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We have studied the onset temperature of the superconductivity Tconset of the organic superconductor (TMTSF)2ClO4, by precisely controlling the direction of the magnetic field H. We compare the results of two samples with nearly the same onset temperature but with different scattering relaxation time τ.
We revealed a complicated interplay of a variety of pair-breaking effects and mechanisms that overcome these pair-breaking effects. In low fields, the linear temperature dependences of the onset curves in the H-T phase diagrams (Fig.1) are governed by the orbital pair-breaking effect. The dips in the in-plane field-angle φ dependence of Tconset, which were only observed in the long-τ sample (Fig.2), provides definitive evidence that the field-induced dimensional crossover enhances the superconductivity if the field direction is more than about 19° away from the a axis. In the high-field regime for H//a, the upturn of the onset curve for the long-τ sample indicates a new superconducting state that overcomes the Pauli pair-breaking effect but is easily suppressed by impurity scatterings. The Pauli effect is also overcome for H//b' by a realization of another state for which the maximum of Tconset(φ) occurs in a direction different from the crystalline axes. The effect on Tconset of tilting the applied field out of the conductive plane suggests that the Pauli effect plays a significant role in determining Tconset.
The most plausible explanation of these results is that (TMTSF)2ClO4 is a singlet superconductor and exhibits Fulde--Ferrell--Larkin--Ovchinnikov (FFLO) states in high fields.
This result is published in the Journal of the Physical Society of Japan.