Anisotropic Thermal Conductivity of Superconducting Lanthanum Cuprate
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ANISOTROPIC THERMAL CONDUCTIVITY OF SUPERCONDUCTING LANTHANUM CUPRATE D.T. MORELLP, G.L. DOLL', J.P. HEREMANS*, H.P. JENSSEN", A. CASSANHO**, and M.S. DRESSELHAUS* * Physics Department, General Motors Research Laboratories, 30500 Mound Road, Warren, MI 48090
"* Center
for Materials Science and Engineering, M.I.T., 77 Massachusetts Avenue, Cam-
bridge, MA 02139 ABSTRACT The thermal conductivities of superconducting, Sr-doped lanthanum copper oxide single crystals have been measured from room temperature to below 100 mK parallel and perpendicular to the copper oxide planes. While the results indicate that the heat conduction is strongly anisotropic, the data have been analyzed in terms of a modified BardeenRickhayzen-Tewordt theory of lattice thermal conductivity. It is shown that while electron scattering plays an important role in limiting the in-plane heat conductivity, this scattering channel is masked by other mechanisms for heat transport across the planes. The thermal conductivity of high temperature superconductors has been extensively studied by many groups, and two excellent reviews of the subject have appeared in the literature. [1,2] Nearly all the experimental investigations to date have however been carried out on sintered materials, and while many aspects of heat conduction in the various sintered materials are qualitatively similar to one another, important differences exist in the magnitude and temperature dependence of the thermal conductivity measured by different workers. It is therefore reasonable to conclude that there are probably effects occurring in the sintered material (no doubt related to their porosity, grain boundary scattering, and inhomogeneity) that often mask the intrinsic behavior of these systems. Thus there is a critical need to extend the experimental information on heat conduction over a large temperature range (from room temperature to sub-Kelvin temperatures) to single crystal samples. This has provided the motivation for the present study. We have therefore undertaken a systematic investigation of the thermal conductivity of the most basic of the high temperature superconductors, the lanthanum (strontium) copper oxide system. On the parent material, La 2 CuO4 , we have reported [3] results on the thermal conductivity K(T) and the susceptibility x(T) from 10 K to 300 K in directions both parallel [110] and perpendicular [001] to the spin orientation axis. The results of K(T) for the undoped material were found to correlate with temperature-induced changes in the magnetic structure as identified by magnetic susceptibility, and with the phonon mean free path as determined by specific heat measurements. Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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Doping with small amounts of strontium (x = 0.05 and x = 0.10 [4]) is found [5] to alter the heat transport in La 2 _zSr.CuO 4 considerably from the case of the undoped, parent and compound. The high temperature range of K(T) for the x = 0.05 sample parallel (11) perpendicular (1) to the copper-oxide
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