Fig. 1: Schematics of the FFLO state, crystal structure, and experimental setup.
From: Emergent anisotropy in the Fulde–Ferrell–Larkin–Ovchinnikov state
a Schematics of ordinary spin-singlet pairing and FFLO pairing. k and σ represent the momentum and spin of the electrons. b Spatial modulation of order parameter Δ(r) in the FFLO state (dashed curve). The normal state (blue) appears at nodes of the superconducting order parameter Δcos(qr). c Quasi-two-dimensional crystal structure of κ-(BEDT-TTF)2Cu(NCS)2. d Fermi surface of κ-(BEDT-TTF)2Cu(NCS)2. The solid and dashed black lines are the first Brillouin zone and the extended Brillouin zone, respectively. The blue and red curves show the Fermi surfaces, and the green arrow indicates the most predominant nesting vector. e Experimental setup for the multidirectional ultrasound measurements using longitudinal sound waves. The magnetic field was rotated from the c-axis to the a*-axis. θ is the polar angle from the c-axis. f Relative change in the sound velocity Δv/v (red, left axis) and attenuation coefficient Δα (blue, right axis) at T = 1.6 K and θ =90° as a function of magnetic field. The black arrow indicates the upper critical field of the superconductivity Hc2.
