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. Lambdaep is McMillan's electron-phonon coupling
parameter.[165] kappa is the Ginzburg-Landau parameter
discussed in Section ; kappa ;SPMlt; 1/sqrt2 indicates type I superconductivity. The density of states,
N(0), shown here was calculated in Section
from the Hc2(theta) data. NA means not applicable.
The most noticeable aspect of Table 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 .)
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 , 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.