1PO2-130 High temperature-low field phase diagram of the vortex matter for a HgBa2CuO4 single crystal
Dimosthenis N. Stamopoulos and Michael Pissas, Institute of Materials Science, NCSR Demokritos, 15310 Ag. Paraskevi, Athens, Greece
Presenting Author: D.N. Stamopoulos
The phase diagram of the vortex matter is investigated by means of local ac-susceptibility and global dc-magnetization measurements for a HgBa2CuO4 single crystal. Our local ac measurements reveal the peak effect very close to Tc and a very sharp diamagnetic step of the order of 100 mK or less at the endpoint of the in phase signal. The related feature in our global dc-magnetization measurements is a change in the slope or a broad step in m(T) curves. We interpret these features as the melting of the flux line lattice. The peak effect, the reversible regime and the low field-high temperature liquid regime have been observed and all the respective lines are presented on the phase diagram.
1PO2-131 Vortex-Glass Transition in the (Ba,K)BiO3 Superconductor at Very Low Temperatures
Pavol Szabo 1, Jozef Kacmarcik 1, Peter Samuely 1, Thierry Klein 2, Jacques Marcus 2, and A.G.M. Jansen 3. 1 Institute of Experimental Physics, Slovak Academy of Sciences, SK-04353 Kosice, Slovakia. 2 Laboratoire d’Etudes des Proprietes Electroniques des Solides, CNRS, F-38042 Grenoble Cedex 9, France. 3 Grenoble High Magnetic Field Laboratory, MPI FKF & CNRS, F-38042 Grenoble Cedex 9, France.
Presenting Author: P. Samuely
Magnetotransport and magnetization measurements have been simultaneously performed on the cubic superconductor (Ba,K)BiO3 in a wide temperature range from the transition temperature Tc = 23 K down to 50 mK in magnetic fields up to 30 T. The irreversibility line in the Magnetic Field vs. Temperature phase diagram inferred from the magnetization measurements is compared with the temperature dependence of the magnetic field where the resistance onsets. A character of the vortex-glass transition is determined.
1PO2-133 Universal Behaviors of Magnetic Properties of Rb3C60 Fullerites
Ming-Fong Tai, Physics Dept., National Chung Cheng University, 160 San Hsing, Ming Hsiung, Chia-yi 621, Taiwan
Presenting Author: M.F. Tai
Superconducting Rb3C60 fullerene crystals with a transition temperature of 30.5 ± 0.2 K were well prepared. Our magnetic measurements show that the magnetization curves scaled by a factor of the thermodynamic critical field, 1.414Hc , exhibit a temperature-independent universal behavior. The universal function is consistent with the prediction of Ginzburg-Landau theory and Hao-Clem variation model. The thermodynamic critical fields Hc at various temperatures were precisely determined from the universal magnetization curve according to the theoretical fitting of Hao-Clem model. The thermodynamic critical field in 10 K < T < 30 K well obeys the power-law relation, Hc(T) = Hco(1-T/Tc)b, where Hc(0) = 2344 Oe, Tc = 29.62 K and b = 0.8682. This result is in good agreement to the result of BCS theory.
*1PO2-134 A Simple Cellular Model for Vortex Avalanches
Kevin E. Bassler 1, and Maya Paczuski 2, 3, Department of
Physics, University of Houston, Houston, Texas 77204-5506, U.S.A., 2
Nordita, Blegdamsvej 17, 2100 Copenhagen, Denmark. 3 Department of
Mathematics, Imperial College, 180 Queen's Gate, London, U.K.
Presenting Author: K.E. Bassler A discrete cellular coarse grained model is used to study the dynamics
of vortices in a dirty superconductor that have been driven into the
marginally stable at the onset of resistivity. The minimal model describes
the system at the scale of the London length lamdba, or larger, and
includes repulsive interactions between vortices and attractive interactions
between vortices and quenched random pinning centers.
Driving the system from the boundary, it self-organizes to a critical
state, characterized by a constant flux density gradient, where the activity
takes place in terms of avalanches spanning all length scales up to the system
size. Scaling relations and universal critical exponents for avalanche
moments and durations that are robust over a range of parameter values will
be presented. The vortices move across the system in preferred channels
that form a self-affine fractal braided pattern reminiscent of the pattern
of braided fluvial rivers, such as the Brahmapurta. Techniques that have
been developed to analyze the scaling properties of those patterns will
be discussed. The results of those techniques will then used to
quantitatively compare the vortex river patterns to fluvial river patterns.
Finally, results for the I-V characteristics obtained by driving the system into the
super-critical regime where the vortices are continually moving will also
be presented.
*1PO2-135 Appearance of Spontaneous Magnetic Field at Tc of YBCO
Raz Carmi, Emil Polturak, Gad Korwn, and Assa Auerbach, Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel.
Presenting Author: E. Polturak
The appearance of a spontaneous field is a direct indication that both time reversal symmetry and parity are broken in the superconducting phase. We report [1] the first observation of a weak magnetic field which is generated spontaneously during the superconducting transition of YBCO. The samples are epitaxial c-axis oriented thin films, and the spontaneous field is sensed using a SQUID magnetometer. The magnitude of the spontaneous field is in the 10-5G range. Particular care was devoted to exclusion of spurious effects, including residual fields within the shielded environment, thermal gradients in the sample, and inductive coupling of the samples to the SQUID. By measuring differently patterned samples, we were able to determine that the signal originates near the edges of the film.
Our data is consistent with a possible symmetry breaking dxy component of magnitude 2.5 x 10-2 of the dx2-y2 order parameter. Another possible interpretation involves spontaneous persistent currents around defects reflecting the unusual nature of HTSC.
[1] cond-mat/0001050.