The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is an exotic superconducting phase formed by Cooper pairs with finite center-of-mass momentum $\mathit{q}$. On theoretical grounds, the superconducting order parameter in the FFLO state is spatially modulated along the $\mathit{q}$ vector, and the emergence of an associated anisotropy is expected at the phase transition from the Abrikosov state to the FFLO state. Here, we report the results of high-resolution magnetostriction measurements for a single crystal of ${\mathrm{CeCoIn}}_5$ around $B\parallel c$. We find two anomalies in the magnetostriction along the $c$ axis, parallel to the magnetic-field orientation. In sharp contrast, this $B_{\mathrm{K}}$ anomaly disappears in the magnetostriction along the $a$-axis direction, perpendicular to the magnetic-field orientation. To explain this uniaxial expansion, we suggest a possibility that the FFLO transition occurs slightly below the upper critical field, and the FFLO modulation vector parallel to the applied magnetic field gives rise to the anisotropic response.

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• FFLO
• Magnetostriction
• Superconducting phase transition
• Superconductivity
• Heavy-fermion systems