HOT TOPICS BROWN BAG IBB

Monday, Feb. 21, 12:30 p.m. – 1:45 p.m., George Bush Grand Ballroom B (GRB)

Chairs: D.M. Ginsberg (U Illinois-Urbana Champaign), K. Kitazawa (U Tokyo)

1BB.1 (*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 d_x2-y2 order parameter. Another possible interpretation involves spontaneous persistent currents around defects reflecting the unusual nature of HTSC.

[1] cond-mat/0001050.

1BB.2 (*4PO2-29) Formation of High Temperature Superconducting Balls

R. Tao 1, X. Zhang 1, X. Tang 1, and P.W. Anderson 2. 1 Department of Physics, Southern Illinois University, Carbondale, IL 62901-4401, USA. 2 Department of Physics, Princeton University, Princeton, NJ 08544, USA.

Presenting Author: R. Tao

Superconducting granular balls, a new phenomenon, have been observed. In nature, there are very few cases that granular particles could aggregate together by themselves to form a round ball. Application of electric field further destroys space’s isotropy. Therefore, it is against common sense that an electric field could drive high Tc superconducting (HTSC) particles together to form a round ball. However, this is our recent discovery. When a strong electric field is applied to a suspension of micron-sized HTSC particles in liquid nitrogen, the HTSC particles quickly aggregate themselves together into macroscopic balls. The ball, about 0.25 mm across and containing several million particles, was quite sturdy; surviving constant heavy collisions with the electrodes. The experiment has established that this effect is uniquely associated with the high temperature superconductivity. Metal particles in this situation would bounce between the two electrodes to transport charges or form chains along the field direction (in ac field). The same HTSC particles, above their superconducting transition temperature, do not ball up but instead form chains. When liquid argon was used to raise the temperature from 77K to pass the Tc of HTSC particles, the force holding these particles together disappeared and the ball broke into pieces in the collision. Our current understanding relates this phenomenon to a new surface tension induced by surface charges on the HTSC particles. High temperature superconductors have a very short c-axis coherence length, xc, shorter than the Thomas-Fermi screening length, ls. After an electric field is applied, HTSC particles pick up charges from the electrodes. These charges stay at the particles’ surface, forming a thin charged layer with a thickness of ls. Since ls > xc, the electric field produced by the charged surface layer affects the layers below. When the field is strong enough, it turns off the Josephson coupling between the interlayers. This loss of Josephson energy becomes a positive surface energy induced by the charged surface layer. Its minimization leads to the ball formation. The new surface tension also brings the wetting phenomenon, producing strong then films on the electrodes.

1BB.3 (*2PO1-115) Local Studies of Instabilities and Memory Effects in NbSe₂

Yossi Paltiel 1, Eli Zeldov 1,2, Youri N. Myasoedov 1, Hadas Shtrikman 1, Shobo Bhattacharya 3,4, Mark J. Higgins 3, Zhili L. Xiao 5, Eva Y. Andrei 5, Peter L. Gammel 2, and David J. Bishop 2. 1 Department of Condensed Matter Physics, The Weizmann Institute of Science, Rehovot 76100, Israel. 2 Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974, USA. 3 NEC Research Institute. 4 Independence Way, Princeton, NJ 08540, USA. 4 Tata Institute of Fundamental Research, Mumbai-400005, India. 5 Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08855, USA.

Presenting Author: Y. Paltiel

Recent studies have shown a number of surprising vortex dynamics phenomena both in low and high temperature superconductors, which include: low frequency noise, slow voltage oscillations, history dependent dynamic response, memory of the direction, amplitude, duration, and frequency of the previously applied current, suppression of a large ac vortex response by a very small dc bias, and a strong frequency dependence. Taken together, these phenomena are incompatible with the current understanding of bulk vortex dynamics. We propose a generic mechanism to account for these observations in terms of the competition between the injection of a disordered vortex phase through the surface barriers at the sample edges, and the annealing of this metastable disorder by the transport current. The model is confirmed by investigating the current distribution across NbSe₂ single crystals using arrays of Hall sensors. For an ac current only narrow regions near the edges are in the pinned disordered phase resulting in a large response. In the presence of a dc bias a wide region of the sample is filled by the disordered phase preventing vortex motion. The resulting spatial variation of the disorder across the sample acts as an active memory of the previously applied current sequence.

1BB.5 (*2PO9-104) Influence of microwave irradiation on excitation of non-Josephson oscillation (NJO) in superconducting Sn films with PSCs and SNS structures

Presenting Author: O. Agafonov

Abstract not available.

1BB.6 (*1PO3-41) The search for superconductivity in PrBa₂Cu₃O_7-d

Fernando M. Araujo-Moreira 1, P.N. Lisboa Filho 1, S.M. Zanetti 2, E.R. Leite 2, A.W. Mombru 3, L. Ghivelder 4, Y.G. Zhao 5, W.A. Ortiz 1, and V.T. Venkatesan 5. 1 Grupo de Supercondutividade e Magnetismo, Departamento de Física, UFSCar, São Carlos-SP, 13565-905 Brazil. 2 Lab. Interdisciplinar de Eletroquímica e Cerâmica, Departamento de Química, UFSCar, São Carlos-SP, 13565-905 Brazil. 3 Lab. de Cristalografia, Facultad de Química, Universidad de la República, Uruguay. 4 Instituto de Fisica, UFRJ, C.P. 68528, Rio de Janeiro, RJ 21945-970, Brazil. 5 Center for Superconductivity Research, Department of Physics, University of Maryland at College Park, MD 20740-4111.

Presenting Author: F.M. Araujo-Moreira

The existence (or not) of superconductivity in PrBa₂Cu₃O_7-d (Pr-123) has been during the years a very polemic subject. Blackstead et al. [1] and Zou et al. [2] have reported superconductivity in thin films and powders, and single crystals of Pr-123, respectively. We have recently reported strong evidence of superconductivity in sintered-polycrystalline samples [3] of Pr-123 and direct confirmation of that property in thin films of the same material [4]. We have characterized the structure of all samples by using x-ray diffraction and scanning electron microscopy techniques. Rietveld refinements were used to determine the crystal quality of the bulk samples. Magnetic characterization was performed by measuring the magnetization and DC magnetic susceptibility as a function of the temperature, T, and the applied magnetic field, H. Measurements were taken in the ranges 2K < T < 400 K and 0 < H < 10⁴ Oe. Thermal characterization was performed by measuring the specific heat in the same range of temperature. In this talk we describe the structural, thermal and magnetic characteristics of superconducting sintered-polycrystalline and thin films of Pr-123 samples and give a descriptive explanation about their unique dependence on the preparation procedure. Our results confirming superconductivity in the Pr-123 system could be associated to superconductivity outside the cuprate-planes strikingly indicating that this property might be originated elsewhere.

[1] Blackstead et al., Phys. Lett. A 207, 109-112 (1995).

[2] Zou et al., Phys. Rev. Lett. 80, 1074 (1998).

[3] F.M. Araujo-Moreira et al., LT22 Conference Proceedings; accepted to be published in Physica B.

[4] F.M. Araujo-Moreira et al.; submitted to Phys. Rev. Letters (1999).

1BB.7 (*4PO4-22) Neutron (p) Resonance in Underdoped Bi2212 and its relation to the electronic spectra as measured by ARPES

Presenting Author: J. Mesot

Abstract not available.

1BB.8 (*4PO8-85) Possible RT Superconductor from PbCO3-2PbO-Ag2O System

Presenting Author: D. Djurek

Abstract not available.