Josephson vortex dynamics and Fulde-Ferrell-Larkin-Ovchinnikov superconductivity in the layered organic superconductor β″−(BEDT−TTF)2SF5CH

Abstract

Interlayer resistance is reported in a layered organic superconductorβ ′′-(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 with T c ≈ 5 K. At 0.7 K in parallel magnetic fields, the interlayer resistance has nonzero values in a wide field region from 2 T to the upper critical field (∼13 T), which can be ascribed to the Josephson vortex (JV) dynamics. Significant hysteresis between 2 T and 8 T, and a step-like structure between 4 T and 8 T are found, both of which become less evident with increasing temperature. In the second field-derivative curves, successive small dips are observed between 10 T and 12 T, which are ascribed to the commensurability (CM) effect between the JV lattice and the wave length λ FFLO of the order parameter oscillation in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. The CM effect is observed only in nearly parallel fields, showing that the FFLO phase is stabilized only in a limited field angle region. The temperature-field phase diagram and λ FFLO values are obtained.

Fermi Surface Structure and Isotropic Stability of Fulde-Ferrell-Larkin-Ovchinnikov Phase in Layered Organic Superconductor β″-(BEDT-TTF)2SF5CH2CF2SO3

“…At 0.5 K, we see a broad dip at ∼6.5 T, corresponding to the resistance increase from the noise level. Above ∼8 T, we see a quasi-periodic dip structure, which is most pronounced at ∼10 T. This structure can be ascribed to the CM effect, which is observable only in the FFLO phase, as discussed in the previous reports [29]. At higher fields, the CM effect is reduced and vanishes above ∼12 T, which corresponds to the melting transition of the JV lattice.…”

“…Above 2.1 K, such astructure is not evident. As has been discussed [27,29], the dips mean relatively strong pinning of the JV lattice, ascribed to the CM effect between the JV lattice and the periodic nodal structure of the gap ∆(r). From the above results, we obtained the temperature-field phase diagram shown in Figure 5.…”

“…The critical field was determined as H c2 ≈ 13 T at 0.7 K from the specific-heat measurements [23]. The finite resistance in the wide field region H c2 can be ascribed to the motion of the JVs in the insulating layers, as has been observed in various organic superconductors [15,28,29]. To clearly see the corrugations in detail, negative second-derivative curves of the resistance are plotted in Figure 4b.…”

“…In our previous studies, we clarified the FFLO phase boundary in terms of the magnetocaloric effect, torque [35], and resistance measurements [29]. We also observed the CM effect in fields almost parallel to the a-axis in the FFLO phase, above ∼9 T. The λ FFLO values, ranging from ∼40 nm to ∼210 nm, were obtained under the assumption of a single q vector perpendicular to the field.…”

“…This commensurability (CM) effect was first predicted by Bulaevskii et al [27]. Thus far, the CM effect has been observed in the FFLO phases for various organic superconductors, λ-(BETS) 2 FeCl 4 [15], β -(BEDT-TTF) 4 [(H 3 O)Ga(C 2 O 4 ) 3 ]C 6 H 5 NO 2 (β -Ga salt) [28], and β -(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 (β -SF 5 salt) [29]. Among them, the highly 2D nature of the β -Ga and β -SF 5 salts with the large anion layers would provide an excellent platform for the FFLO studies, since JV dynamics play an essential role in the CM effects.…”