Organic superconductors in one and two dimensions

I will discuss superconductivity in two classes of materials in the seminar. The first class is the quasi-two-dimensional organic charge transfer salts (BEDT-TTF)₂X. The phenomenology of the superconducting state is still a matter of contention. I will critically review the experimental situation [1], focusing on the key experimental results that may distinguish between rival theories of superconductivity, particularly probes of the pairing symmetry and measurements of the superfluid stiffness. I will then discuss some strongly correlated theories of superconductivity, in particular the resonating valence bond (RVB) theory of superconductivity [2]. I will show that this theory predicts that magnetic frustration can drive the superconductor into a state with broken time reversal symmetry [3].

Secondly I will discuss the nature of the superconducting state in the Bechgaard salts, which have long been thought to be triplet superconductors. However, triplet superconductors may exhibit a wide range of distinct superconducting phases. Therefore, the question remains - which triplet phase(s) is (are) realised in the Bechgaard salts. We present a group theoretical analysis [1] of the superconducting state of the Bechgaard salts, e.g., (TMTSF)₂PF₆ or (TMTSF)₂ClO₄. We show that there are eight symmetry distinct superconducting states. Of these only the (fully gapped, even frequency, p-wave, triplet) ‘polar state' is consistent with the full range of the experiments on the Bechgaard salts. The gap of the polar state is d(k)~(ψ_uk,0,0), where ψ_uk may be any odd parity function that is translationally invariant.

We predict that a Freedericksz transition between the BW and polar phases occurs at low fields [1]. The change between the BW and polar states is caused by moving from the regime where the orientation of d(k) is determined by the orientation of the magnetic field to the regime where this is determine by the microscopic properties of the crystal. We show that this change is associated with symmetry breaking therefore is associated with a phase transition rather than a crossover. As such it is highly analogous to the Freedericks transition associated with the change the orientation of the Frank director in liquid crystal confined in a slab geometry [2]. We will argue that recent experiments measuring the Knight shift and NMR relaxation rate in (TMTSF)₂ClO₄ [3] have confirmed our prediction.

[1] B.J. Powell and R.H. McKenzie, J. Phys.: Condens. Matter 18, R827 (2006).

[2] B.J. Powell and R.H. McKenzie, Phys. Rev. Lett. 94, 047004 (2005).

[3] B.J. Powell and R.H. McKenzie, Phys. Rev. Lett. 98, 027005 (2007).

[4] B.J. Powell, arXiv:cond-mat/0606188.

[3] J. Shinagawa, et al., Phys. Rev. Lett., 98, 147002 (2007).