Introduction

Frequent reference has been made in Chapters and to differences between the measured properties of gold and pink specimens of C₄KHg. In Table , 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. Lambda_ep is McMillan's electron-phonon coupling parameter.[165] kappa is the Ginzburg-Landau parameter discussed in Section ; kappa ;SPM_lt; 1/sqrt2 indicates type I superconductivity. The density of states, N(0), shown here was calculated in Section from the H_c2(theta) data. NA means not applicable.

The most noticeable aspect of Table is the lack of knowledge for the pink phase about the properties (theta_D, Lambda_ep, N(0)) that are most directly connected to T_c, the zero-field superconducting transition temperature. Clearly a specific heat experiment on a T_c = 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 ( I_c = 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 T_c 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- T_c samples contain the beta phase, although neutron scattering suggests that this might be the case. (See Section .)

With just the information in the table on hand, it is hard to make a judgement about the reason for the lower- T_c 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 , 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 T_c from 0.15 K[141] to 0.22 K.[125] Dramatic increases in T_c 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 T_c enhancement in the elemental superconductors Pd, Al, and Th[226]. The mechanism of the hydrogen-related T_c enhancement in these materials is discussed below.