New details in the superconducting phase diagram of λ-(BETS)₂GaCl₄

Abstract: New low-noise rf penetration depth measurements of the high-field superconducting state in the quasi-2d organic superconductor $\lambda$-(BETS)₂GaCl₄ are presented and compared to previous measurements of the same material [Coniglio, et al., Phys. Rev. B 83, 224507 (2011); Mielke, et al., J. Phys.: Condens. Matter 13 (2001)]. The new data show very clear indication of a phase transition within the superconducting state, with the position of the $H_P$ phase line significantly lower than in less clean samples, while the $H_{c2}$ phase line is unchanged. Shubnikov-de Haas oscillations, previously never seen below 32 T in this material, are observed at fields as low as 13 T, indicating that there is less scattering in these new samples. $H_{c2}$ is usually sensitive to spin-orbit scattering, suggesting that the unchanged upper critical field is not traditional, but rather the destruction of a FFLO state. In contrast, $H_P$ should still be sensitive to changes of the spin-orbit scattering rate, consistent with the new data.