Abstract Body

Superconductivity is a macroscopic condensation state of electrons pairs. In the conventional superconducting state, the paired electrons have wavenumbers of the same magnitude with different signs like k and −k, and thus, the center-of-mass momentum of the electron pair is zero. On the other hand, the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state [1,2] has a finite center-of-mass momentum q because of the unbalanced pairing, k and −k+q. The finite q introduces the additional term cos(qr) to the superconducting order parameter. This periodic term induces the alternate appearance of the superconductivity and normal state. Therefore, one of the main features of the FFLO state is that the superconductivity oscillates macroscopically in real space. Since this periodic spatial modulation yields emergent anisotropy, the FFLO state is regarded as nematic superconductivity when the modulation breaks the rotational symmetry of the underlying lattice. However, it has been challenging to detect this feature due to experimental difficulties and severe conditions for the emergence of the FFLO state.

In order to investigate the emergent anisotropy unique to the FFLO state, we have investigated one of the organic superconductors κ-(BEDT-TTF)2Cu(NCS)2employing ultrasound measurements in pulsed magnetic fields. When applying magnetic fields parallel to the c-axis in the conducting plane, the relative change in the sound velocity Δv/v along the b-axis shows an anomaly at HFFLO=21.3 T. According to numerous previous reports, this anomaly is attributable to the transition to the FFLO state. As the direction of sound-wave propagation reflects the anisotropy of elastic properties, the formation of the FFLO pattern should be detected by the sound-wave direction dependence. In our study [3], we found that a direction-dependent anomaly is significant only in the FFLO state. The anisotropic response of the acoustic property is attributed to the nematic-like behavior of the FFLO state.

Also, we have performed further ultrasound measurements to examine the nature of the FFLO state by varying parameters, such as the magnetic field, field angle, and temperature, as well as the amount of disorder, in order to precisely discuss the intrinsic nature of the FFLO state.

References

[1] P. Fulde and R. A. Ferrell, Phys. Rev. 35, A550 (1964). [2] A. I. Larkin and Y. N. Ovchinnikov, Zh. Eksp. Teor. Fiz. 47, 1136 (1964). [3] S. Imajo, T. Nomura, Y. Kohama, and K. Kindo, Nat. Commun. 13, 5590 (2022).

Acknowledgment

We acknowledge the supports by Japan Society for the Promotion of Science KAKENHI Grant 20K14406 and 22H04466.