Weak interband-coupling superconductivity in the filled skutterudite LaPt4Ge12

The superconducting pairing state of LaPt4Ge12 is studied by measuring the magnetic penetration depth lambda(T, B) and superfluid density rho(s)(T) using a tunnel-diode-oscillator (TDO)-based method and transverse-field muon-spin rotation (TF-mu SR) spectroscopy. The penetration depth follows an exponential-type temperature dependence at T << T-c, but increases linearly with magnetic field at T = 1.5 K. A detailed analysis demonstrates that both lambda(L) (T) and rho(TDO)(s)(T), measured in the Meissner state using the TDO method, are well described by a two-gap gamma model with gap sizes of Delta(1)(0) = 1.31k(B)T(c) and Delta(2)(0) = 1.80k(B)T(c), and weak interband coupling. In contrast, rho(mu SR)(s) (T), derived from the mu SR data, can be fitted by a single-gap BCS model with a gap close to Delta(2)(0). We conclude that LaPt4Ge12 is a marginal two-gap superconductor and the small gap Delta(1) seems to be suppressed by a small magnetic field applied in the mu SR experiments. In comparison, the 4f electrons in PrPt4Ge12 may enhance the interband coupling and, therefore, give rise to more robust multiband superconductivity.