Modulation vector of the Fulde-Ferrell-Larkin-Ovchinnikov state in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>CeCoIn</mml:mi><mml:mn>5</mml:mn></mml:msub></mml:math> revealed by high-resolution magnetostriction measurements
Shunichiro Kittaka, Yohei Kono, Kaito Tsunashima, Daisuke Kimoto, M. Yokoyama, Yusei Shimizu, Toshiro Sakakibara, Minoru Yamashita, Kazushige Machida
Abstract
The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is an exotic superconducting phase formed by Cooper pairs with finite center-of-mass momentum $\mathbit{q}$. On theoretical grounds, the superconducting order parameter in the FFLO state is spatially modulated along the $\mathbit{q}$ 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 $B\ensuremath{\parallel}c$. 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.