Upper critical field in the organic superconductorβ″−(ET)2SF5CH

Abstract

We report upper critical-field measurements in the metal-free-all-organic superconductor ␤Љ-͑ET͒ 2 SF 5 CH 2 CF 2 SO 3 obtained from measuring the in-plane penetration depth using the tunnel diode oscillator technique. For magnetic field applied parallel to the conducting planes the low-temperature upper critical fields are found to exceed the Pauli limiting field calculated by using a semiempirical method. Furthermore, we found a signature that could be the phase transition between the superconducting vortex state and the Fulde-FerrellLarkin-Ovchinnikov state in the form of a kink just below the upper critical field and only at temperatures below 1.23 K.In this Rapid Communication we report the upper critical field H c2 versus temperature phase diagram of ␤Љ-͑ET͒ 2 SF 5 CH 2 CF 2 SO 3 over temperatures ranging from 0.45 to 4.0 K and find an enhancement of H c2 over the Pauli limiting field H p at temperatures below 1.23 K. An additional lower-field phase transition that appears as a kink just below H c2 and only at temperatures below 1.23 K has been observed. We suggest that this lower-field phase transition could be a signature of the superconducting vortex state ͑SCVS͒-to-Fulde-Ferrell-Larkin-Ovchinnikov ͑FFLO͒ phase transition.Since the first discovery of organic superconductors in 1980, 1 their unique features, such as low dimensionality and cleanness due to self-organizing, have drawn a lot of interest. 2 Due to these features, several theoretical studies have suggested organic superconductors as possible candidates for the FFLO state. [3][4][5] The FFLO state is an additional superconducting state that stabilizes above H p when the superconducting order parameter lowers its free energy by becoming spatially inhomogeneous in a high magnetic field. It was predicted by Fulde et al. 6 and independently by Larkin et al. 7 in 1964. Subsequent theoretical studies have shown that heavy fermion and quasitwo-dimensional organic superconductors are possible candidates for the FFLO state. [3][4][5] In the past decade several heavy fermion materials, such as CeRu 2 ͑Ref. 8͒ and UPd 2 Al 3 , 9 were suggested to have a FFLO state, but these observations were later found to be due to flux motion 10 or to be inconsistent with theoretical models. 11 A more unambiguous observation of the FFLO state was reported in CeCoIn 5 by measuring the specific heat and magnetization 12 and was confirmed by subsequent specific-heat 13 and penetration depth measurements. 14 In organic superconductors there have been several reports of the possible existence of the FFLO state. [15][16][17][18] In the metal-free-all-organic superconductor ␤Љ-͑ET͒ 2 SF 5 CH 2 CF 2 SO 3 the enhancement of H c2 above the value of H p has been observed at low temperatures. 19 This observation raised the possibility of the existence of the FFLO state. The material also satisfies two important conditions necessary to become a candidate for the FFLO state. One is that the material be in the Pauli limit, which means that the orbital limiting field H orb is much gre...

Magnetic torque evidence for the Fulde-Ferrell-Larkin-Ovchinnikov state in the layered organic superconductorκ−(BEDT−TTF)2CuBergk1, Demuer2, Sheikin3 et al. 2011Phys. Rev. B10481170View full textAdd to dashboardCite

“…22 A pronounced upturn of H c2 at low temperatures and indications for a second phase transition within the superconducting state close to the Pauli limit could also be determined. The high-field phase was interpreted as the FFLO state.…”

Emerging evidence for FFLO states in layered organic superconductors (Review Article)

“…These well-studied materials [8][9][10][11][12] are strong type-II spin-singlet superconductors of high purity with long mean free pathes. During the last years, there indeed have been a number of reports giving growing evidence for the existence of an FFLO state in 2D organic superconductors [13][14][15][16][17][18][19][20].…”

Orbital effect for the Fulde-Ferrell-Larkin-Ovchinnikov phase in a quasi-two-dimensional superconductor in a parallel magnetic field

“…[27,28] is that the orbital effect is of the relative order of t 2 ⊥ /T 2 c0 1. From the experimental side, plenty of experimental works on Q2D organic and some other superconductors have been performed [29][30][31][32][33][34][35][36][37][38][39][40][41] to establish the possible existence of the FFLO phase in a parallel magnetic field.The goal of our paper is to consider the orbital effect in a parallel magnetic field in a Q2D conductor at zero temperature, in contrast to Refs. [27,28].…”