Physica C 162-164 (1989) 508-509 North-Holland EVIDENCE FOR AN INTRINSIC LINEAR TERM IN THE T-DEPENDENCE OF THE THERMAL CONDUCqTvrrY OF SUPERCONDUCTING YBa2Cu30 r AND Bi2Sr2CaCu2Os SINGLE CRYSTALS G. SPARN, M. BAEN1TZ, S. HORN, F. STEGLICH, W. ASSMUS*, T. WOLF s, A. KAPITULNIK ÷, Z.X. ZHAO # Institut for Festk6rperphysik, Technlsehe Hochschule Darmstadt and SFB 252, D-6100 Darmstadt, F.R.G. ; *Universit~it Frankfurt and SFB 252, D-6000 Frankfurt, F.R.G.; SKernforsehnngszentrum Karlsruhe, 1TP, D-7500 Karlsruhe, F.R.G.; ÷Dpt. of Appl.Phys., Stanford University, CA-94305, USA; #Institute of Physics, Academia Sinica, Beijing, P.R.China The thermal conductivity (T) of superconducting YBa2Cu30 z and Bi2SrzCaCuzOa single crystals contains an intrinsic non-phononie contribution for T<0.5K which variesq.inear in temperature and very likely reflects the presence of light normal carriers far below T e . INTRODUCTION. Linear terms in the low temperature specific heat of high temperature superconductors (HTSC) have been found to depend strongly on temperature and magnetic field and have thus been attr~uted to spin glass effects caused by local defects. 1 This would mask any possible intrinsic linear term -tiT.While such local spin-glass excitations do not contribute to heat transport, the low energy excitations (LEE) giving rise to ~tiT may be observable in a thermal conductivity (TC) experiment. In fact, TC results on polycrystalline oxide superconductors like YBa2Cu30 r (YBCO), (La 1.xSrx)CuO4 or BaPb.rsBi.250 3 show a contribution linear in T at very low temperatures in addition to the ordinary phonon contribution ~T~ (boundary scattering) as T-*0 .2,5 Consequently an expression ,c(T)=aT + bT3 fits the data points very well for T< 1K as reported earlier by us and other groups. 2'3'4 The fact that this linear term is only present for superconducting samples, while it is absent in samples where superconductivity is suppressed (e.g. YBCO?_t, a>0.5), proves its relation to the superconducting state of these materials.Moereover, Cohn et al.6 have shown that when oxygen is extracted from and reloaded into a YBCO ceramic, the aT term reversibly vanishes and reappears, although the size and distribution of the grains did not change. The latter observations seem to be inconsistent with an interpretation of the low-T TC data in terms of both phonon (Rayleigh) scatteringr and extrinsic TC contributions by normal metallic impurity phases. To substantiate these previous conclusions, we present in this paper results of TC measurements demonstrating the existence of a linear term in shade crystals of YBCO and BSCCO. SAMPLES. We have investigated four samples, prepared by different groups, i.e. flux grown and followed by an anneal in an oxygen flow for longer than one week.The dimensions of the specimens are: lxlx2.5 mm 3 for YBCO (I); 1.54x2x3 mm 3 forYBCO (II); 1.9xl.6x5.2 mm 3 for BSCCO (HI) and 1.2mm in diameter and 7mm long for BSCCO (IV). RESULTS. In Figs.la and lb we plot the TC data of YBCO samples (I) and (II) and BSCCO sample (HI) versus temperature on a double logarithmic scale .The TC curves of the two BSCCO crystals measured are virtually identical, confLrming their intrinsic nature.The solid lines represent a fit of ~(T)= aT + bT 3 to the TC data measured within the a/b-plane.The straight lines correspond to linear and cubic T-dependence, respectively, and describe the asymptotic behavior of the measured curve (for the coefficients refer to Table 1). This fit describes the TC of all samples below T=300 mK with a deviation of less than 5%.A fit to a T 2 dependence as used by Graebner et al.r produces a much larger deviation of approximately 15%, with the departure in digression largest at low temperature. It is obvious from Table 1 that the linear term is about the same size in all samples, but becomes less apparent as the grain size increases, i.e., going from polycrystal (V) to single crystal samples.The flattening of ~(T) (for temperatures T> 0.5K) in the single crystal data of Fig.1 compared to polycrystal data 2 is ascribed to a larger dislocation density in the former: the phonon-dislocation cross section varies as T -2 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)