Magnetic penetration depth in single crystals of SrPd2Ge2 superconductor

Abstract

The in-plane magnetic penetration depth lambda(m)(T) was measured in a single crystal of SrPd2Ge2 superconductor in a dilution refrigerator down to T = 60 mK and in magnetic fields up to H-dc = 1 T by using a tunnel diode resonator. The London penetration depth lambda saturates exponentially approaching T -> 0 indicating fully gapped superconductivity. The thermodynamic Rutgers formula was used to estimate lambda(0) = 426 +/- 60 nm which was used to calculate the superfluid density, rho(s)(T) = lambda(2)(0)/lambda(2)(T). Analysis of rho(s)(T) in the full temperature range shows that it is best described by a single-gap behavior, perhaps with somewhat stronger coupling. In a magnetic field, the measured penetration depth is given by the Campbell penetration depth which was used to calculate the theoretical critical current density j(c). For H <= 0.45 T, the strongest pinning is achieved not at the lowest, but at some intermediate temperature, probably due to matching effect between temperature-dependent coherence length and relevant pinning length scale. Finally, we find compelling evidence for surface superconductivity. Combining all measurements, the entire H-T phase diagram of SrPd2Ge2 is constructed with an estimated H-c2(0) = 0.4817 T. DOI: 10.1103/PhysRevB.87.094515