2PO9-50 Application of the Photoacoustic Method for Characterization of YBa₂Cu₃O_7-x

Pantelija M. Nikolic 1, Dragan M. Todorovic 1, Stevan S. Vujatovic 1, Stevan Djuric 1, Dana, G. Vasiljevic-Radovic 1, Katarina T. Radulovic 1, Vladimir Blagojevi 2, Dragan Urosevic 3, Jovan Elazar 2, Pene Mihajlovic 1, and Aleksandar Bojicic 1. 1 Joint Laboratory for Advanced Materials of SASA, P.O. Box 745, Belgrade 11000, Yugoslavia. 2 Faculty of Electronic Engineering, University of Belgrade, Belgrade, 11000, Yugoslavia. 3 Mathematical Institute of SASA, Belgrade, 11000, Yugoslavia.

Presenting Author: P.M. Nikolic

The possibility of simultaneous determination of thermal diffusivity, optical absorption coefficient and some transport properties of polycrystalline YBa₂Cu₃O_7-x pellets using a photoacoustic (PA) method was investigated. Both the amplitude and phase PA signals were measured at room temperature as a function of the modulation frequency of the laser He-Cd (25 mW), in the frequency range between 10 Hz and 3000 Hz. The normalized experimental PA amplitude and phase spectra for three different thicknesses (480 m m, 410 m m and 345 m m) of the YBa₂Cu₃O_7-x sample were then numerically analyzed by fitting the experimental curves with theoretically calculated PA signals of this superconducting material.

The thermal diffusivity coefficient D_T» 2 x 10^-7 m²/s, the optical absorption coefficient a » 8000 m^-1, the electron diffusion coefficient D» 0.2 x 10^-3 m²/s, the thermal conductivity K nearly equal to 1 W/ms and the excess carrier lift time t » m s, were determined at room temperature, using both the amplitude and phase PA diagrams.

2PO9-51 Normal-state behavior of the resistivity and the magnetic susceptibility in the superconducting YBa₂(Cu_1-xLi_x)₃O_7-d system (0.0£ x£ 0.06): an evidence of enhanced scattering by the spin fluctuations

S. García 1, R. Cobas 1, J.E. Musa 2, and E.M. Baggio-Saitovitch 2. 1 Laboratorio de Superconductividad, Facultad de Física, Universidad de la Habana, San Lázaro y L, 10400, Cuba. 2 Centro Brasileiro de Pesquisas Físicas, Rua Xavier Sigaud 150, Urca, 222090-180, Rio de Janeiro, Brazil.

Presenting Author: S. Garcia

A systematic study of the normal-state resistivity and magnetic susceptibility of the YBa₂(Cu_1-xLi_x)₃O_7-d system with 0.0£ x£ 0.06 was performed. The normal-state resistivity was found to increase with the Li content, while an upturn in the curves appears as the superconducting transition is approached, becoming more intense with doping. This deviation from linearity was properly accounted by a logarithmic term, whose coefficient C smoothly increases with x. The critical temperature T_c was found to be gradually suppressed at a rate of 5.5K at.% of Li. An estimation of the mean free path for the most resistive sample gives l » 35 Å, which is larger that the coherence length, indicating that nonmagnetic disorder at the CuO₂ planes is not the source of this effect. However, the observed variations in T_c clearly correlate with C, suggesting that the excitation which interacts with the carriers in the normal state has a leading role as a pair-breaking mechanism. Fits of the normal-state magnetic susceptibility curves at zero external magnetic field evidence that the Pauli contribution is gradually reduced with doping, while a small induced magnetic moment (m _ind ~ 5 x 10^-4 m _B) emerges, pointing to an enhancement of the antiferromagnetic correlations. A similar effect is observed as the external dc field is increased up to 9 T for a fixed concentration. Under the assumption that the logarithmic anomaly comes from scattering of the carriers by the spin fluctuations at the CuO₂ planes, a consistent explanation is presented in terms of the effect of disorder on the spin dynamics. Other possible sources for the lnT behavior were considered and found to be inconsistent.

2PO9-52 E-J Characteristics of (Y_1-xPrx)Ba₂Cu₃O_7-d Thin Films Near S-N Transition

Chunguang Li 1, Duo Jin 1, Yusheng He 1, Guangcheng Xiong 2, Guijun Lian 2, and Daole Yin 2. 1 Institute of Physics & Center for Condensed Matter Physics, CAS, P.O. Box 2711, Beijing, 100080, P.R. China. 2 Department of Physics, Peking University, Beijing, 100871, P.R. China.

Presenting Author: D. Jin

Study of resistivity-temperature and isothermal E-J characteristics of (Y_1-xPrx)Ba₂Cu₃O_7-d (x=0, 0.1, 0.2 and 0.3) thin films in different magnetic fields up to 4 Tesla has been carried out. At magnetic fields below upper critical field H_c2(T), a nearly constant fluxflow resistivity was observed for J<<J_d, the depairing current density, as predicted by Bardeen and Stephen. Special attention was paid to the regime when J approaches J_d. At this regime the resistivity grows rapidly and eventually saturates, reaching the normal state resistivity (n. Features of this crossover is discussed with Ginzburg-Landau theory and a phenomenological expression is derived, where (f₀ is the constant fluxflow resistivity.

Excellent fits to the measured E-J characteristics of (Y_1-xPrx)Ba₂Cu₃O_7-d thin films by using above expression can be obtained, and the values of the dimensionless factor p and the depairing current J_d can be then deduced, suggesting a possible new way of measuring the critical parameters (J_d, H_c2) from E~J curves.

*2PO9-53 The Condensation Energy and Upper Critical Field in Y_0.8Ca_0.2Ba₂Cu₃O_6.7 from Specific Heat Measurement

J.L. Luo 1,2, J.W. Loram 1, J.R. Cooper 1, and J. Tallon 1. 1 IRC in Superconductivity, University of Cambridge, Cambridge CB3, 0HE, United Kingdom. 2 Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China.

Presenting Author: J.W. Loram

The temperature dependence of the upper critical field (H_C2) is still being understood for high T_c cuprates because of strong thermal fluctuations around T_c and extremely high H_C2 value at lower temperatures. Using a high-resolution differential technique, we determined the electronic specific heat of optimal doped Y_0.8Ca_0.2Ba₂Cu₃O_6.7 from liquid Helium temperature to room temperature in applied magnetic field up to 13T. From the temperature and field dependence of the superconducting free energy density, obtained by integrating the electronic entropy, the magnitude and temperature dependences of H_C1 and H_C2 are deduced using the London model. The variation of these quantities with oxygen content is also discussed.

2PO9-54 Comparison of the electromagnetic pulse emission from YBaCuO and (Y,Pr)BaCuO excited by femtosecond laser pulses

H. Wald 1, C. Steigmeier 1, P. Seidel 1, S. Nashima 2, M. Tonouchi 2, and M. Hangyo 2. 1 Institut fuer Festkoerperphysik, Friedrich-Schiller-Universitaet Jena, Helmholtzweg 5, D-07743 Jena, Germany. 2 Res. Ctr. Supercond. Mat. and Electron., Osaka University, Osaka 565-0871, Japan.

Presenting Author: P. Seidel

Femtosecond laser pulses can modulate the supercurrent by breaking the current carrying Cooper pairs, which generate an electromagnetic pulse with their recombination on a picosecond time scale. The electrodynamic properties of the superconductors YBaCuO (YBCO) and Pr-doped YBCO (YPBCO) make both materials suitable for devices using such a terahertz (THz) source. We measured transmission of YBCO and YPBCO thin films in a frequency range from 0.25 to 1.5 THz at temperatures between 10 to 300 K. We compared these results to the observed THz radiation emitted from laser excited dipole antennas. Amplitude and shape at different temperatures and different bias currents were investigated. An influence of the flux motion in the films on the emitted radiation amplitude was observed. The higher transmission of YPBCO suggested a higher THz radiation signal power, what could be observed.

2PO9-55 Transport Properties of Pr_0.5Ca_0.5(Ba_1-xSr_x)₂Cu₃O_7-y (x=0, 0.1, 0.2, 0.3) Epitaxial Thin Films

Z.H. Wang, G.J. Lian, Y.F. Sun, and G.C. Xiong, Department of Physics and Mesoscopic Physics National Laboratory, Peking University, Beijing 100871, P.R. China

Presenting Author: Z.H. Wang

Pr_0.5Ca_0.5(Ba_1-xSr_x)₂Cu₃O_7-y (PCBSCO)(x=0, 0.1, 0.2, 0.3) epitaxial thin films have been prepared on SrTiO₃, LaAlO₃, and Y-ZrO₂ single crystal substrates by pulsed laser deposition method. By 10% Sr substituting for Ba, the T_c(R=0) of the Pr_0.5Ca_0.5Ba₂Cu₃O_7-y films increases from 35K(x=0) to 39K(x=1). The resistance measurments showed that PCBSCO (x=0.1) films have smaller resistivity compared with the PCBSCO(x=0) films. Unfortunately, when increasing the substitution concentration x continuously to 0.2 and 0.3, the resistivity of the films begins to increase and the superconductivity is gradually suppressed. The effects of the oxygen content on the superconductivity of PCBSCO have also been studied. The optimum annealing condition for T_c was found to be in an oxygen pressures around 0.6 atm at 350. Annealed in higher and lower oxygen pressures will result in lower T_c(R=0).

2PO9-56 Electrostatic Modulation of Superconductivity in Epitaxial Pb(Zr_xTi_1-x)O₃/GdBa₂Cu₃O_7-x Heterostructures

C.H. Ahn 2, S. Gariglio 1, P. Paruch 1, T. Tybell 1, L. Antognazza 1, and J.-M. Triscone 1. 1 DPMC, University of Geneva, 1211 Geneva 4, Switzerland. 2 Department of Applied Physics, Yale University, New Haven, CT 06520-8284.

Presenting Author: C.H. Ahn

Using the polarization field of the ferroelectric oxide Pb(Zr_xTi_1-x)O₃ in epitaxial Pb(Zr_xTi_1-x)O₃/GdBa₂Cu₃O_7-x heterostructures, we have electrostatically modulated superconductivity in the high temperature superconductor GdBa₂Cu₃O_7-x, shifting the doping level without introducing chemical or structural disorder. For slightly underdoped samples, a change in the normal state resistivity of 50% and a uniform shift of the superconducting transition of 7 K were observed. In more underdoped samples, an insulating state was induced. This approach allows one to switch reversibly between superconducting and insulating behavior, demonstrating the route to a nonvolatile superconducting switch, a device that is dissipationless in one of its states.

C.H. Ahn, S. Gariglio, P. Paruch, T. Tybell, L. Antognazza, J.-M. Triscone, Science 284, 1152 (1999).

2PO9-57 Anisotropy and transport properties of (Bi₂Sr₂CuO_6+d )_m/(CaCuO₂)_n multilayers obtained by molecular beam epitaxy

Matteo Salvato, Carmine Attanasio, Gerardina Carbone, Albino Montella, Tiziana Di Luccio, Serghej L. Prischepa, and Luigi Maritato, Dipartimento di Fisica and INFM Università di Salerno, 84081, Baronissi-Salerno, Italy

Presenting Author: M. Salvato

Structural and transport properties of (Bi₂Sr₂CuO_6+d )_m/(CaCuO₂)_n multilayers, obtained by molecular beam epitaxy alternately stacking m layers of Bi₂Sr₂CuO_6+d with n layers of CaCuO₂, have been studied. The study of the RHEED patterns and the observation of the RHEED intensity oscillations, together with the X-ray analyses, indicate that a layered structure has been achieved. The behavior of the critical temperature T_c vs. n seems to confirm a charge transfer mechanism between the Bi₂Sr₂CuO_6+d and the CaCuO₂ layers. The irreversibility lines have been extracted from the resistivity measurements performed in perpendicular and parallel external magnetic field and the superconducting anisotropy parameter g has been estimated for different samples.

2PO9-58 Transport properties in Nd_1+xBa_2-xCu₃O_7-d thin films in high magnetic fields

Matteo Salvato 1, Carmine Attanasio 1, Serghej L. Prischepa 1, Luigi Maritato 1, and Marco Salluzzo 2. 1 Dipartimento di Fisica and INFM Università di Salerno, 84081, Baronissi-Salerno, Italy. 2 Dipartimento di Fisica Università di Napoli, Piazzale Tecchio 80100, Napoli, Italy.

Presenting Author: M. Salvato

Transport measurements in external applied magnetic field up to 9 T have been performed on Nd_1+xBa_2-xCu₃O_7-d thin films. The samples have been obtained by sputtering with different Nd concentrations and show a dependence of the critical temperature on their stoichiometry. The pinning energy U vs. the external magnetic field B has been extracted for the different samples. The role of Nd/Ba disorder on the dependence of the U(B) shapes has been discussed. An explanation of this behavior has been proposed on the basis of the pinning theory.

2PO9-59 The significant drop in the resistance around 200K in superconducting Hg-based compounds.

Huimin Shao 1, C.J. Wang 1, K.F. Liang 2, S.A. Aruna 1, D. Su 1, T. Yang 1, J.S. Zhu 1, Y.M. Cai 3, and P.C.W. Fung 2. 1 Department of Physics and National Laboratory of Solid State Microstructures, Center for Advanced Studies in Science and Technology of Microstructures, Nanjing University, Nanjing, 210093, P.R. China. 2 Department of Physics and Center of Materials Science, The City University of Hong Kong, Hong Kong, China. 3 Department of Basic Sciences, Nanjing University of Chemical Technology, Nanjing, 210009, China.

Presenting Author: C.J. Wang

A significant drop in the resistance around 250K [J.L. Tholence et al. Phys. Lett. A 184(1994) 215] and 230K [Y.S. Yao et al. Physica C 234 (1994) 39] were repeatedly observed in the above experiments on superconducting Hg-compounds. We report here the electrical resistivity measurements on pure (as determined by X-ray powder technique) Hg-1223 phase, which has shown a partial or total drop in resistivity around 200K but with no significant observation of diamagnetic signal. We are sure that the drop in resistance for our Hg-based compounds at 200K is not due to the presence of liquid Mercury or Hg-Au alloys in the sample since our samples did not contain any Hg-Au alloys. So we therefore rather believe that the drop in resistivity is due to superconductivity or other mechanisms.