The linear and nonlinear response to a microwave electromagnetic field of two c-axis oriented polycrystalline samples of the newly discovered superconductor CaC6 (Tc = 11.5 K) is studied in the superconducting state down to 2 K. The surface resistanc
e Rs and the third order intermodulation distortion, arising from a two-tone excitation, have been measured as a function of temperature and microwave circulating power. Experiments are carried out using a dielectrically loaded copper cavity operating at 7 GHz in a hot finger configuration. The results confirm recent experimental findings that CaC6 behaves as a weakly-coupled, fully gapped, superconductor. The weak power dependence of Rs encourages a further investigation of this novel superconductor as a possible alternative to Nb in specific microwave applications.
We report a study of the temperature dependence of the surface resistance RS in the graphite intercalated compound (GIC) CaC6, where superconductivity at 11.5 K was recently discovered. Experiments are carried out using a copper dielectrically loaded
cavity operating at 7 GHz in a hot finger configuration. Bulk CaC6 samples have been synthesized from highly oriented pyrolytic graphite. Microwave data allows to extract unique information on the quasiparticle density and on the nature of pairing in superconductors. The analysis of RS(T) confirms our recent experimental findings that CaC6 behaves as a weakly-coupled, fully gapped, superconductor.
The two tone intermodulation distortion arising in MgB2 thin films synthesized by hybrid physical-chemical vapour deposition is studied in order to probe the influence of the two bands on the nonlinear response of this superconductor. The measurement
s are carried out by using a dielectrically loaded copper cavity operating at 7 GHz. Microwave data on samples having critical temperatures above 41 K, very low resistivity values, and residual resistivity ratio larger than 10, are shown. The dependence of the nonlinear surface losses and of the third order intermodulation products on the power feeding the cavity and on the temperature is analyzed. At low power, the signal arising from distortion versus temperature shows the intrinsic s-wave behavior expected for this compound. Data are compared with measurements performed on Nb and YBCO thin films using the same technique.
The temperature dependence of the in-plane magnetic penetration depth, $lambda_{ab}(T)$, has been measured in a c-axis oriented polycrystalline CaC$_{6}$ bulk sample using a high-resolution mutual inductance technique. A clear exponential behavior of
$lambda_{ab}(T)$ has been observed at low temperatures, strongly suggesting isotropic s-wave pairing. Data fit using the standard BCS theory yields $lambda_{ab}(0)=(720pm 80)$ Angstroem and $Delta(0)=(1.79pm 0.08)$ meV. The ratio $2Delta(0)/k_{_B}T_{c}=(3.6pm 0.2)$ gives indication for a conventional weakly coupled superconductor.
