Abrikosov-to-Josephson vortex lattice crossover in heavy fermion CeCoIn₅

Abstract

We present torque magnetization measurements on the quasi-2D heavy fermion superconductor CeCoIn₅ at temperatures down to 20 mK and magnetic fields up to 18 T. At orientations with the magnetic field perpendicular to the conducting planes, a prominent vortex lattice peak effect is present at around 0.5H _c2. The peak effect gradually disappears upon rotating the field into the plane parallel orientation. We interpret the absence of the peak effect for the plane parallel case as a transformation of the Abrikosov lattice into a Josephson vortex state, favored by the Pauli paramagnetic limit in CeCoIn₅ together with the unusually large condensation energy. Additionally, we do not observe flux avalanches as found in organic superconductors and suggest that the complete absence of vortex activity in the plane parallel field orientation is crucial for the formation of Fulde–Ferrell–Larkin–Ovchinnikov superconductivity in CeCoIn₅.

Acknowledgments

This work was supported by the National Science Foundation under Cooperative Agreement No. DMR-0084173, an NHMFL In-House Research Program, the State of Florida, and the US Department of Energy. We would like to thank C. J. Olson and A. E. Koshelev for stimulating discussions about vortex behavior in anisotropic superconductors.