Critical Field Measurements on Superconducting Graphite-KHG Multilayers
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CRITICAL FIELD MEASUREMENTS ON SUPERCONDUCTING GRAPHITE-KHG MULTILAYERSa A. CHAIKEN*, P.M. TEDROW*", and G. DRESSELHAUS** *Massachusetts Institute of Technology, Cambridge, MA 02139 "sFrancis Bitter National Magnet Laboratoryb, Cambridge, MA 02139 aSupported by AFOSR Contract #F49620-83-C-0011. bSupported by NSF. ABSTRACT Upper critical fields of graphite-KHg multilayers with 10A periodicity were measured as a function of angle and temperature. The Hc2 (0, T) data were compared to the anisotropic Ginzburg-Landau model and were found to be in qualitative agreement, except at the lowest reduced temperatures, where significant deviations are found. The primary deviations from the anisotropic GL model are first, that the values of the critical field at the lowest temperatures are found to be higher than that predicted by the model, and secondly, the critical field anistropy ratio is found to be temperaturedependent. These deviations are discussed in light of more detailed models of anisotropic superconductivity. INTRODUCTION The class of layered superconducting materials has assumed increased importance as a result of the recent discovery of the high-Tc perovskite superconductors, which possess highly anisotropic properties.[1] High-Tc perovskites belong to the group of chemically grown multilayers, which includes graphite intercalation compounds (GIC's), and transition metal dichalcogenides and their intercalation compounds. Another new group of layered superconductors could be termed synthetically structured, since they are prepared using sputtering and evaporation techniques. [2] There axe several striking similarities in the superconducting properties of these various layered materials, such as Tc-dependence on layer thickness[3] and unusual temperature variation of the upper critical field H, 2 .[4] An understanding of the nature of these and other anomalies in the superconductivity of one type of layered material, the superconducting GIC's, may give some insight into the properties of the entire class. The graphite intercalation compounds are particularly interesting to study because of their unique structure, which in the stage n compound consists of n graphite layers in alternation with an intercalant metal unit (here, a trilayer of K and Hg). The structure[5] of C 4KHg, a typical superconducting GIC, is shown in Figure 1.
Figure 1: Structure of the stage I KHg-GIC C4 KHg. Mat. Res. Soc. Symp. Proc. Vol. 103. 01988 Materials Research Society
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The upper critical fields of the synthetically layered superconductors have been among their most widely studied properties, since H, 2 parallel to the layer planes is strongly affected by the layering.[2] A thorough experimental study of H12 in KHgand RbHg-GIC's was performed by lye and Tanuma,[6] who interpreted their data in terms of an anisotropic version of the standard Ginzburg-Landau theory. [2] This model describes the critical field anisotropy in terms of a single parameter
Hc2= _=[mr]1/
' Hc2,
_U
(1)
Lmc J
which, since e is related to the effective m
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