Zero-Field Tc Measurements in C4KHg

The preparation for the zero-field T_c 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 ). 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 ). Those that were well-staged and uniform in color were taken to the National Magnet Laboratory for zero-field T_c and critical field measurements. The zero-field T_c measurements were performed using the same cryogenics, electronics, and thermometry that were employed in the critical field studies. This apparatus is described in Section .

T_c 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 T_c 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.

T_c 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 . The transition width, Delta T_c, 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 T_c/T_c. This ratio is a T_c-independent measure of how narrow a specimen's superconducting transition is. Delta T_c/ T_c ranged from about 0.2 for poorly ordered samples to about 10^-2 for well-ordered specimens.

  
Figure: Experimental definition of T_c.

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

If the beta phase is responsible for T_c 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- T_c 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 show, these broadened lineshapes have not been observed.

  
Table: Transition temperatures T_c and reduced widths Delta T_c/T_c 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 . 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 T_c as the x-rays are, as Table 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 , it is clear that gold samples have lower T_c's than pink ones. Furthermore, this T_c 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 . The differences in the critical fields of the two types of C₄KHg are detailed in Chapter .