The angular and temperature dependence of the upper critical field Hc2 of C4KHg was previously measured by Iye and Tanuma.[30] The zero-field transition temperature of their samples was about 0.8 K. The new study reported here has two goals. One goal is to determine if the superconducting properties of the gold mixed-phase C4KHg samples are similar to those previously reported.[30] The other aim is to measure the critical fields of the pink single-phase C4KHg to see what can be learned about their differences with the gold mixed-phase samples.
After characterization for staging and phase purity, the GIC's were affixed to copper sample holders using Apiezon N grease. These sample holders were designed so that the GIC's could not move during the Hc2 measurements. The sample holders were mounted inside pyrex tubes which were then filled with purified helium gas to insure good thermal contact with the bath at cryogenic temperatures. After mounting, the (00l) x-ray spectrum was taken again to insure that no sample deterioration had occurred during mounting.
The superconducting transitions were measured by monitoring the ac susceptibility of the sample as the temperature or magnetic field was swept. A standard ac inductance bridge[1] with excitation field 0.2 Oe at 490 Hz was used to monitor the magnetic susceptibility. Temperature readings were obtained from a calibrated germanium resistor (Lake Shore GR-200A-100), and from a capacitive pressure sensor (MKS Baratron) which monitored the 3He vapor pressure. Given the accuracy with which the transition height and dimensions of the intercalated specimen are known, it can be stated that the superconducting areal fraction of the GIC's used in this experiment was typically (80 ± 20)%. Tc, the zero-field transition temperature, 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. The transition width, delta Tc, was defined as the interval between the 10% and 90% completion temperatures of the transition. The dimensionless (Tc-independent) figure of merit used to characterize sample quality is delta Tc/Tc. delta Tc/ Tc ranged from about 0.2 for poorly ordered samples to about 10-2 for well-ordered specimens.[10] Hc2 was defined as the field where a line drawn tangent to the most linear part of the transition intersects the level upper portion of the field sweep, the same definition as was used in Ref. [30]. The angle theta that was varied in the angular dependence of Hc2 measurements was defined the angle between the applied field and the graphite c-axis. (Note that this angle is the complement of that usually called theta in the thin-film superconductivity literature.)