High-resolution magnetic penetration depth and inhomogeneities in locally noncentrosymmetric SrPtAs

We present a magnetic-penetration-depth study on polycrystalline and granular samples of SrPtAs, a pnictide superconductor with a hexagonal structure containing PtAs layers that individually break inversion symmetry (local noncentrosymmetry). Compact samples show a clear-cut $s$-wave-type BCS behavior, which we consider to be the intrinsic penetration depth of SrPtAs. Granular samples display a sample-dependent second diamagnetic drop, attributed to the intergrain coupling. Our experimental results point to a nodeless isotropic superconducting energy gap in SrPtAs, which puts strong constraints on the driven mechanism for superconductivity and the order parameter symmetry of this compound.

Figure 1

SEM images of (a) polycrystalline sample A, (b) a typical area of sample A, (c) polycrystalline sample B, (d) a typical area of sample B, and (e) a typical zone of the powdered sample. Sample B is evidently granular, whereas sample A looks compact. Note that the magnification in (b) is larger than in (d).

Figure 2

Magnetic penetration depth of polycrystalline SrPtAs. The main panel exhibits the low-temperature behavior of polycrystalline sample A. The solid blue line corresponds to the $s$-wave BCS model. The inset shows the behavior in the temperature region below $T_c$.

Figure 3

Magnetic penetration depth of granular samples of SrPtAs. The main panel exhibits the low-temperature behavior of samples B, C, and a powder, in which second diamagnetic drops are observed. The inset displays the low-temperature region for sample D that has a third drop. It is seen that the extra drops take place at different temperatures in the various samples, which suggests their extrinsic (nonsuperconducting) origin.

Figure 4

Superfluid density of SrPtAs as a function of temperature compared to a BCS model with an isotropic energy gap.