Weak Interband-Coupling Superconductivity in the Filled Skutterudite LaPt4Ge12

TL;DR

This study reveals that LaPt4Ge12 exhibits weak interband-coupling two-gap superconductivity, with different measurements indicating a marginal two-gap nature and sensitivity of the small gap to magnetic fields.

Contribution

It provides detailed experimental evidence for weak interband coupling in LaPt4Ge12 and compares its superconducting properties to PrPt4Ge12, highlighting the role of 4f-electrons.

Findings

LaPt4Ge12 is a marginal two-gap superconductor.

The small gap is easily suppressed by magnetic fields.

Weak interband coupling is demonstrated through multiple measurement techniques.

Abstract

The superconducting pairing state of LaPt$_{4}$Ge$_{12}$ is studied by measuring the magnetic penetration depth $\lambda(T,B)$ and the superfluid density $\rho_s(T)$ using a tunnel-diode-oscillator (TDO)-based method and by transverse field muon-spin rotation ($\mu$SR) spectroscopy. $\lambda(T)$ follows an exponential-type temperature dependence at $T\ll T_{c}$, but its zero-temperature value $\lambda(0)$ increases linearly with magnetic field. Detailed analyses demonstrate that both $\lambda(T)$ and the corresponding $\rho_{s}(T)$, measured in the Meissner state by 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 a very weak interband coupling. In contrast, $\rho_s(T)$, derived from the $\mu \rm{SR}$ data taken in a small field, can be fitted by a single-gap BCS model with a gap close to $\Delta_2(0)$. We conclude that LaPt$_{4}$Ge$_{12}$ is a marginal two-gap superconductor and the small gap $\Delta_1$ seems to be destroyed by a small magnetic field. In comparison, in PrPt$_4$Ge$_{12}$ the $4f$-electrons may enhance the interband coupling and, therefore, give rise to more robust multiband superconductivity.