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Experimental Method


The techniques used in the Tc measurements on the CsBi-GIC's were nearly the same as those used for the KHg-GIC's, described in Section gif . There were several differences, however. One was that the CsBi samples were routinely cleaved before the Tc measurement so that any transition observed would be due to the bulk of the sample, and not a spurious effect arising from alloy adsorbed on the surface. Lagrange and coworkers do not report whether they cleaved their samples or not. KHg-GIC's very rarely had any adsorbed alloy since their intercalation is accomplished through a vapor-phase reaction, in contrast to the contact reaction necessary to intercalate the MBi-GIC's. Therefore, KHg samples were not generally cleaved before Tc measurements.

Because cleaving is a difficult operation to perform in the glovebox on small samples, the CsBi-GIC's were cleaved in air. Handling in air was also necessary to perform the weight-uptake measurements described in Section gif . Because of the compounds' excellent stability in air,[145] the brief exposures to air were not thought to influence the superconductivity experiments. The work of the Nancy group does not state whether their samples were exposed to air before the low-temperature experiments.

The CsBi-GIC's were place in glass ampoules for the low-temperature experiments. They were not fixed in metal sample-holders as the KHg-GIC's were [See Section gif ], though, because no critical field measurements were performed on them. The inductive Tc measurements were performed on the CsBi-GIC's in the same manner as on the KHg-GIC's [See Section gif]. The temperature range of the inductive measurements was from 4.2 K to about 0.4 K, just as for the KHg experiments.

The air stability of the CsBi compounds allowed resistive Tc measurements to be performed easily on them. Contacts were made with silver paint on the edges of the GIC in a standard four-probe geometry. The samples were then encapsulated in a cell filled with helium gas for good thermal contact at low temperatures. Both zero-field temperature sweeps and fixed-temperature field sweeps were performed on CsBi alpha- and alpha + beta-phase samples. The temperature range of the resistive measurements was from liquid helium temperature (4.2 K) to about 1.1 K. Current densities used were on the order of 10-1-10-2 A/cm2. The results did not depend on the current density employed. The values of the normal-state resistivity obtained from the temperature sweeps were estimated to be uncertain by about 25% due to remanent fields in the Bitter magnets used for the Shubnikov-de Haas measurements.

next up previous contents
Next: Results Up: The Quest for Previous: The Quest for (Alison Chaiken)
Wed Oct 11 22:59:57 PDT 1995