Weak interband-coupling superconductivity in the filled skutteruditeLaPt4Ge12

Abstract

The superconducting pairing state of LaPt4Ge12 is studied by measuring the magnetic penetration depth λ(T, B) and the superfluid density ρs(T ) using a tunnel-diode-oscillator (TDO)-based method and by transverse field muon-spin rotation (µSR) spectroscopy. λ(T ) follows an exponential-type temperature dependence at T ≪ Tc, but its zero-temperature value λ(0) increases linearly with magnetic field. Detailed analyses demonstrate that both λ(T ) and the corresponding ρs(T ), measured in the Meissner state by the TDO method are well described by a two-gap γ model with gap sizes of ∆1(0) = 1.31kB Tc and ∆2(0) = 1.80kB Tc and a very weak interband coupling. In contrast, ρs(T ), derived from the µSR data taken in a small field, can be fitted by a single-gap BCS model with a gap close to ∆2(0). We conclude that LaPt4Ge12 is a marginal two-gap superconductor and the small gap ∆1 seems to be destroyed by a small magnetic field. In comparison, in PrPt4Ge12 the 4f -electrons may enhance the interband coupling and, therefore, give rise to more robust multiband superconductivity.The discovery of superconductivity (SC) in a series of Pr-based skutterudite compounds, i.e., PrT 4 X 12 (T =Fe, Ru, Os, and X =pnicogen), has attracted considerable interests. Despite of intensive investigations in the physical properties of these compounds, their superconducting order parameter and, therefore, the pairing mechanism remain highly controversial [1]: nodal SC was evidenced in some of these compounds, while conventional BCS SC was recognized in others. Even for the same compound, most prominently, the heavy-fermion superconductor PrOs 4 Sb 12 , the gap symmetry is still under debate. Here, the early measurements provided evidence of point nodes with a possible triplet pairing state for PrOs 4 Sb 12 [2][3][4]. With improving sample homogeneity and lowering the accessible temperatures, recent measurements of thermal conductivity demonstrated that PrOs 4 Sb 12 belongs to multiband superconductors [5,6]. Exploration of other filled skutterudite superconductors may help to elucidate the superconducting pairing state and the question of its universality.

Recent progress on superconductors with time-reversal symmetry breaking

“…The early measurements of the spe-cific heat and µSR suggest nodal superconductivity for PrPt 4 Ge 12 104 . Later measurements of the London penetration depth 105,106 , specific heat, thermal transport 92 , NQR 107 and photoemission spectroscopy 108 demonstrate that both PrPt 4 Ge 12 and LaPt 4 Ge 12 behave more like conventional BCS SCs with a fully opened energy gap. The smooth evolution of the superconducting transition between the Pr-based end compounds with TRS breaking, and the time reversal preserving La-based materials, is difficult to account for and requires further studies.…”

Superconducting gap structure of the skutteruditeLaPt4Ge12probed by specific heat and thermal transport

“…This value is in resonable agreement with the results from photoelectron spectroscopy: ∆ 0 = 1.95 k B T c 16 , and NMR: ∆ 0 = 1.92 k B T c 12 . Since superconductivity in other skutterudites is discussed in terms of multiband superconductivity 13,[24][25][26] and there are results in favor of this interpretation for LaPt 4 Ge 12 as well 21 , we also consider this possibility based on our specific-heat data. In the case of two gaps, the specific heat might be described by a weighted sum of the contributions from both gaps within the standard two-gap α-model…”

“…Additionally, no time-reversal symmetry breaking is observed in LaPt 4 Ge 12 by µSR 14 . Only one recent study using µSR and tunnel diode spectroscopy for penetration-depth measurements reports indications of multiband superconductivity in LaPt 4 Ge 12 21 . In order to develop a conclusive picture, it is necessary to shed more light on the superconducting properties of LaPt 4 Ge 12 .…”

Electron-phonon superconductivity in the filled skutterudites LaRu4P12, LaRu4As12 , and Tütüncü1, Karaca2, Srivastava3 2017Phys. Rev. B

“…This discovery is very interesting because, so far, X elements are restricted to volatile and toxic pnictogens (X = P, As, Sb) rather than Ge. Furthermore, new germanium-platinum compounds LaPt 4 Ge 12 and PrPt 4 Ge 12 with the filled-skutterudite crystal structure have been synthesized by experimenters [49][50][51][52][53][54]. Magnetic susceptibility, specific heat, and electrical resistivity measurements indicate superconductivity in LaPt 4 Ge 12 and PrPt 4 Ge 12 below 8.3 K [49].…”