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Zero-Field Tc Measurements in C4KHg


The preparation for the zero-field Tc measurements was as follows. After a batch of KHg-GIC's was removed from the furnace in its intercalation ampoule, the individual GIC's were transferred inside an inert-gas-filled glovebox to 4mm Pyrex tubes. When the specimens were intended for critical field studies, they were mounted in special sample holders before placement in the glass tubes (see Section gif ). Once the specimens were inside the glass tubes, the tubes were attached to a stopcock via a Cajon quick-connect vacuum fitting. They were then taken out of the glovebox and attached to a pumping station, where they could be evacuated to pressures on the order of 10-5 torr with a diffusion pump. Once the ampoules were evacuated, they were filled with about 200 torr of He gas which had been passed through a liquid-nitrogen cold trap to remove water. This gas was intended to provide thermal contact of the sample to the glass tube, thus making the cryogenic temperature measurements more accurate. The He-filled tubes were then sealed off with a gas/oxygen torch.

Once the GIC's were in individual tubes, they were characterized for staging fidelity with (00l) x-rays (see Section gif). Those that were well-staged and uniform in color were taken to the National Magnet Laboratory for zero-field Tc and critical field measurements. The zero-field Tc measurements were performed using the same cryogenics, electronics, and thermometry that were employed in the critical field studies. This apparatus is described in Section gif .

Tc was determined by measuring the voltage from an inductance bridge as a function of temperature from 4.2 K down to 0.4 K. For materials in which a higher Tc than 4.2 K seemed plausible, higher temperatures were achieved by bleeding warm He gas into the liquid helium cryostat. The transition was easily recognized as a large reproducible decrease in the inductive voltage. Signal-to-noise ratios of 1000 were not uncommon.

Tc was defined to be the intersection of a line drawn tangent to the most linear part of the transition with the level upper part of the transition. This definition, which is a standard one for inductive measurements,[120] is illustrated by Figure gif . The transition width, Delta Tc, was defined as the interval between the 10% and 90% completion temperatures of the transitions. The dimensionless figure of merit used to characterize sample quality is Delta Tc/Tc. This ratio is a Tc-independent measure of how narrow a specimen's superconducting transition is. Delta Tc/ Tc ranged from about 0.2 for poorly ordered samples to about 10-2 for well-ordered specimens.

Figure: Experimental definition of Tc.

Tc and Delta Tc/Tc are reported for a number of samples in Table gif . Delta Tc/Tc mostly increases with decreasing Tc, which suggests that the low- Tc GIC's are less homogeneous than the higher- Tc ones. The observation of broader transitions in lower- Tc samples is in keeping with the idea that the lower- Tc specimens contain both the alpha and beta phases, while the higher- Tc specimens contain only the alpha phase.

If the beta phase is responsible for Tc suppression in C4KHg, one might expect that samples might sometimes have two separate superconducting transitions, the first at 1.5 K and a second at 0.8 K. Yet two separate transitions have never been observed in a C4KHg sample, although very broad transitions have sometimes been seen. An interpretation for the lack of two transitions is suggested by the TEM work performed by Timp[246]. The TEM micrographs consistently showed that the in-plane phases were intermixed on a microscopic scale. If the regions of beta phase are small and are surrounded by alpha-phase, they may well be proximity-coupled to the alpha-phase regions. Proximity coupling between the lower- and higher- Tc regions of the specimen can give a single transition, just as is observed. Caution is necessary when discussing the intermixture of phases, though, since a high-enough degree of admixture along the c-axis would give broadened (00l) x-ray lineshapes.[109] As the spectra in Section gif show, these broadened lineshapes have not been observed.

Table: Transition temperatures Tc and reduced widths Delta Tc/Tc for a sampling of C4KHg specimens.

A sceptic might claim that the color of the C4KHg samples is a property only of the surface layers, and is not really linked to the superconductivity. This is not an unreasonable idea since the (sqrt3 × sqrt3)R30° superlattice has been tentatively identified as a surface phase in Section gif . However, the gold color is strongly correlated with the presence of the beta phase in the (00l) x-rays. Furthermore, the color is as good a predictor of Tc as the x-rays are, as Table gif shows. Lagrange and coworkers have also reported the existence of the gold color in association with the beta phase.[147] They report that repeated cleaving of a ``gold'' sample will expose both pink and gold surfaces in turn.

From these considerations and the evidence in Table gif , it is clear that gold samples have lower Tc's than pink ones. Furthermore, this Tc depression seems to be linked to the presence of the beta phase. The possible effect of the two phases of C4KHg on superconductivity measurements is discussed further in Chapter gif . The differences in the critical fields of the two types of C4KHg are detailed in Chapter gif .

next up previous contents
Next: Discussion and Conclusions Up: Normal-State Characterization of Previous: Chemical Analysis of (Alison Chaiken)
Wed Oct 11 22:59:57 PDT 1995