Campbell penetration depth in low carrier density superconductor YPtBi

TL;DR

This study measures the magnetic penetration depth in YPtBi, revealing a low pinning strength and critical current density, indicative of low Cooper pair density in this low carrier superconductor.

Contribution

It provides the first detailed measurement of the Campbell penetration depth in YPtBi, highlighting its unusual vortex pinning behavior and low superfluid density.

Findings

Pinning strength suppressed rapidly with magnetic field.

Critical current density is orders of magnitude lower than conventional superconductors.

The vortex pinning behavior deviates from typical B^{1/2} dependence.

Abstract

Magnetic penetration depth, $\lambda_{m}$, was measured as a function of temperature and magnetic field in single crystals of low carrier density superconductor YPtBi by using a tunnel-diode oscillator technique. Measurements in zero DC magnetic field yield London penetration depth, $\lambda_{L}\left(T\right)$, but in the applied field the signal includes the Campbell penetration depth, $\lambda_{C}\left(T\right)$, which is the characteristic length of the attenuation of small excitation field, $H_{ac}$, into the Abrikosov vortex lattice due to its elasticity. Whereas the magnetic field dependent $\lambda_C$ exhibit $\lambda_{C}\sim B^{p}$ with $p=1/2$ in most of the conventional and unconventional superconductors, we found that $p\approx 0.23\ll1/2$ in YPtBi due to rapid suppression of the pinning strength. From the measured $\lambda_{C}(T,H)$, the critical current density is $j_{c}\approx40\,\mathrm{A}/\mathrm{cm^{2}}$ at 75 mK. This is orders of magnitude lower than that of conventional superconductors of comparable $T_{c}$. Since the pinning centers (lattice defects) and vortex structure are not expected to be much different in YPtBi, this observation is direct evidence of the low density of the Cooper pairs because $j_{c}\propto n_s$.