The origin of the difference in the superconducting critical temperatures of the β H and β L phases of ( BEDT - TTF ) 2 I 3

Incommensurate lattice fluctuations are present in the βL phase (Tc ∼ 1.5 K) of ET2I3 (where ET is BEDT-TTF bis(ethylenedithio)tetrathiafulvalene) but are absent in the βH phase (Tc ∼ 7 K). We propose that the disorder in the conformational degrees of freedom of the terminal ethylene groups of the ET molecules, which is required to stabilise the lattice fluctuations, increases the quasiparticle scattering rate and that this leads to the observed difference in the superconducting critical temperatures, Tc, of the two phases. We calculate the dependence of Tc on the interlayer residual resistivity. Our theory has no free parameters. Our predictions are shown to be consistent with experiment. We describe experiments to conclusively test our hypothesis. It has long been known that the application of hydrostatic pressure, P, to β-ET2I3 has a dramatic effect on the superconducting critical temperature, Tc. At ambient pressure Tc ∼ 1.5 K but when the applied pressure reaches P ∼ 1 kbar a discontinuous increase in Tc (∼ 7 K) is observed. The low Tc state (P 1 kbar) is denoted the βL phase and the high Tc state (P 1 kbar) is labelled the βH phase. When the pressure on the βH phase is decreased the material does not return to the βL phase but rather Tc is seen to further increase. Below T ∼ 130 K the resistivity of the βH phase is found to undergo a discontinuous decrease while no such anomaly is found in the βL phase [1]. Incommensurate lattice fluctuations have been observed in the βL phase but they are absent in the βH phase below T ∼ 130 K [2]. The incommensurate lattice fluctuations are stabilised by variations in the conformational ordering of the terminal ethylene groups of the ET molecules and thus can only exist in the presence of disorder [3]. For a recent review of this phenomenology see [4]. Although the change in resistivity between the βL and βH phases and the incommensurate lattice fluctuations have been studied both theoretically [3] and experimentally [1, 2, 4], no explanation of the change in Tc has been forthcoming. In this paper we propose that difference in the Tc’s of the two phases is due to disorder, we will show that current data is consistent with our explanation and suggest experiments which could clearly determine whether or not this is the correct explanation of the difference in Tc. In a recent paper it was shown [5] that intrinsically non-magnetic disorder decreases the Tc of a wide range of the superconducting salts of ET in line with the Abrikosov–Gorkov (AG) formula: