2PO9-70 Fluctuation conductivity and way of pairing in a Bi-2212 thin film

Stamatis K. Patapis, Physics Department, Solid State Section, University of Athens, Panepistimiopolis, Athens GR 15784, Greece

Presenting Author: S.K. Patapis

There are many experiments to investigate the way of pairing (s or d) in the new high temperature superconductors, paraconductivity measurements are not included in the above group. Although, fluctuation conductivity behavior may indicate the presence or absence of one way of pairing. Detailed measurements of this kind made in a thin film of Bi-2212 showed the absence of the Maki-Thompson effect (present in YBaCuO thin films) and consequently the absence of the s-pairing mechanism.

2PO9-71 New Resistance Anomaly in Superconducting Zn-doped Bi₂Sr₂CaCu₂O₈ Single Crystals

Yuqin Zhou, Zhaojia Chen, Ping Zheng, Duo Jin, and Nanlin Wang, Institute of physics Center for Condensed Matter physics, Chinese Academy of Sciences, P.O. Box 2711, Beijing, P.R. China

Presenting Author: D. Jin

We have measured the magnetoresistance of some Zn-doped superconducting single crystals. It is found that for a small driving current (<20MA) the resistance displays a very sharp rise just below T_c and before the zero-resistance state is reached. The resistance anomaly is quenched by a magnetic field. We propose that the resistance peak is a manifestation of quasi-reentrant behavior resulting from an intrinsic granularity of p-type high temperature superconductors.

2PO9-72 Permanent Photo-Induced Changes in the Transport Properties of Bi₂Sr₂CaCu₂O_8+d Single Crystals

Bing Zhao, Wenhai Song, Jianmin Dai, Weidong Huang, Minghua Pu, Xingcai Wu, Yuping Sun, and Jiaju Du, Institute of Solid State Physics, Academia Sinica, P.O. Box 1129, Hefei 230031, China

Presenting Author: B. Zhao

We have observed permanent photo-induced changes in the transport properties of Bi₂Sr₂CaCu₂O_8+d single crystals. Both normal-state conductivity and T_c for underdoped and overdoped single crystals decrease after illuminating with He-Ne laser light at 20K for 1 hour. No relaxation is observed when the temperature is elevated to room temperature. It is believed that photo doping causes a displacement of oxygen atoms in Bi-O double layers in BSCCO and local structure distortion which will lead to the localization of carrier. Our experimental results can be explained based on the competition of above two photo-induced effects.

2PO9-73 Transport properties and electron paramagnetic resonance study of Bi₂Sr₂Ca_1-xY_xCu₂O_z system

Gaojie Xu 1,2, Zejun Din 1, Zhiqiang Mao 2, and Yuheng Zhang 2. 1 Department of Astronomy and Applied Physics, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China. 2 Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China.

Presenting Author: G. Xu

Polycrystalline samples of Bi₂Sr₂Ca_1-xY_xCu₂O_z system have been prepared by the conventional solid-state reaction method. The charge transport and magnetic behavior have been studied by resistivity and electron spin resonance (ESR). Experimental results indicate that Y³⁺ doping induces the rapid increase of resistivity. And the ESR signal which has been revealed to come from Cu²⁺ spins intensifies with Y content increasing. The relation between structural distortion and Cu²⁺ spins localization, as well as the Cu²⁺ spin scattering and transport properties are discussed in this paper.

2PO9-74 Experimental Evidence for the Generation and Annihilation of Free Vortices at the Edges in BSCCO (2212) whiskers.

I.G. Gorlova 1, V.Ya. Pokrovskii 1, V.N. Timofeev 2, and S.G. Zybtsev 1. 1 Institute of Radioengineering and Electronics of Russian Academy of Sciences, Mokhovaya 11, 103907 Moscow, Russia. 2 Baikov Institute of Metallurgy of Russian Academy of Sciences, Leninskii Prospect 49,117911 Moscow, Russia.

Presenting Author: I.G. Gorlova

It has been shown theoretically that the sample edges provide generation of additional free vortices, and higher dissipation is expected in small samples of 2D superconductors.

We have measured temperature dependencies of the resistance and current-voltage characteristics for BSCCO (2212) single crystal whiskers of different width, w=1-15 m m, as well as for whiskers with width steps. Higher resistivity, r , of narrow sectors of BSCCO whiskers was observed below the Berezinskii-Kosterlitz-Thouless transition temperature, T_BKT. However, the r (T) curves for narrow and wide sectors intersect at T=T_i, slightly above T_BKT, and at higher T the resistivity of the narrow sample is less. This effect was not predicted theoretically.

We explain the results by both generation and annihilation of free vortices at the edges. As the annihilation contribution is proportional to free vortices concentration, it prevails above T_BKT, resulting in the lower resistance of the narrow sector. At T_i the generation and annihilation effects of the edges in the narrow sector compensate each other. It was found that T_i decreases with increasing current, supporting the idea of free vortices annihilation contribution.

2PO9-75 Influence of Thermodynamic Fluctuations on the Thermoelectric Coefficient in Ag-doped Bi-samples

Alvaro Marino and Julio Rodriguez, Physics Dept., Universidad Nacional de Colombia, Bogota, Colombia

Presenting Author: A. Marino

We present a study of the thermoelectric coefficient L(T) of polycrystalline Ag-doped BSCCO superconducting samples. Polycrystalline samples of nominal composition (Bi_1.6Pb_0.4)Sr₂Ca₂Cu_3(1-x)Ag_3xO_10+d and (Bi_1.6Pb_0.4)Sr₂Ca₃Cu_4(1-x)Ag_4xO_12+d with 0<x<0.1 were analyzed. For low level doping (0£ x£ 0.005) the exponential results suggest that in the mean field region, L(T) be not affected by thermodynamic fluctuations of the superconducting order parameter (OPF).

2PO9-76 Magnetoconductivity measurements in Bi(2223) tape: evidence of critical fluctuations

Marina Putti 1, Dimitri V. Livanov 2, Andrei A. Varlamov 2, Maria R. Cimberle 1, Carlo Ferdeghini 1, and Antonio S. Siri 1. 1 INFM/CNR, Dipartimento di Fisica, via Dodecaneso 33, 16146 Genova, Italy. 2 Department of Theoretical Physics, Moscow State Institute of Steel and Alloys, 117936 Moscow, Russia.

Presenting Author: M. Putti

Magnetoresistivity measurements are performed in a Bi(2223) tape prepared by power in tube technique. Fluctuation-induced magnetoconductivity is studied above the zero-field critical temperature T_c(0) and for moderate magnetic fields of 0.04, 2, 4, 8 T. It is found that the Gaussian approximation for superconducting fluctuations underestimates the negative fluctuation magnetoconductance drastically in the temperature range T_c(0) < T < T_c(0)+20 K Taking into account the critical fluctuation contribution on the base of self-consistent Hartree approximation makes it possible to explain quantitively the data in terms of Aslamazov-Larkin contribution only. With a reasonable set of relevant microscopic parameters consistent with hose obtained in the zero-field fit, we describe the data at different magnetic field in the temperature range T_c(0)+0.1K < T < T_c(0)+20 K. From the analysis of magnetoconductivity data we have found the magnitude of zero-field Ginzburg number in Bi(2223) compound 0.09 ± 0.01, the effective anisotropy parameter 10^-3 and the upper critical field of the order of 150 Tesla.

2PO9-77 Excess Conductivity in BSCCO Crystals: Effects of a Short-Wavelength Cutoff

E. Silva 1, D. Neri 1, M. Esposito 2, R. Fastampa 2, M. Giura 2, S. Sarti 2. 1 Dip.di Fisica "E. Amaldi" and Unita' INFM, Universita' di Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy. 2 Dip. di Fisica and Unita' INFM, Universita' "La Sapienza", 00185 Roma, Italy.

Presenting Author: S. Sarti

We present measurements of the (a,b) plane resistivity above T_c in several BSCCO crystals. Measurements were taken through a multi-terminal technique, in order to accurately extract the in-plane resistivity. The measurements were taken as a function of the temperature, up to 250 K. The data do not follow simple Aslamazov-Larkin laws in temperature ranges wider than 1K. We perform a Ginzburg-Landau calculation in order to include the effects of a short-wavelength cutoff in the fluctuational spectrum in a nearly-two-dimensional superconductor. The resulting expression recovers the microscopic theory both in the vicinity of T_c (Aslamazov-Larkin limit) and far away from T_c. The theoretical result describes quantitatively the data in all crystals from (approximately) T_c-1K up to T = 1.3 T_c. Close to T_c, the departure of the theoretical expression from the data is in agreement with the prediction of the 2D-3D dimensional crossover. The cutoff is of order unity (in units of the coherence length), as expected. We find a clear correlation between the value of the cutoff and the critical temperature.

2PO9-78 Experimental Current-Voltage Characteristics of Bi2223/Ag Tapes and BSCCO Current Leads

Mario De Salve, Paolo Ferracin, Raffaella Gerboni, and Giuseppe Ruscica, Department of Energetic, Politecnico di Torino

Presenting Author: M.B. De Salve

The paper presents and analyses several tests aimed at representing the electrical characteristics of some samples of commercial Bi2223/Ag tapes (length: 0.1-1.15 m; nominal cross section: 3´0.3 mm) and of a cave cylindrical sample of pure BSCCO, (length: 12 cm; external diameter: 12 mm; internal diameter: 9.7 mm), in zero applied magnetic field.

The tests were developed using a cryogenic system based on a Gifford-McMahon cycle. The base stand, made of aluminum and teflon parts, is constituted by round and square rods assembled to create a cradle. Three different choices were made to hold the samples:

1. short samples are blocked with simple clamps; 2. long samples are rolled up on small insulated cylinders and fixed with chucks; and 3. the cylindrical sample is secured with special channel-designed clamps.

Two kind of tests were performed: at a constant dc-current value, we monitored the cooling-down and the switching-off phases (range: 300-40K); at a certain temperature (range: 120-50K), we varied the current value from 0.5 to 5 A (steps: 0.5 A).

As result, it was possible to estimate:

• the electrical resistivity of BSCCO and its fraction in the tapes;

• the critical current densities in the 70-105K temperature range.

2PO9-79 Percolation study in Superconductor-CuO binary mixture

D.R. Mishra, Dipten Bhattacharya, H.S. Zadeh, P.N. Dheer and R.G. Sharma, National Physical Laboratory, New Delhi-110012, India

Presenting Author: D.R. Mishra

The random binary mixture of BPSCCO/YBCO superconductors with CuO insulator was studied for wide range of compositions. Samples were prepared using usual solid state reaction technique, and were characterised by X-ray diffraction (XRD), SEM, resistivity, thermoelectric power (TEP) and ac magnetisation measurements. Attempts has been made to explain the percolation behaviour from the standard 3D lattice network model vis-a-vis disordered continuum model. The estimation of the exponent 't' was done using the conductivity pattern c=c'(P-P_c)^t, c being conductivity, P, the concentration and P_c , the percolation threshold value. The estimated values for 't' reveals compliance to the 3D lattice model in the case of BPSCCO+CuO mixture while compliance to the continuum model in the case of YBCO+CuO mixture. The percolation threshold in the thermoelectric power P_cs is found to be lower than the P_c. The reason could be the fact that the network for percolation along the direction of heat flow develops at a much lower 'P' where as for conduction a 3D chain is required which is formed only when a sizeable 'P' (=P_c) is reached. Samples with as low as 20% (by Weight) of the superconducting phase showed superconducting transition in resistivity whereas the percolation threshold is only ~10% in the case of thermoelectric power data.