Superconductivity in Graphite Intercalation Compounds

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186 In contrast to C8M, charge transfer in the case of C6Li and stage 2 alkalimetal GICs is believed to be unity [11], i.e., empty intercalant bands. For ternary GICs, the amount of charge transfer is not yet known. However, considering the electron affinity of Hg and Tl, the intercalant MHg and MTIl.5 should act as weak donors, so that charge transfer should be rather small. Thus, for the ternary GICs, we again have a coexistence of two distinct electron systems. It is noticeable that all of the superconductive GICs have two electron systems basically independent of each other. A natural question is then, "Which of the two is responsible for the superconductivity?" This question is of particular interest for CAM,of which neither constituents are superconductive. In thi:s paper, we discuss the superconductivity in GICs with these unique features, primarily on the basis of our experimental results on the ternary GICs.' Emphasis is placed upon the two-dimensional anisotropy these GICs exhibit. And it shall be demonstrated that the two-dimensionality is controlled by stage and pressure. EXPERIMENTAL Samples of C4 nMHg (M:K,Rb,n=1,2) and C4 KTI1 5 were prepared from chips of highly oriented pyrolytic graphite (HOPG) of Onion Carbide ZYA grade. Our sample preparation method, which essentially follows the procedures reported earlier [12,13], is described in Refs. 7 and 9. The layer structures ofC4 KHg and C8KHg are shown in Fig. 1.

Cg K

cKgH

stoge 1

C4KHg

Fig.

stage 2

cHKHg

1. Crystalline structures of C4 nKHg.

These ternary GICs are air-sensitive, although apparently not as sensitive as C•M. The samples were protected against exposure to air, either by wrapping with Parafilm (American Can Co.) or by immersing into degassed silicone oil (in the case of pressure experiments). The reproducibility of the data assured that a reasonably good sample protection was achieved by either of the methods. After each experiment, the staging fidelity was checked by X-ray diffraction measurements. A 3He evaporation cryostat was used to attain low temperatures down to 0.4K. Temperature was measured by a carbon resistor thermometer calibrated against 4 14eand 3 He saturated vapor pressure. Superconductive transitions were detected by the measurement of low frequency (typically 27Hz) a.c. susceptibility. The amplitude of the modulation field was typically 0.20Oe. A static magnetic field was applied by an electromagnet in directions varying from parallel to perpendicular with respect to the c-axis of the sample. Superconductivity measurements were also carried out at elevated pressures up to 6 kbar, by use of a clamp-type piston-cylinder cell. The "frozen" pressure was measured by a manganin wire pressure gauge. Details of the pressure experiment are given in Ref. 14.

187 RESULTS We have investigated 17 samples of ternary GICs. For five out of them, we have also studied the effect of pressure. Angular dependence of Hc2. C8 K is reported to exhibit either type I or type II superconductivity depending on the magnetic field orienta

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