*1PO2-80 to 1PO2-89

*1PO2-80 Commensurability Oscillations and Double Peak-Effect in YBa₂Cu₃O_y

Alexander A. Zhukov 1, Heinz Küpfer 2, Peter A.J. de Groot 1, and Thomas Wolf 2. 1 Department of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, United Kingdom. 2 Forschungszentrum Karlsruhe, Institute für Technische Physik, D-76021 Karlsruhe, Germany.

Presenting Author: A.A. Zhukov

Using a high angular resolution (0.01Deg) vector magnetometer we studied the components of the magnetic moment parallel and transverse to the CuO-planes in YBa₂Cu₃O_y single crystals with different oxygen contents (6.76£ y£ 6.999). The transverse magnetisation Mc is governed by the vortex lock-in to the CuO-planes. For the magnetic field direction within the lock-in angle (~0.2Deg) the longitudinal magnetisation Mab shows oscillatory behaviour originating from commensurability between the inter-vortex distance and the CuO-layer spacing. The oscillation period provides a means for locally probing of anisotropy in samples with different oxygen contents. We find that the anisotropy of effective mass varies from 26 to 120 when the oxygen content changes from y=6.999 to y=6.76. In the locked state two sharp peaks are found in the temperature dependence of the irreversible magnetisation Mab. Using the angular dependences of the magnetisation and the oscillations we show that the high temperature peak marks the transition between a nematic vortex liquid and a smectic vortex phase induced by intrinsic pinning. The low temperature transition is connected with the melting from a vortex solid to a smectic phase. The angular phase diagram is mapped.

1PO2-81 Entropy Driven Fragmentation of the Locked Vortex State

Presenting Author: A.A. Zhukov

Using a high angular resolution (0.01Deg) vector magnetometer we have studied the components of the magnetic moment parallel and transverse to the CuO-planes induced by vortex lock-in phenomena in YBa₂Cu₃O_y single crystals. The full shielding of the transverse field component that originates from the infinite tilt modulus is observed for the lowest fields only. The transverse susceptibility decreases linearly with the magnetic field. We show that this is connected with entropy induced fragmentation of the locked vortex state by the thermally-activated kinks. We introduce a simple model of this phenomenon that accounts for the reduced shielding as well as describes the angular behaviour of the critical current

1PO2-82 Tilted to combined vortex lattice phase transition in Bi₂Sr₂CaCu₂O₈

Abdel Fattah Khoder 1, Michel Couach 2, Bernard Revaz 3, and Alain Junod 3. 1 Université Libanaise, Faculte des Sciences,B.P. 826, Tripoli, Lebanon. 2 CEA Grenoble, Département de Recherche Fondamentale sur la Matière Condensée, 38054 Grenoble Cedex 9, France. 3 Université de Genève, Département de Physique de la Matière Condensée, CH-1211 Genève 4, Switzerland.

Presenting Author: A.F. Khoder

The vortex lattice in the high T_c layered superconductors offers many possible configurations for magnetic field close to the layers. The very existence of each vortex lattice such as staircae tilted or Abrikosov and Josephson combined, is very senstiive to the system anisotropy g , its penetration depth l and also to external parameters as the applied field strength and the tilt angle of H relative to the layers. In this work, we give experimental evidences of change in the dynamic behavior of vortices as seen by a.c. susceptibility, explained by lattice phase transition from tilted to combined configuration. The obtained phase diagrams are in agreement with theoretical expectations of Bulaevskii et al.

1PO2-83 Angular Dependence of the Lower Critical Field in Layered (s+d)-Wave Superconductors

Alexey S. Bystrov, Alexander S. Mel'nikov, and Denis A. Ryzhov, Institute for Physics of Microstructures, Russian Academy of Sciences, 603600, Nizhny Novgorod, GSP-105, Russia

Presenting Author: D.A. Ryzhov

The structure of a single tilted vortex line consisting of pancake vortices coupled by Josephson strings in layered (s+d)-wave superconductors has been analyzed within the Lawrence-Doniach model generalized for two order parameter components. The angular dependence of the lower critical field H_c1 has been derived for different ranges of angles between directions of the magnetic field and crystal axes. The four-fold symmetry of H_c1 is shown to be associated with the s-wave order parameter component induced in Josephson string regions.

1PO2-84 Relaxation of magnetic flux at a field nearly parallel to the CuO plane of the YBCO crystal

Yury I. Talanov, Rustem I. Khasanov, and Tatiana S. Shaposhnikova, Kazan Institute for Technical Physics, Kazan 420029, Russia

Presenting Author: Yu.I. Talanov

YBaCuO single crystals with the critical temperature 92K were used to study the relaxation of magnetic flux induced by a magnetic field applied at a small angle (0.1 - 6 degrees) to the basal plane. The local field at the crystal surface (typical size 1x2 sq.mm) was measured by Hall probe with the sensitive area 0.2x0.5 sq.mm. Measurements were carried in the temperature range from 30 to 70K, and at magnetic field - up to 8 kOe. Unusual nonmonotonic temporal variation of magnetic flux at the field direction nearly parallel to the CuO plane (an angle less than 1 degree) was observed. Namely, after switching off the applied field a magnetic flux near the crystal edge increases at first (during about 10 min.), and then decreases. Such behavior is due to the motion of vortex kinks toward crystal edges. It causes the normal component of remanent field to increase there.

1PO2-85 Anomalous Angular Dependence of Magnetization near H½ ½ ab-plane in YBa₂Cu₃O_7-_dSingle 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

The angular dependence of the magnetization M(q ) of a twinned YBa₂Cu₃O_7-_dsingle crystal is measured in the magnetic fields H up to 70 kOe using SQUID magnetometer (Quantum Design Inc.). The angle q is defined as the angle between the ab-plane and the direction of H. M(q ) near q =0° , i.e. H½ ½ ab-plane, shows

the anomalous behaviors as follows. Below T < 75 K, M(q ) shows a dip at all magnetic fields of 10 kOe < H < 70 kOe. At T = 80 K, the dip disappears and changes to a broad peak at H = 50 kOe. Above T < 82 K, the dip is only observed below 20 kOe. The dip is interpreted as a result of the lock-in transition of the three dimensional (3D) vortices, because the dip width becomes narrow with increasing H; this is consistent with the results expected from the lock-in transition. On the other hand, the broad peak is similar to that observed in La_2-xSr_xCuO₄ single crystals reported earlier. Thus, it seems that the broad peak is resulted from the quasi-2D vortices. Our results indicate that the dimensionality of the vortex system changes with increasing H near H½ ½ ab-plane.

1PO2-86 Mixed State Behavior of Superconducting NbN/AlN Multilayers

El-Hadi S. Sadki 1, Zoe H. Barber 1,2, Stephen J. Lloyd 3, Mark G. Blamire 1, Archie M. Campbell 1,2. 1 IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom. 2 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. 3 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom.

Presenting Author: E.S. Sadki

The in-plane properties of a single layer of the low-T_c superconductor, NbN, and multilayers, consisting of 10 NbN/AlN repeats, have been studied using transport measurements in magnetic fields applied perpendicular and parallel to the layer normal. The effects of changing the insulating (AlN) layer thickness has been systematically investigated and a NbN film was also studied for comparison. Unlike the NbN, the resistivity transition of NbN/AlN was found to broaden with increasing magnetic field. The upper critical fields for field parallel to the layer show a dimensional crossover in the superconductivity. The irreversibility lines of the NbN/AlN multilayers and their evolution with increasing anisotropy are similar to those observed in high-T_c materials.

1PO2-87 New Angular Dependence of Vortex Melting Transition in Single Crystal Bi-2212

Jovan Mirkovic 1,2 Emiko Sugahara 1, and Kazuo Kadowaki 1,3. 1 Institute of Materials Science, University of Tsukuba, Tsukuba 305-8573, Japan. 2 Faculty of Sciences, University of Montenegro, 81000 Podgorica, Yugoslavia. 3 CREST, Japan Science and Technology Corporation (JST), Japan.

Presenting Author: J. Mirkovic

The vortex states in oblique magnetic fields in single crystal Bi-2212 were investigated by means of resistivity measurements using the Corbino geometry which excludes surface barriers and provides good quality of bulk resistivity data. The first-order vortex lattice melting transition, identified by the sharp resistance drop, demonstrated the complex behavior in magnetic fields tilted away from the c-axis and various vortex phases were distinguished. As the in-plane field Hab was increased, both melting transition temperature Tm and perpendicular component of melting field Hm//c decreased linearly indicating crossed pancake vortices lattice and Josephson vortex lattice as proposed recently by Koshelev. Above certain value of Hab*, Tm and Hm//c sharply changed behavior and became independent of the in-plane field Hab, what may suggest a quasi-decoupled vortex solid phase. However, as magnetic field H further approached the ab-plane, we identified the new vortex phases and novel Hm//c versus Hm//ab phase diagram in the whole angular range including orientation H//ab, where the melting transition possibly changes its character from the 1st order to to 2nd order phase transition.

1PO2-89 Vortex Matter Phase Transitions under Tilted Fields in Pristine and Pb-Substituted Bi₂Sr₂CaCu₂O_8+y

T. Tamegai, S. Ooi, K. Itaka, and T. Shibauchi, Department of Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Presenting Author: T. Tamegai

Vortex matter phase diagram in Bi₂Sr₂CaCu₂O_8+y under fields tilted away from the c-axis is studied by using Hall probes. Vortex lattice melting (Hm(q )) and peak effect (Hp(q )) are found to follow a novel angular scaling; transition fields linearly depends on the in-plane component of the field. This novel scaling is valid in the vortex solid phase up to doping-level-dependent angle q *. At angles q > 60 deg., a new anomaly, H_p*, appears at fields lower than H_p. Contrary to H_pcosq , H_p*cosq increases with q and shows a maximum near q *. With lowering anisotropy by carrier doping, q * decreases. In Pb-substituted Bi₂Sr₂CaCu₂O_8+y, we also observed similar anomalies. However, in spite of comparable magnitude of anisotropy inferred from H_m in pristine and Pb-substituted crystals, q *'s in these systems have quite different values. This indicates that there is another factor governing H_p(q ) and H_p*(q ). Above q *, H_pcosq begins to increase. However, at angles close to 90 deg., H_p(q )cosq recovers the normal behavior, namely, it decreases again towards zero at q = 90 deg. In this angle region, H_p(q ) seems to follow the conventional scaling law with g ~ 50. Another anomaly near H = 0 becomes prominent in Pb-substituted Bi₂Sr₂CaCu₂O_8+y. We discuss the origins of these anomalies based on the characteristic vortex configuration under tilted fields.