Superconductivity and Intralayer Structure in Potassium Amalgam-GIC
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G. TIMP,*a B.S. ELMAN,§O M.S. DRESSELHAUS*O and P. TEDROWt Massachusetts Institute of Technology, Cambridge, MA 02139,
USA
ABSTRACT We report on the structural, lattice and electronic properties of stage 1,2,3 potassium-amalgam GIG and the relation of these properties to the observed superconducting transition temperature. The ultramicrostructure of the intercalant domains is investigated using lattice fringe imaging. Our observation of macroscopic (V3xV3),(2x2) and (V3x2) domains is consistent with the basal plane zonefolded phenomena observed in the Raman spectra. Shubnikovde Haas measurements of the Fermi surface show high frequency (>1000 T) oscillations in stage 2, identified with the intercalant band.
INTRODUCTION It is possible by varying the stage index to study the effects of interlayer coupling on the superconducting to normal transition in graphite intercalation compounds (GIG), provided the fidelity of the in-plane superlattice can be maintained. This opportunity has been tacitly exploited by recent susceptibility measurements [1,21 involving stage 1 and 2 potassium amalgam-GIG, a type II anisotropic superconductor. The intralayer structure of potassium amalgam-GIG vs stage is especially interesting because of the observed anomalous stage dependence of the superconducting transition temperature (Tc = 0.70 K for the stage I compound, Tc = 1.90 K for the stage 2 compound [1-31) and because compounds of stage n > 3 have recently been fabricated [4,5]. Previous work [1] has suggested that dimensional crossover might occur for stage n = 3, where the c-axis coherence distance would be less than the superlattice repeat distance. It is shown in this work that the stage 1, 2 and 3 potassium amalgam-GIG exhibit a variety of commensurate a-axis intercalant ordering schemes depending on preparation conditions. The extent and type of ordering are manifested in the measured structural, lattice and electronic characteristics. We review below our investigation of the ultramicrostructure of potassium amalgam-GIG with high resolution electron microscopy. These results are significant because of the discovery of large, dislocation-free intercalant domains, with different in-plane ordering, thus supporting our observation of in-plane zone folding phenomena in the Raman spectra. The Fermi surface of the stage 2 and 3 compounds has been studied with the Shubnikov-de Haas technique to elucidate details of the electronic structure. Finally, the relationship between these results and the superconducting transition is delineated with a report of the critical temperatures. *Department of Electrical Engineering and Computer Science. §Department of Physics. °Center for Materials Science and Engineering. lFrancis Bitter National Magnet Laboratory.
mat. Res. Soc. Symp. Proc. Vol. 20 (1983)
zElsevier Science Publishing Co.,
Inc.
202
EXPERIMENTAL DETAILS Our preparation of the potassium amalgam-GIC follows the method of Lagrange et al. [31 and utilizes a two-zone technique. To prepare the stage 1 compound, 0 pristine KHg metal
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