The in-plane magnetic penetration depth ${\lambda }_m\left(T\right)$ was measured in a single crystal 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 $\lambda$ saturates exponentially approaching $T\to 0$ indicating fully gapped superconductivity. The thermodynamic Rutgers formula was used to estimate $\lambda \left(0\right)=426\pm 60$ nm which was used to calculate the superfluid density, ${\rho }_s\left(T\right)={\lambda }^2\left(0\right)/{\lambda }^2\left(T\right)$. Analysis of ${\rho }_s\left(T\right)$ 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 length scale. Finally, we find compelling evidence for surface superconductivity. Combining all measurements, the entire $H$-$T$ phase diagram of SrPd${}_2$Ge${}_2$ is constructed with an estimated $H_{c2}\left(0\right)=0.4817$ T.

• Received 25 January 2013

DOI:https://doi.org/10.1103/PhysRevB.87.094515