Magnetic field induced lattice anomaly inside the superconducting state of CeCoIn${}_5$: evidence of the proposed Fulde-Ferrell-Larkin-Ovchinnikov state

Abstract

We report high magnetic field linear magnetostriction experiments on CeCoIn${}_5$ single crystals. Two features are remarkable: (i) a sharp discontinuity in all the crystallographic axes associated with the upper superconducting critical field $B_{c2}$ that becomes less pronounced as the temperature increases; (ii) a distinctive second order-like feature observed only along the c-axis in the high field (10 T $\lesssim B\le B_{c2}$) low temperature ($T\lesssim$ 0.35 K) region. This second order transition is observed only when the magnetic field lies within 20${}^o$ of the ab-planes and there is no signature of it above $B_{c2}$, which raises questions regarding its interpretation as a field induced magnetically ordered phase. Good agreement with previous results suggests that this anomaly is related to the transition to the Fulde-Ferrel-Larkin-Ovchinnikov superconducting state.

3 figures, 5 pages