Superconducting phases in Bechgaard salts in an applied magnetic field

The superconducting critical fields in quasi-1d organic Bechgaard salts are very large. Such large values are not compatible with a uniform singlet phase. Therefore, two different order parameter symmetries have been proposed to account for these large values of the critical fields: (i) a non-uniform singlet FFLO phase or (ii) a triplet phase. The real three-dimensional character of the crystal structure has to be taken properly into account. Experimental data indicate the following orders of magnitude for the three transfer integrals along the main crystal axis: ta=3000 K, tb=300 K and tc in the 5-10 K range. The spin effect, as well as the orbital effect has been taken into account to calculate the critical field. We find that the critical field strongly decreases as a function of tc, as a result of a strong orbital pair breaking. A value of tc=5 K is enough to reduce the FFLO critical field to about 0.3 T. Therefore, a finite and even not very large Pauli limit exists in the FFLO phase. On the contrary, a re-entrant superconductivity is still present in the triplet phase. Thus, we expect that, in large enough applied field, a singlet-triplet field-induced phase transition should occur. The NMR experimental data of Shinagawa et al. seem a good indication of this phenomenon.