2PO1-50 Interlayer Phase Coherence in the Vortex Matter Phases of Bi₂Sr₂CaCu₂O_8+y

T. Shibauchi 1,2, T. Nakano 3, M. Sato 3, T. Kisu 3, N. Kameda 3, N. Okuda 3, S. Ooi 3, and T. Tamegai 3. 1 IBM T. J. Watson Research Center, Yorktown Heights, NY 10598, USA. 2 MST-STC, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. 3 Department of Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

Presenting Author: T. Shibauchi

The vortex matter phase diagram in Bi₂Sr₂CaCu₂O_8+y (BSCCO) was studied by the magnetization measurements and by Josephson plasma resonance (JPR). Above the first-order transition (FOT) line, sample-moving magnetization measurements show a clear but scan-length dependent anomaly [1] suggesting the existence of a surface-barrier related transition (Tx line), which is consistent with a report by Fuchs et al. [2]. We found that the Tx line continues up to the highest field we measured (5 T) without saturation. To clarify the nature of each vortex phases, the interlayer phase coherence in BSCCO was quantitatively determined in a wide field range by using JPR, covering below and above the FOT and the second peak lines. At the FOT line, a novel frequency-independent JPR with sharp resonance width is found, indicating that the interlayer phase coherence abruptly changes at the FOT [1]. Similar results have been obtained in the organic layered superconductor k -(BEDT-TTF)₂Cu[N(CN)₂]Br [3]. We also found a strong change in the interlayer phase coherence at the second peak line, indicating the decoupling nature of these lines. Above the FOT line, however, we found that the interlayer phase coherence shows smooth 1/H dependence with no anomaly at the Tx line. These results clearly indicate that the decoupling occurs at the FOT not at the Tx line, and therefore can rule out existence of the vortex line liquid state.

[1] T. Shibauchi et al., Phys. Rev. Lett. 83, 1010 (1999).

[2] D.T. Fuchs et al., Phys. Rev. Lett. 80, 4971 (1998).

[3] T. Shibauchi et al., Phys. Rev. B 57, R5622 (1998).

2PO1-51 Magnetic Decay due to Vortex Phase Boundary Motion in Bi₂Sr₂CaCu₂O₈ Crystals

M. Konczykowski 1, C.J. Van Der Beek 1, S. Colson 1, M.V. Indenbom 1,2, P.H. Kes 3, and E. Zeldov 4. 1 Laboratoire des Solides Irradies, Ecole Polytechnique, 91128 Palaiseau, France. 2 Institute of Solid State Physics, 142432 Chernogolovka, Russia. 3 Kamerlingh Onnes Laboratorium, Rijksuniversiteit Leiden, The Netherlands. 4 Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel,

Presenting Author: M. Konczykowski

Magnetic relaxation in clean Bi₂Sr₂CaCu₂O₈ (BSCCO) crystals was investigated by the Hall-array technique. A new regime of decay of the irreversible magnetization was identified for local induction values close to the "second peak field" at which the bulk critical current density steeply increases. We identify a time window within which the decay of the local induction is controlled by the slow propagation of the phase transformation front across the sample.

2PO1-52 Supercooling of the high field vortex phase in single crystalline BSCCO

Cornelis J. Van Der Beek 1, Sylvain Colson 1, Marcin Konczykowski 1, and Mikhail V. Indenbom 2. 1 Laboratoire des Solides Irradies, Ecole Polytechnique, 91128 Palaiseau, France. 2 Institute of Solid State Physics, 142432 Chernogolovka, Russia.

Presenting Author: C.J. Van Der Beek

We present a detailed study of flux dynamics during vortex penetration and exit in clean, optimally doped Bi₂Sr₂CaCu₂O₈ (BSCCO) single crystals, in magnetic induction values close to the value B_sp at which the the second magnetization peak occurs. Time-resolved magneto-optical images, displaying the relaxation of the quenched flux distribution after the rapid decrease of the magnetic field from values much above B_sp to a value much below it, show that it is possible to supercool the high-field vortex phase to inductions less than half B_sp. We present dynamical measurements on the supercooled phase and discuss the consequences of this observation for the mixed state phase diagram.

2PO1-53 3D-melting features of the irreversibility line in overdoped Bi₂Sr₂CuO₆ at ultra-low temperature and high magnetic field

Andrea Morello 1,2, Louis Jansen 1, Renato S. Gonnelli 2, and Sergey I. Vedeneev 3. 1 High Magnetic Field Laboratory, Max-Planck Institut fur Festkorperforschung and Centre National de la Recherche Scientifique, BP166, 38042 Grenoble Cedex 9, France. 2 INFM - Dipartimento di Fisica, Politecnico di Torino, c.so Duca degli Abruzzi 24, 10129 Torino, Italy. 3 P.N. Lebedev Physical Institute, Russian Academy of Sciences, SU-117924 Moscow, Russia.

Presenting Author: A. Morello

The layered high-T_c superconductor Bi₂Sr₂CuO₆ has low enough critical temperature to allow the investigation of the whole B-T phase diagram. By means of a capacitive torquemeter, we have measured the irreversible magnetization of an overdoped Bi₂Sr₂CuO₆ single crystal up to B = 28 T and down to T = 60 mK, and extracted the irreversibility line B_irr(T). The 3D anisotropic vortex lattice melting provides an excellent fit to the data in the whole investigated temperature range; using the Lindemann criterion, we find the reasonable value of the Lindemann number c_L = 0.13. For a comparison, we discuss the applicability of 2D melting, quantum melting and flux creep models. Qualitatively, B_irr(T) is very similar to the resistive upper critical field reported for a Bi₂Sr₂CuO₆ thin film: this confirms that flux lattice melting is likely to play an important role in magnetoresistance experiments.

2PO1-54 Disentangled vortex liquid phase in BSCCO-2212 single crystal highly overdoped with oxygen

D. Darminto 1,2, M.O. Tjia 1, and A.A. Menovsky 3. 1 Physics Department, Bandung Institute of Technology, Jl. Ganesa 10, Bandung 40132, Indonesia. 2 Physics Department, Sepuluh Nopember Institute of Technology, Keputih-Sukolilo, Surabaya 60111, Indonesia. 3 Van der Waals - Zeeman Institute, The University of Amsterdam, Valcknierstraat 65, 1018XE Amsterdam, The Netherlands.

Presenting Author: M.O. Tjia

Measurements of in-plane as well as out-of-plane resistivity of a BSCCO-2212 single crystal highly overdoped with oxygen have been performed in an external magnetic field covering the range between 100 Oe and 40 kOe. The data show a large reduction of anisotropy with respect to the undoped or less heavily doped species on the one hand, while showing a more gradual nature of the vortex melting processes on the other hand, signifying an additional enhancement of disorder in the vortex system. Further analysis of the experimental result reveals the occurrence of vortex line disentanglement in its liquid phase, indicating the presence of effective pinning effect in the viscous vortex liquid before entering the flux flow regime at higher temperature and magnetic field.

2PO1-55 Melting line near T_c(H=0) in YBa₂Cu₃O_7-x single crystal

M. Pissas, Institute of Materials Science, NCSR Demokritos, 15310, Aghia Paraskevi, Athens, Greece, and R. Liang, Department of Physics, The University of British Columbia, British Columbia, Canada V6T 1Z1

Presenting Author: M. Pissas

We report on local Hall probe ac-susceptibility measurements near the T_c and for low dc-magnetic fields in an untwinned YBa₂Cu₃O_7-x single crystal. A sharp step in the real part of local ac-magnetic induction (B') is interpreted as a liquid to solid transition of the flux lattice.

In the range 0<(T_c-T)/T_c<0.0084 we find that, the temperature where the sharp drop occurs varies with the dc-magnetic field, as H_m(T)=2051(T_c-T)^2a with a=2/3.

This result is consistent with the critical behavior predicted from the 3D XY model.

In all the temperature sweeping measurements of B'(T) and B"(T) (the imaginary part of the local ac magnetic induction), under constant dc-magnetic field, we observed three peaks in B"(T) and three discrete modes in B'(T).

2PO1-56 The Mechanical Study on Vortex Phase Diagram in the Mixed State of HTSC

Z. Li, J.S. Zhu, X.B. Wu, and Y.N. Yang, National Laboratory of Solid State Microstructure, Department of Physics, Nanjing University, Nanjing 210093, P.R. China

Presenting Author: Y.N. Li

The mechanical dissipation was measured using a vibrating need technique in various measuring conditions in a melted textile YBCO sample. A dissipation peak associated with vortex motion was observed below T_c. The mechanism of this dissipation peak was discussed. Special attention was paid on the high temperature side of dissipation peak, vortex liquid region. A crossover transition from vortex entanglement to vortex disentanglement was obtained from the vanish point of dissipation on the dissipation curve. The barrier of vortex entanglement was also estimated, the theoretic prediction is consistent with the experiment results.

On the based of what is mentioned above, a vortex phase diagram, showed the transition from the vortex solid to the vortex liquid and from the vortex entanglement to the vortex disentanglement, was given out from the mechanical measurement at different field for the melted textile YBCO sample.

2PO1-57 Flux Creep and Critical Current Density of The Vortex Liquid Phase in YBa₂Cu₃O₇-d Film

L. Shan 1, Z.H. Wang 1, X.N. Xu 1, X. Jin 1, L.J. Shen 2, and C.C. Lam 2. 1 Department of Physics and National Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210008, China. 2 Department of Physics and Materials Science, City University of Hong Kong, Hong Kong.

Presenting Author: L. Shan

The current-voltage characteristics (CVC's) of YBa₂Cu₃O_7-d epitaxial film were determined at various temperatures and magnetic fields around vortex-glass transition. Temperature and magnetic field dependencies of the pinning potential U₀(T,H) and critical current density J_c(T,H), and current, temperature and magnetic field dependencies of the effective pinning potential U_eff(H,T,J) were derived from the CVC's. It is shown that the nonlinear response regime of the vortex-liquid (NRVL) should be divided into two parts in H-T phase diagram with respect to different flux dynamic characteristics, respectively.

2PO1-58 Thermodynamic Study of the Critical Points in the Vortex Phase Diagram of High Purity YBa₂Cu₃O_7-d Single Crystals.

K. Deligiannis 1, M. Charalambous 1, J. Chaussy 1, R. Gagnon 2, and L. Taillefer 2. 1 CRTBT-CNRS, 25 Avenue des Martyrs, 38000 Grenoble, France. 2 Physics Department, McGill University, Montreal, Quebec.

Presenting Author: K. Deligiannis

We present a calorimetric study of high purity YBa₂Cu₃O_7-d single crystals, using a state of the art AC calorimetric set-up with a specific heat resolution 0.5 nJ/K and a relative resolution dC/C ~ 10^-4. Our investigation is focused on the existence of critical points which bound the melting transition line in the H-T vortex phase diagram from above (upper critical point) and below (lower critical point) as well as on the effect of oxygen concentration. We derive the solid-to-liquid entropy difference for a large magnetic fields regime, a precious parameter for the understanding of the melting mechanism.