μSR studies of intercalated HfNCl superconductor

Muon spin relaxation measurements of the magnetic field penetration depth λ have been performed in a quasi two-dimensional (2-d) superconductor based on HfNCl intercalated with Li and organic molecules. The relaxation rate σ(T→0)∼0.5μs−1 was obtained with transverse fields applied perpendicular to the aligned HfN planes. Using the relation σ∝λ−2∝ns2d/m∗ (2-d superconducting carrier density/effective mass), we deduced the 2-d Fermi temperature TF2d. The ratio Tc/TF2d for the HfNCl-(Li) system is very close to those in two other quasi-2-d superconductors, YBa2Cu3O7 and (BEDT-TTF)2-X. Compared to the Kosterlitz–Thouless (KT) transition temperature TKT=TF2d/8 for the limit of strong coupling, Tc's of these systems are reduced by a factor of 3–4, indicating the importance of carrier overlapping and/or 3-d coupling.