Campbell penetration depth in a single crystal of heavy fermion superconductor CeCoIn$_5$

Abstract

The temperature and magnetic field dependent magnetic penetration depth, $\lambda_m(T,H)$, was measured in a single crystal of a heavy fermion superconductor CeCoIn$_5$ using a frequency-domain tunnel diode resonator. In addition to the London penetration depth, which yields the superfluid density, measurements in a finite DC magnetic field provide Campbell penetration depth, $\lambda_C(T,H)$, which is directly linked to the true (unrelaxed) critical current density, $J_c$. The measured $\lambda_C(H)$ in CeCoIn$_5$ deviates significantly from the conventional $\sim \sqrt{H}$ behavior, and its slope changes abruptly at the characteristic magnetic field values. Considering that our sample is in the clean limit, we interpret this deviation as a fingerprint of the vortex lattice symmetry change. The temperature dependence $J_c(T)$ of CeCoIn$_5$ calculated from $\lambda_C(T)$ is nearly $T$-linear over the entire temperature range, also in stark contrast to expectations in a conventional type-II superconductor. Our results provide new evidence for unconventional superconductivity in CeCoIn$_5$ from the never-before-measured Campbell penetration depth.