London penetration depth and coherence peak in ammonia-intercalated fulleride superconductors

The compound (NH3)(x)NaRb2C60 (x similar to 1.6) has been studied by H-1 nuclear magnetic resonance. The proton line shows no shift at any temperature. while lineshape analysis of the low-temperature (30 K) spectra reveals the presence of a Knight shift interaction of pure anisotropic nature, whose amplitude has been estimated. Relaxation measurements show a single-exponential recovery of the magnetization at all investigated temperatures and fields. A Korringa behavior is observed below 65 K and a clear Hebel-Slichter coherence peak is detected just below T-c. The London penetration depth can be estimated from the line broadening observed below T-c, which yields lambda = 825 nm.