Tomoyuki Naito 1, Hideo Iwasaki 1, Terukazu Nishizaki 2, Kenji Shibata 2, and Norio Kobayashi 2. 1 School of Materials Science, Japan Advances Institute Science and Technology, Hokuriku, Tatsunokuchi 923-1292, Ishikawa, Japan. 2 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.

Presenting Author: T. Naito

It is well known that two theories have been suggested for explaining the superconducting fluctuation of the high-T_c superconductors in a magnetic field: one is the lowest-Landau-level (LLL) theory, another one is the three dimensional(3D) XY theory. To examine the validity of both models, we measure the c-axis conductivity for an untwinned YBa₂Cu₂O_7-d single crystal in the magnetic fields up to 100 kOe in parallel to the c-axis. The experimental results for the magnetic fields of 10 kOe < H < 100 kOe are not scaled by the 3D XY prediction but well described by the LLL theory. In the LLL scaling, however, the data taken at 10 kOe slightly deviates from the universal scaling curve. These results indicate that the critical behavior of the untwinned YBa₂Cu₂O_7-d single crystal is described by the LLL theory above the magnetic fields of 10 kOe. We also point out the possibility that the crossover of the critical behavior from the LLL region to the 3D XY one occurs below 20 kOe.

1PO2-108 Testing a cosmological scenario using high T_c superconductors

Raz Carmi, Emil Polturak, and Gad Koren, Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel

Presenting Author: E. Polturak

As one cools a superconductor through T_c, one first forms disjoint superconducting regions, each one having a random value of the phase of the order parameter. As these regions coalesce, the sum of the phase differences around some loop made of many regions can exceed 2p , and a topological defect (a flux quantum) will be formed. This idea was originally proposed by Kibble and Zurek in the context of string creation in the early universe. Topological defects (strings) are predicted to appear during a quench of the universe into an ordered state (Big Bang). The implication is that a superconductor quench-cooled through T_c should generate spontaneous flux of random sign and magnitude. We describe an experimental search for this effect, using thin films of YBa₂Cu₃O_7-d . A high T_c - SQUID magnetometer is used to look for the spontaneously generated flux. In addition, we also looked for the effect in a single loop made of initially decoupled superconducting sections. To realize this system, the loop was made of 214 Josephson junctions connected in series. We see no spontaneous flux associated with this scenario in the films down to a level of 10^-3 of the predictions . However, we do see the effect in the Josephson junction loop. Possible reasons for this will be discussed.