Campbell penetration depth in low carrier density superconductor YPtBi

Magnetic penetration depth, ${\ensuremath{\lambda}}_m$, 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, ${\ensuremath{\lambda}}_L(T)$, but in the applied field the signal includes the Campbell penetration depth, ${\ensuremath{\lambda}}_C(T)$, which is the characteristic length of the attenuation of small excitation field, ${H}_{\mathrm{AC}}$, into the Abrikosov vortex lattice due to its elasticity. Whereas the magnetic field dependent ${\ensuremath{\lambda}}_C$ exhibit ${\ensuremath{\lambda}}_C\ensuremath{\sim}{B}^p$ with $p=1/2$ in most of the conventional and unconventional superconductors, we found that $p\ensuremath{\approx}0.23\ensuremath{\ll}1/2$ in YPtBi due to rapid suppression of the pinning strength. From the measured ${\ensuremath{\lambda}}_C(T,H)$, the critical current density is ${j}_c\ensuremath{\approx}40\phantom{\rule{4pt}{0ex}}\mathrm{A}/{\mathrm{cm}}²$ at 75 mK. This is orders of magnitude lower than that of conventional superconductors of comparable ${T}_c$.