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

TL;DR

This study measures the magnetic penetration depth in SrPd₂Ge₂ single crystals, revealing fully gapped superconductivity, estimating key parameters, and exploring vortex pinning and surface superconductivity effects.

Contribution

It provides detailed experimental data on the penetration depth and superfluid density, demonstrating single-gap behavior and surface superconductivity in SrPd₂Ge₂.

Findings

Exponential saturation of λ(T) indicating fully gapped superconductivity

Estimated λ(0) = 426 nm and Hc2(0) ≈ 0.48 T

Evidence of surface superconductivity and non-monotonic pinning behavior

Abstract

The in-plane magnetic penetration depth, $\lambda_m(T)$, 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, $\lambda$, saturates exponentially approaching $T\rightarrow 0$ indicating fully gapped superconductivity. The thermodynamic Rutgers formula was used to estimate $\lambda(0) = 426$ 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 $H \le 0.45$ 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.