Magnetic penetration depth in single crystals of SrPd${}_2$Ge${}_2$ superconductor

Abstract

The in-plane magnetic penetration depth, $\hat{I}{»}_m(T)$, was measured in single crystals of SrPd${}_2$Ge${}_2$ 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, $\hat{I}»$, saturates exponentially approaching $T\to 0$ indicating fully gapped superconductivity. The thermodynamic Rutgers formula was used to estimate $\hat{I}»(0)=426$ nm which was used to calculate the superfluid density, $\ddot{I}{}_s(T)=\hat{I}{»}^2(0)/\hat{I}{»}^2(T)$. Analysis of $\ddot{I}{}_s(T)$ 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 $H\le 0.45$ 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 lengthscale. Finally, we find a compelling evidence for surface superconductivity. Combining all measurements, the entire $H$-$T$ phase diagram of SrPd${}_2$Ge${}_2$ is constructed with an estimated $H_{c2}(0)=0.4817$ T.