Multiband superconductivity in PrPt4Ge12single crystals

Abstract

We report measurements of the London penetration depth ∆λ(T ) and the electronic specific heat Ce(T ) on high-quality single crystals of the filled-skutterudite superconductor PrPt4Ge12 (Tc ≃8K). Both quantities show a weak temperature dependence at T ≪ Tc, following ∆λ ∼ T n (n ≃ 3.2) and Ce/T ∼ T 2.8 . Such temperature dependences deviate from both conventional s-wave type and nodal superconductivity. A detailed analysis indicates that the superfluid density ρs(T ), derived from the penetration depth, as well as the electronic specific heat can be consistently described in terms of a two-gap model, providing strong evidence of multiband superconductivity for PrPt4Ge12. Recently, a series of new skutterudite superconductors with a germanium-platinum framework, i.e., M Pt 4 Ge 12 (M =Sr, Ba, La, Pr), were successfully synthesized [10,11]. Among all the Pr-filled variants, PrPt 4 Ge 12 shows an unexpectedly high transition temperature of T c =7. [19]. However, these experiments were performed on polycrystalline samples at relatively high temperatures, which could not make a clear assertion on the gap symmetry. The reasons underlying such discrepancies of the gap structure in PrPt 4 Ge 12 are not yet clear, and further measurements, in particular those based on high-quality single crystals, are badly needed.In this Letter, we probe the superconducting gap symmetry of PrPt 4 Ge 12 by measuring the London penetration depth ∆λ(T ) and the specific heat C p (T ) of highquality single crystals. Precise measurements of the penetration depth changes at low temperatures show ∆λ ∼ T n with n ≃ 3.2, indicating that PrPt 4 Ge 12 is actually neither a simple BCS nor a nodal superconductor. A detailed analysis of the superfluid density ρ s (T ), converted from λ(T ), and the electronic specific heat C e (T ) provide strong evidence of two-band SC for PrPt 4 Ge 12 .High-quality single crystals of PrPt 4 Ge 12 were synthesized by using multi-step thermal treatments [20]. Powder X-ray diffraction indicates the presence of a small amount of foreign phases. Energy-dispersive Xray (EDX) analysis confirms that all the crystals have a stoichiometric composition and the impurity phases, mainly PtGe 2 and free Ge, are located at the crystal surfaces [20]. In our measurements, the crystals were mechanically polished to get rid of these surface contaminations. Precise measurements of the resonant frequency shift ∆f (T ) were performed by utilizing a tunnel diode oscillator (TDO) based, self-inductance method at an operating frequency of 7 MHz down to about 0.5K

Weak interband-coupling superconductivity in the filled skutteruditeLaPt4Ge12

“…On the other hand, the derived interband coupling λ 12 =0.07 is more than one order of magnitude larger than that of LaPt 4 Ge 4 . The enhanced interband coupling leads to a more robust small energy gap, showing the same type of temperature dependence for the two energy gaps [15]. For PrPt 4 Ge 12 , the gap parameters derived from the γ-model [∆ 1 (0) = 0.88k B T c , ∆ 2 (0) = 2.17k B T c and γ = 0.15] are compatible with those fitted by the α-model [15].…”

“…The enhanced interband coupling leads to a more robust small energy gap, showing the same type of temperature dependence for the two energy gaps [15]. For PrPt 4 Ge 12 , the gap parameters derived from the γ-model [∆ 1 (0) = 0.88k B T c , ∆ 2 (0) = 2.17k B T c and γ = 0.15] are compatible with those fitted by the α-model [15]. These results suggest that the Pr-4f electrons might not play a crucial role in the formation of SC, but enhance the interband coupling in the M Pt 4 Ge 12 compounds.…”

“…Resembling those of PrOs 4 Sb 12 , zero-field µSR experiments showed evidence of time-reversal symmetry (TRS) breaking in the superconducting state of PrPt 4 Ge 12 , but not for LaPt 4 Ge 12 [11][12][13]. However, more recent experiments suggested multi-gap SC for PrPt 4 Ge 12 [14][15][16]. Furthermore, a coherence peak was revealed in the nuclear spin-lattice relaxation rate 1/T 1 just below T c for PrPt 4 Ge 12 , which is unexpectedly suppressed in LaPt 4 Ge 12 [17].…”

“…It was previously shown that the sister compound, PrPt 4 Ge 12 , is also a two-gap superconductor featuring a relatively small admixture of a second gap with a small ∆ 0 /k B T [15]. As a comparison, we plotted the superfluid densities ρ TDO s (T ) of LaPt 4 Ge 12 and PrPt 4 Ge 12 in Fig.…”

“…Following the same methods used in Ref. 15 for PrPt 4 Ge 12 , we also fit ρ TDO s (T ) of LaPt 4 Ge 12 with various gap functions (see Fig. 4).…”