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Frequent reference has been made in Chapters gif and gif to differences between the measured properties of gold and pink specimens of C4KHg. In Table gif, a summary appears of the results of previous chapters regarding these two sample types. Although several distinctions between the pink and gold C4KHg have been pointed out, so far there has been no attempt at an explanation. Such an understanding is clearly very important for an understanding of superconductivity in GIC's since the two types of C4KHg have different superconducting properties.

Table: A summary of the known differences between the pink and gold C4KHg. The numbers here are representative of a typical sample of its type, although some variation was observed from sample to sample. Lambdaep is McMillan's electron-phonon coupling parameter.[165] kappa is the Ginzburg-Landau parameter discussed in Section gif; kappa ;SPMlt; 1/sqrt2 indicates type I superconductivity. The density of states, N(0), shown here was calculated in Section gif from the Hc2(theta) data. NA means not applicable.

The most noticeable aspect of Table gif is the lack of knowledge for the pink phase about the properties (thetaD, Lambdaep, N(0)) that are most directly connected to Tc, the zero-field superconducting transition temperature. Clearly a specific heat experiment on a Tc = 1.5 K C4KHg sample is highly desirable. (The specific heat experiment of Alexander et al. did not indicate superconductivity in C4KHg down to 0.8 K.[8] The samples used were said to be ``pink-copper'' in color.[8,148])

The other block to progress is the impossibility of preparing pure beta-phase ( Ic = 10.83 Å) samples.[147] From the information in the table it is hard to discern whether the presence of the beta phase is by itself sufficient to depress Tc from 1.5 to 0.8 K. The gold samples might also be more disordered in-plane than the pink ones, for example.[247] Also, there is no proof that all low- Tc samples contain the beta phase, although neutron scattering suggests that this might be the case. (See Section gif .)

With just the information in the table on hand, it is hard to make a judgement about the reason for the lower- Tc in the gold C4KHg samples. More experimental results are needed to confidently identify a cause. Optical transmission and resistivity measurements were attempted, as discussed in Section gif , but these experiments were unsuccessful. Hydrogenation experiments are an appealing alternative because of the interesting effects of hydrogen chemisorption in the closely related binary compound C8K.[79,62] In particular, hydrogen chemisorption in C8K raises Tc from 0.15 K[141] to 0.22 K.[125] Dramatic increases in Tc due to hydrogenation are also observed in many of the transition metal dichalcogenides,[90] which have similar upper critical field curves to the ternary GIC's. Hydrogen can also cause a Tc enhancement in the elemental superconductors Pd, Al, and Th[226]. The mechanism of the hydrogen-related Tc enhancement in these materials is discussed below.

next up previous contents
Next: Hydrogenation and Superconductivity Up: Hydrogenation Experiments on Previous: Hydrogenation Experiments on (Alison Chaiken)
Wed Oct 11 22:59:57 PDT 1995