*2PO9-30 Mixed-state Hall effect of high-temperature superconductors under small and large Lorentz force

Wolfgang Lang 1, Wolfgang Gob 1, Ionut Puica 2, Roman Rossler 3, Johannes D. Pedarnig 3, Dieter Bauerle 3, and Roman Sobolewski 4. 1 Institut für Materialphysik, Universität Wien, Kopernikusgasse 15, A-1060 Wien, Austria. 2 Physics Department, Polytechnic University of Bucharest, 77206 Bucharest 6, Romania. 3 Angewandte Physik, Johannes-Kepler-Universität Linz, A-4040 Linz, Austria. 4 Department of Electrical and Computer Engineering, University of Rochester, Rochester, NY 14627-0231, USA.

Presenting Author: W. Lang

In many high-temperature superconductors (HTS) the mixed-state Hall effect has a sign opposite to that in the normal state. Some highly anisotropic HTS even exhibit a double sign reversal of the mixed-state Hall effect. Most experimental and theoretical studies to date have investigated on this anomaly in moderate magnetic fields between about 1 and 20 T and in low current densities.

We report our measurements of the Hall effect in thin films of YBCO and 2212-BSCCO with various doping levels, patterned to microbridge structures with potential probes. Large Lorentz forces on the vortices were achieved with high currents, applied in short pulses down to 300 ns. In this experiment the Lorentz force overcomes the pinning potential even at temperatures well below the critical temperature. The Hall conductivity increases steeply towards low temperatures and indicates that the scattering rate of the quasiparticles in the superconducting state is drastically reduced.

On the other hand, measurements of YBCO thin films in low magnetic fields down to 5 mT reveal a significant increase of the negative anomaly of the Hall coefficient and, surprisingly, the evolution of the double-sign change. The results are compared to several recent theoretical approaches.

2PO9-31 Giant enhancement of quasi-particle thermal Hall conductivity in high-purity crystals of YBaCuO *

Y. Zhang 1, N.P. Ong 1, D.A. Bonn 2, R. Liang 2, and W.N. Hardy 2. 1 Dept. of Physics, Princeton University, NJ 08544. 2 University of British Columbia, Vancouver, BC, Canada V6T 4M1.

Presenting Author: Y. Zhang

The synthesis of BaZrCO₃ crucibles has resulted in the growth of crystals of YBaCuO with very high purity. We have measured the quasiparticle (qp) a-axis thermal conductivity k_a and thermal Hall conductivity k_xy below T_c in fields B up to 14 Tesla. In zero B, the maximum value of ka exceeds 45 W/mK (compared with 26 W/mK typical of previous untwinned crystals). The most striking enhancement is observed in the weak-field kxy, which increases by a factor of 1000 between 93 K and 40 K. The behavior of kxy below 30 K appears to be qualitatively distinct from previous results. We will describe these results and discuss qp behavior in the limit of very long mean-free-paths.

*Supported by the U.S. Office of Naval Research.

*2PO9-32 The Wiedemann-Franz ratio in YBaCuO determined from thermal Hall conductivity*

Z.A. Xu 1, Y. Zhang 1, N.P. Ong 1, K. Krishana 1, R. Gagnon 2, and L. Taillefer 3. 1 Dept. of Physics, Princeton University, Princeton, NJ 08544. 2 McGill University, Montreal, Quebec, Canada. 3 University of Toronto, Toronto, Ontario, Canada M5S 1A7.

Presenting Author: Z.A. Xu

The Wiedemann Franz ratio, expressed as the Lorenz number L = ke/T*s , provides important information on the charge and entropy carriers. In the normal state of the cuprates, L is not known reliably because the phonon conductivity dominates the electronic thermal conductivity ke. To get around the phonon background, we have measured with high precision the thermal Hall conductivity k_xy (which does not involve phonons), and formed the Hall-Lorenz number L_xy = k_xy/Ts _xy. The method was checked against high purity copper. We find that L_xy in YBCO is linear in T and strongly suppressed from the Sommerfeld value, in conflict with previous claims. Implications of the finding will be discussed.

*Supported by the U.S. Office of Naval Research and National Science Foundation

2PO9-33 Universal T_c depression by irradiation in underdoped and overdoped cuprates

F. Rullier-Albenque 1, P.A. Vieillefond 1, H. Alloul 1, A. Tyler 2, P. Lejay 3, and F. Marucco 4. 1 Laboratoire des Solides Irradiés, CEA/CNRS, Ecole Polytechnique, 91128 Palaiseau cedex, France. 2 IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 OHE, United Kingdom. 3 CRTBT, CNRS, BP 166X, 38042 Grenoble, France. 4 LEMHE, Université Paris-Sud, 91405 Orsay cedex, France.

Presenting Author: F. Rullier-Albenque

Point defects were introduced by electron irradiation at low temperatures in the CuO₂ planes of YBa₂Cu₃O_7-d and Tl₂Ba₂CuO_6+x single crystals with various hole dopings n [1]. The variations of the critical temperature T_c and of the normal state resistivity r _2D(T) were followed in a large range of defect concentrations (from ~10^-3 to ~10^-2). For all these compounds the r (T) curves are found to be perfectly parallel, i.e. the Matthiessen’s rule is very well verified, allowing to determine very precisely the resistivity increase D r _2D due to irradiation in-plane defects.

A remarkable scaling between the decrease in T_c and D r _2D x n is found for all of the samples investigated here. This demonstrates that the hole doping is the relevant parameter to describe the transport properties from the underdoped to the overdoped regime, in contradiction with a recent suggestion of a change in the number of carriers from n to 1-n at the optimal doping [2,3]. Moreover, this universal correlation strongly suggests that D T_c is due to impurity scattering in a d-wave superconductor.

[1] F. Rullier-Albenque et al, Cond-Mat/9908405.

[2] Y. Fukuzumi et al., Phys. Rev. Lett. 76, 684 (1996).

[3] N. Nagaosa and P.A. Lee, Phys. Rev. Lett 79, 3755 (1997).

2PO9-34 Magnetic Field Induced Quasiparticle Transport in YBCO

R.W. Hill 1, Christian Lupien 1, Louis Taillefer 1, May Chiao 2, and Robert Gagnon 3. 1 Canadian Institute for Advanced Research and Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7. 2 Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom. 3 Department of Physics, McGill University, Montreal, Quebec, Canada H3A 2T8.

Presenting Author: R.W. Hill

The response of low temperature excitations to a magnetic field in the vortex state of a d-wave superconductor has been studied using measurements of thermal transport in optimally doped YBa₂Cu₃O_6.9. In contrast to s-wave superconductors we find the electronic thermal conductivity increases when fields up to 8T are applied. These results are consistent with theories which describe the increased occupation of extended quasiparticle states at the nodes of the superconducting gap in the presence of vortices. Moreover measurements on pure and Zn doped samples show the magnitude of the effect to be directly related to the quasiparticle scattering rate.

2PO9-35 Large, Linear C-Axis Magnetoresistance in YBCO

Fedor F. Balakirev 1, Yoichi Ando 2, Albert Passner 3, Jonathan B. Betts 1, Lynn F. Schneemeyer 3, Kouji Segawa 2, and Gregory S. Boebinger 1. 1 National High Magnetic Field Laboratory, Los Alamos National Laboratory, P.O. Box 1663, MS E536, Los Alamos, NM 87545, USA. 2 Central Research Institute of Electric Power Industry, 2-11-1 Iwato-kita, Komae, Tokyo 201-8511, Japan. 3 Bell Laboratories, Lucent Technologies, 700 Mountain Avenue, Murray Hill, NJ 07974.

Presenting Author: F.B. Balakirev

Underdoped YBCO single crystals exhibit a large, positive c-axis magnetoresistance that is strikingly linear over a wide magnetic field range up to 60T. This linear magnetoresistance is observed at temperatures near or below T_c, once sufficient magnetic field has been applied to suppress superconductivity. The measurements are done in the longitudinal geometry, with current and magnetic field both along the c-axis (I½ ½ B½ ½ c). The linear magnetoresistance can be large, giving as much as a 35% increase in resistance between 30T and 60T. It also exhibits very strong temperature dependence, increasing by as much as an order of magnitude as the temperature is reduced from 100K to 10K. The linear magnetoresistance is observed over a broad range of carrier concentrations in the underdoped regime, beginning as a relatively small effect in samples near optimal doping (T_c ~ 90K) and becoming larger as T_c is reduced to ~60K. The linear magnetoresistance abruptly vanishes in heavily underdoped samples (T_c < ~55K), which instead exhibit a small negative magnetoresistance similar to that reported in c-axis transport measurements of other cuprates, including Bi-2201, Bi-2212, and (La,Sr)₂CuO₄. These observations suggest a link between the large, linear c-axis magnetoresistance and partial doping of the Cu-O chains in YBCO, since both of these phenomena occur over roughly the same range of T_c in the underdoped regime.

2PO9-36 Magnetoresistivity of Strongly Underdoped YBa₂Cu₃O_x Single Crystals

E. Cimpoiasu 1, G.A. Levin 1, C.C. Almasan 1, Hong Zheng 2, and B.W. Veal 2. 1 Department of Physics, Kent State University, Kent, OH, 44242, USA. 2 Material Science Division, Argonne National Laboratory, Argonne, IL 60439, USA.

Presenting Author: E. Cimpoiasu

We report magnetoresistivity measurements on strongly underdoped YBa₂Cu₃O_6.36 single crystals in fields H up to 14 T, with H ½ ½ c-axis. The c-axis magnetoresistivity D r _c was determined for 100 K < T < 270 K using two techniques: the four point method and the flux transformer geometry, which allows simultaneous determination of r _c and r _ab. We observe several unusual features: the out-of-plane magnetoresistivity is negative and D r _c/r _c decreases in absolute value with decreasing temperature, in contrast to the opposite trend followed by conventional metals and most cuprates. The in-plane magnetoresistivity D r _ab/r _ab is positive and nonmonotonic in T, displaying a maximum at T » 200 K. We show that there is a direct correlation between the T-dependence of magnetoresistivities and the corresponding T-dependence of the zero-field temperature coefficients; i.e. D r _ab,c µ dr _ab,c/dT. Indeed, (dr _c/dT)/r _c is negative and decreases in absolute value with decreasing temperature, while (dr _ab/dT)r _ab is positive and nonmonotonic in temperature, with a maximum around 200 K. These correlations can be understood in terms of the interlayer incoherence and reduction of the in-plane phase coherence length with increasing magnetic field.

This research was supported by the NSF under Grant No. DMR-9801990 at KSU and the DOE under Contract No. W-31-109-ENG-38 at ANL.

2PO9-37 Noise properties of d-wave Superconducting Junctions

B.C. Yao, H.W. Cheng, S.H. Tsai, C.C. Chi, and M.K. Wu. Material Science Center and Department of Physics, Tsing-Hua University, Hsinchu, Taiwan 300, R.O.C.

Presenting Author: B.C. Yao

We have experimentally investigated the noise in a normal metal-d-wave superconductor (NS) junction with a {110} and {100} oriented interfaces at 99 kHz frequency where the noise power spectrum is white. The NS junctions are silver-YBa₂Cu₃O_y epitaxial thin films in planar junction geometry. The observed noise is significantly different from those in the normal metal-s-wave superconductor junction. The measured noise is compared to the existing theory base on scattering formalism. We have also measured the quasi-particle noise of 45° bi-epitaxial grain boundary YBa₂Cu₃O_y Josephson Junctions, i.e. YBCO/YSZ and YBCO/YBCO/YSZ, in c-axis parallel geometry. The measured noise contribution associated with the Andreev reflection is discussed.

2PO9-38 Size Effect of 1/f Noise in PrBa₂Cu₃O₇

Yan Gao, Yanjing Bei, Guijun Lian, Guangcheng Xiong, and Shousheng Yan, Department of Physics, Peking University, Beijing 100871, P.R. China

Presenting Author: S. Yan

Abnormally high level of 1/f noise in the normal state of bulk YBaCu₃O₇(YBCO) samples have been observed. For small-size samples, low noise level was reported recently. PrBa₂Cu₃O₇(PrBCO) has similar structure but exhibits quite different temperature dependence of conductivity. It's noise behavior will be interesting. In this work, the 1/f noise power spectral densities of c-axis oriented PrBCO thin films and a series of microbridge samples, patterned using standard photolithography, of widths 2-200m m and the length/width ratio of 2.5:1 were measured at temperatures from 80K to 300K. The noise level of bulk film decreases slightly with decreasing temperature, which is consistent with the reported YBCO results, but the noise level of all microbridges show a contrary temperature dependence. The room-temperature noise level, which is characterized by g value in Hooge's formula, of the microbridge samples drop with decreasing width from 10³ for 200m m bridge, which is at the same level in bulk films as well as in bulk YBCO films, to 10^-3 order for 2m m bridge, which is in the range of g value in semiconductors. Some possible mechanisms are also briefly mentioned.

2PO9-39 Anomalous thermal expansion behaviors in Sm-Ba-Cu-O superconductors

Masahiro Okaji 1, Naofumi Yamada 1, Atsushi Mase 2, Hiroshi Ikuta 3, and Uichiro Mizutani 2. 1 National Research Laboratory of Metrology, 1-1-4, Umezono, Tsukuba, Ibaraki 305-8563 Japan. 2 Dept. of Crystalline Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan. 3 CIRSE, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan.

Presenting Author: M. Okaji

The linear thermal expansion coefficients (LTECs) of c-axis oriented Sm-Ba-Cu-O superconductors have been measured in the range of 10 - 300 K by means of a high sensitive interferometric dilatometer. Four 123 phase Sm-Ba-Cu-O specimens including 25 wt% 211 phase were prepared with 2, 5, 10, and 20 wt% Ag2O ingredient. The 211 phase micro particles were introduced as pinning centers, and Ag2O was added as a mechanical reinforcement. The material is expected to be applied as strong superconducting permanent magnet, therefore it is useful to study its thermo-mechanical properties such as thermal expansion at cryogenic temperatures.

The LTEC along c-axis showed about 1.5 times larger than that along ab-plane for all specimens. The anisotropy in the LTEC is not strong compared to those of the other High-T_c superconductors such as Y-Ba-Cu-Os and Bi-Sr-Ba-Cu-Os. It is notable that the LTECs showed a bump along c-axis and a dent along ab-plane around 180 - 250 K especially for the 2 wt% and 5 wt% Ag₂O specimens. The anomalies became unclear for the 10 wt% and 20 wt% Ag₂O specimens and also with thermal cycles between room and cryogenic temperatures. These results suggest that an existence of the Ag₂O ingredient may moderate the observed macroscopic deformation or dimensional instability.