Campbell penetration depth in low carrier density superconductor YPtBi
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Magnetic penetration depth, λm, 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, λL(T), but in the applied field the signal includes the Campbell penetration depth, λC(T), which is the characteristic length of the attenuation of small excitation field, HAC, into the Abrikosov vortex lattice due to its elasticity. Whereas the magnetic field dependent λC exhibit λC∼Bp with p=1/2 in most of the conventional and unconventional superconductors, we found that p≈0.23≪1/2 in YPtBi due to rapid suppression of the pinning strength. From the measured λC(T,H), the critical current density is jc≈40A/cm2 at 75 mK. This is orders of magnitude lower than that of conventional superconductors of comparable Tc.