1PO2-110 Irreversibility fields of low-T_c superconductors

D.N. Zheng 1, P.J. Wu 1, Y.M. Ni 1, A.M. Campbell 2, and Z.X. Zhao 1. 1 National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, P.R. China. 2 Interdisciplinary Research Centre in Superconductivity, University of Cambridge Madingley Road, Cambridge CB3 0HE, United Kingdom.

Presenting Author: P.J. Wu

Systematic measurements of dc magnetization have been carried out on several low-T_c superconductors including niobium alloys, Chevrel phase compound PbMo₆S₈ and bismuthate BaPb_0.75Bi_0.25O₃ to investigate the irreversibility line in low-T_c materials. The results show that there exits an observable region below the mean-field critical field, B_c2, where the magnetization is reversible during a cycle of increasing and decreasing field. These experimental observations indicate that the irreversibility line is not unique to high temperature oxides but also exists in low-T_c superconductors although much closer to B_c2 in some cases. For the niobium alloyed samples, in addition to dc magnetization measurements, ac susceptibility and magnetoresistivity measurements have also been performed on the same samples as alternative techniques to probe the irreversibility line to determine the best way of distinguishing a genuine thermally activated reversibility from a finite, but low critical current density. The results showed that the collapse of the dc magnetic hysteresis, the onset of the diamagnetic ac susceptibility (or the peak of the ac loss) and the zero resistance occurred at nearly the same field, namely, the irreversibility field, B_irr.

1PO2-112 Vortex Melting and Decoupling Transitions in YBa₂Cu₄O₈ single Crystals

Xianggang Qiu 1,3, V.V. Moshchalkov 1, Y. Bruynseraede 1, J. Karpinski 2, and Y. Segawa 3. 1 Laboratorium voor Vaste-Stoffysika en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium. 2 Laboratorium für Festkörperphysik, ETH-Hönggerberg, CH-8093, Zürich, Switzerland. 3 Photodynamics Research Center, The Institute of Physics and Chemistry (RIKEN), 19-1399 Koeji, Nagamachi, Aoba-ku, Sendai, Japan.

Presenting Author: X.G. Qiu

The mixed state in-plane (r _ab) and out-of-plane (r _c) resistivity of high quality single crystals of YBa₂Cu₄O₈ have been measured, using the quasi-flux transformer configuration. A simultaneous sharp drop associated with the vortex lattice melting is observed in both the in-plane and out-of-plane resistive transitions. The vortices just above the melting temperature are three-dimensional line liquid with its correlation length along the c direction l_c £ t, the sample thickness. At a characteristic temperature T_d, the vortices lose their correlation along the c direction and they dissolve into two dimensional pancake vortices. The temperature at which the resistive peak in R_c appears corresponds to the decoupling temperature T_d. An H-T phase diagram for the YBa₂Cu₄O₈ single crystal is obtained.

1PO2-113 Vortex pinning in untwinned YBCO

Alain Pautrat 1, Christophe Goupil 1, Charles Simon 1, Norbert Lukte-Entrup 2, Bernard Placais 2, Patrice Mathieu 2, Yvan Simon 2, Alexandre I. Rykov 3, and Setsuo Tajima 3. 1 Laboratoire CRISMAT-ISMRA, 14 050 Caen cedex, France. 2 Laboratoire de matière condensée de l'Ecole Normale Supérieure, 75005 Paris, France. 3 Superconductivity Research Laboratory, International Superconductivity Technology Center, Tokyo 135, Japan.

Presenting Author: A. Pautrat

We present here transport measurements showing that in clean untwinned YBCO, critical currents values are explained without taking into account any bulk pinning effect. Localization of sub critical current achieved by means of a pick-up coil shows that it only flows close to the surface as it should be in the case of pinning by surface irregularities. No bulk current, which implies bulk pinning, was observed below the irreversibility line. Extensive measurements of AC penetration depth in a large sample allowed us to investigate the frequency spectrum of the depinning transition. The result was in perfect agreement with a two modes penetration controlled by surface pinning and free flow in the bulk. Those three complementary experiments suggest that interpretations of pinning and critical currents in terms of collective bulk pinning theories are questionable. It suggests also that the irreversibility line in untwinned YBCO is due to the disparition of surface pinning, and not by a melting of the vortex lattice, in accordance with previous measurements showing no evidence of vortex entanglement in the so called "liquid" phase.

1PO2-114 Irreversibility in Type-II Superconductors: Geometric Barrier versus Pinning

Ernst Helmut Brandt, Max-Planck-Institut Metallforschung, P.O. Box 800665, D-70506 Stuttgart, Germany

Presenting Author: E.H. Brandt

The usual cause for magnetic irreversibility of type-II superonductors is pinning of vortices by inhomogeneities in the material. But also in pin-free materials a pronounced irreversibility may be caused by two types of surface barriers: The microscopic Bean-Livingston Barrier for the the penetration of single vortex lines, and the geometric barrier for flux penetration at the edges of superconductors of non-ellipsoidl shape, e.g., films of constant thickness in a perpendicular magnetic field. A novel method allows to calculate the distributions of magnetic field and current in superconductors of arbitrary shape, with any given constitutive laws which describe pinning, depinning, flux flow, and the reversible equilibrium magnetization curve which would be there in absence of pinning. Irreversible magnetization curves are presented for strips and disks or cylinders with rectangular cross section of various aspect ratios. Obtained are the field of first flux entry, where the magnetization is maximum, and a reversibility field above which the magnetization coincides with the irreversible magnetization of an ideal ellipsoid. The results are compared with the reversible behavior of ideal ellipsoids.

1PO2-115 Overcritical states in magnetically shielded superconductor strips

Yuri A. Genenko 1, Alexander Usoskin 2, Alexei Snezhko 3, and Herbert C. Freyhardt 4. 1 Dept. Materials Science, Technical University of Darmstadt, D-64287 Darmstadt, Germany. 2 Zentrum fuer Funktionswerkstoffe gGmbH, D-37073 Goettingen, Germany. 3 Physics Department, Charles University, 180 00 Prague 8, Czech Republic. 4 Materials Science Inst., University of Goettingen, D-37073, Germany.

Presenting Author: Yu.A. Genenko

A soft magnetic surrounding strongly affects the transport current distribution within a flux-free superconductor strip protected by an edge barrier against magnetic flux entry. Exact analytical solutions and numerical studies show that, in special geometries of the surrounding, the current peaks at the strip edges are strongly suppressed and the current is redistributed towards the center of the strip which results in large (1-2 orders of the magnitude) enhancements of the self-field critical current [1].

A hard superconductor strip partly filled with a magnetic flux in the critical state [2,3] exhibits a similar behavior in the flux-free region where the current distribution proves to be very sensitive to the magnetic surrounding too. Special configurations of the surrounding magnet allows the strip to carry the loss-free transport current several times exceeding the critical current of the isolated strip.

Numerical studies reveal a weak dependence of the effect on the magnetic permeablity but a strong dependence on the geometry of the magnetic environment.

[1] Yu.A. Genenko, A.Usoskin, and H.C.Freyhardt, Phys.Rev.Lett., 1999, in print.

[2] E.H.Brandt, and M.V. Indenbom, Phys.Rev.B 48, 12893 (1993).

[3] E.Zeldov, J.R.Clem, M.McElfresh and M.Darwin, Phys.Rev.B 49, 9802 (1994).

1PO2-116 Temperature dependence of the lower critical field for a HgBa₂CuO₄ single crystal

D. Stamopoulos and M. Pissas, Institute of Materials Science, NCSR Demokritos, 15310, Aghia Paraskevi, Athens, Greece

Presenting Author: D. Stamopoulos

The temperature dependence of the lower critical field (Hc1), above which vortices penetrate into a type II superconductor, is of great interest since is related to the symmetry of the pairing wave function. Using global dc magnetization measurements for a HgBa₂CuO₄ single crystal we estimated the lower critical field as a function of temperature. The influence of surface barriers on the penetration of vortices is also investigated. By lowering the sweep rate of the applied magnetic field we minimized the contribution of the surface barriers on the penetration mechanism. A linear temperature dependence of Hc1 is revealed down to 5 K. Possible interpretations are discussed.

1PO2-117 Influence of surface irregularities on barriers for vortex entry in type-II superconductors

Alexey Yu. Aladyshkin, Alexander S. Mel'nikov, Iosif D. Tokman, and Ilya A. Shereshevsky, Institute for Physics of the Microstructures, Russian Academy of Sciences, GSP-105, Nizhny Novgorod, 603600, Russia

Presenting Author: A.Yu. Aladyshkin

We have investigated the influence of defects (e.g. cracks or twins) on the Bean-Livingston (BL) surface energy barrier. Such surface imperfections result in a decrease of the critical field H* of the first vortex entry into a superconductor: H*~g H_cm , where H_cm is the critical field of the BL barrier suppression in the ideal type-II superconductors, g <1. The suppression is shown to be the strongest for defects like cracks with dimensions L ~ l (l is the London penetration depth), when an external magnetic field is parallel to the crack edge. The conditions for vortex nucleation near the wedge-like crack edge and vortex penetration in the bulk are found for an arbitrary wedge angle q . The maximum suppression of BL barrier occurs for thin cracks (q <<1): H*~(x /l )^1/2 H_cm, where x is the coherence length.

1PO2-118 Off equilibrium magnetic properties in a system of repulsive particles

Mario Nicodemi and Henrik Jeldtoft Jensen, Imperial College, London, United Kingdom.

Presenting Author: M. Nicodemi

We study the properties of a simple lattice model of repulsive particles diffusing in a pinning landscape, which shows many features similar to those observed in the vortex physics of superconductors, such as the equilibrium phase diagram, creep dynamics, hysteresis of magnetisation loops (including the second peak feature). The model is analytically and numerically treatable and offers a comprehensive schematic framework of the phenomenology.

1PO2-119 Reversible Magnetization Measurements in the cubic (K,Ba)BiO₃ superconductor

Isabelle Joumard 1, Thierry Klein 1, Agustin Conde-Gallardo 1, Jacques Marcus 1, and Andre Sulpice 2. 1 LEPES-CNRS, 25 Avenue des Martyrs, BP 166, 38 042 Grenoble, France. 2 CRTBT-CNRS, 25 Avenue des Martyrs, BP 166, 38 042 Grenoble, France.

Presenting Author: I. Joumard

It is now well established that thermal fluctuations play a significant role in both dynamic and thermodynamic properties of high T_c superconductors. We have shown that, despite much smaller thermal fluctuations (the Ginzburg number Gi~ 10^-4) the vortex solid in the fully isotropic (K,Ba)BiO₃ system (T_c~32K for optimally doped samples) melts into a liquid through a second order phase transition which can be well described by the vortex glass scaling theory.

Magnetization measurements have been performed on the cubic (K,Ba)BiO₃ superconductor. The reversible part of the magnetization varies linearly with lnH in both liquid and solid phases. In contrast to strongly anisotropic systems the reversible magnetization curves M(T,H) never cross each other but the slope clearly deviates from the standard London model in a large temperature range below T_c.