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Effect of Hydrogenation on Superconductivity in the KHg-GIC's


Tc measurements on KHg are summarized in Table gif . 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 gifb). 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 gif a). The effect of hydrogen on a specimen with an initially broad transition but a fairly high Tc is shown in Figure gifc). 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 gif ). 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 gif for more details.) Most of the transitions were on the order of 4 to 8 muV high. The Tc's quoted in Table gif 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 gif 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 gif, 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 gif. 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 gif 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 gif tend to underline the differences between the pink and gold C4KHg GIC's that were summarized in Table gif . 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.

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Next: Possible Explanations for Up: Hydrogenation Experiments on Previous: Experimental Methods: Hydrogenation (Alison Chaiken)
Wed Oct 11 22:59:57 PDT 1995