Tc measurements on KHg are summarized in
Table . The hydrogenation had very little effect on the
superconductivity of pink specimens, which initially had a narrow
transition and Tc 1.5 K. The transition of a pink C4KHg sample before and after hydrogenation is shown in
Figure
b). On the other hand, hydrogenation had two major
effects on the gold samples. The first effect was to narrow the
superconducting transition, decreasing Delta Tc/ Tc by as
much as a factor of two. In addition, Tc in the hydrogenated gold
samples was about 1.5 K, just as in the pink specimens. Superconducting
transitions before and after hydrogenation for a gold sample are shown in
Figure
a). The effect of hydrogen on a specimen with an
initially broad transition but a fairly high Tc is shown in
Figure
c). Notice that the 3 transitions before
hydrogenation are quite different from one another, whereas afterward the
three all have Tc 1.5 K and Delta Tc/ Tc on
the order of 10-2.
A valid question about these experiments is whether a small
fraction of the sample volume might account for the changes seen in the
superconducting transition. This concern is especially important because
the inductive measurements are sensitive to a transition in as little as
2% of the typical sample volume (see Section ). Also,
the small size of the hydrogen uptake might lead one to suspect that only a
tiny portion of the sample was hydrogenated. Because of the sensitivity of
the transition height to the sample's position within the secondary coil,
and the lack of information about the sample's exact dimensions after
intercalation, it is not possible to say with any certainty what the actual
areal fraction of superconductivity before and after hydrogenation is.
However, the superconducting transitions measured both before and after
hydrogenation were always at least 2 muV high, a considerable fraction
of the 5 muV expected for the typical sample cross-sectional area. (See
the discussion of Figure
for more details.) Most of the
transitions were on the order of 4 to 8 muV high. The Tc's
quoted in Table
can be said with certainty to
correspond to at least 40% of the areal fraction, and probably they
represent true bulk superconductivity. Therefore the hydrogen-induced Tc enhancement in C4KHg is undoubtedly a bulk effect.
Table: Effect of
hydrogenation on Tc in KHg-GIC's. All the Tc's are for C4KHg, except for the blue 1.879 K sample, which was C8KHg.
dagger indicates the second hydrogenation of a previously
hydrogenated sample. * indicates that deuterium rather than
hydrogen was added. S indicates that the remeasurement of a
transition one year after hydrogenation.
Figure: Superconducting transitions before and after hydrogenation in
three types of C4KHg samples. a) A gold sample. Tc
increases from 0.88 K to 1.54 K, and Delta Tc/ Tc decreases
from 7.3× 10-2 to 7.8× 10-2. b) A pink sample. Tc is almost constant; Delta Tc/ Tc decreases from
4.7× 10-2 to 2.2× 10-2. c) A copper-colored sample.
Tc increase from 1.32 K to 1.50 K; Delta Tc/ Tc
decreases from 0.138 to 6.47× 10-2.
Another issue related to sample homogeneity is the question of
multiple superconducting transitions. It is reasonable to ask whether the
hydrogenated Tc = 1.54 K specimen (whose transition appears in
Figure a)) did not still have a transition at 0.88 K, its
pre-hydrogenation Tc. The answer is no, within the limits of the
sensitivity of the apparatus. A second, lower-temperature transition was
always checked for and never found in samples whose Tc was increased
by hydrogenation. The lack of a second transition is additional evidence
that hydrogen increased the bulk Tc.
A related hypothesis is that the transition narrowing is merely due to suppression of the superconductivity of the lower- Tc phase. This assertion cannot be correct since Tc enhancement was also seen in GIC's with initially narrow transitions.
Because of the slow kinetics of hydrogen uptake in HOPG-based
GIC's, one might wonder whether Tc would increase above 1.5 K
through further hydrogen additions. As is shown in Table ,
a second 5-minute hydrogen exposure had almost no effect on Tc or
Delta Tc/ Tc. As another check of time-dependent effects,
the superconducting transition of one hydrogenated GIC was remeasured after
a wait of one year. The Tc increased only from 1.52 K to 1.54 K,
which is within the experimental uncertainty. The (00l) x-rays also
did not show any change over this period.
In order to get more clues about the origin of the Tc
enhancement, one copper-colored C4KHg was exposed to deuterium
rather than hydrogen. Deuterium appears to have the same effect as
hydrogen, as is shown in Table . The lack of any isotope
effect associated with the hydrogenation suggests that the Tc
enhancement is not due to optic phonons, the mechanism responsible for the
Tc enhancement in the transition metals.[68]
Table also shows that one C4KHg specimen
showed only a small Tc increase as a result of hydrogenation, from
0.72 K to 0.94 K. The reason why the final Tc of this sample was
not 1.5 K is not known. The fact that this sample had the lowest Tc
of any of the measured samples suggests that it may have been too
disordered to improve substantially upon hydrogenation. Or alternatively,
the hydrogen uptake rate in this specimen may have been especially slow.
In light of the results on C4KHg, the apparent decrease of Tc in C8KHg by hydrogen addition is quite surprising. It would be a mistake to make very much of this result since hydrogen was added to only one stage 2 sample. The hydrogenation of C8KHg deserves further investigation.
The data in Table tend to underline the
differences between the pink and gold C4KHg GIC's that were
summarized in Table
. These experiments show that adding
hydrogen to a gold sample seems to change its superconducting properties
into those of a pink sample. Hydrogenation of a sample with a broad
transition seems to turn the lower- Tc material that forms the foot
of the transition into Tc = 1.5 K material. Hydrogen appear to have
no effect on regions that initially have Tc = 1.5 K. The striking
aspect of these results is thus not that hydrogen raises Tc but that
hydrogen appears to be transforming one type of C4KHg into another
since the end state appears to be always the same.
By what means does hydrogen transform the lower- Tc material into Tc = 1.5 K material? And what is the essential difference between the lower and higher- Tc material? The next section contains some speculative attempts at answers to these questions.