Friday, March 4, 2011

New superconducting state in organics

particularly interesting. This material has a qualitatively different band structure from many of the kappa-(BEDT-TTF)2X family of organic charge transfer salts. In terms of the relevant tight-binding model t' > t so that it is closer to the limit of weakly coupled chains rather than to the square lattice [for background see the PRL referenced below and/or this recent review, which will appear in Reports of Progress in Physics]. The figure below shows the band structure and the Fermi surface for the new kappa-BETS material.

The observed pressure-temperature phase diagram is below. The system is always at half-filling and so the insulating state is a Mott-Hubbard insulator.
What is the symmetry of the pairing in the superconducting state.
Based on calculations reported in a PRL by Ben Powell and I, the superconductivity will have A_1 symmetry with "accidental" nodes in the energy gap. In contrast, in other kappa organics (which have t' < 0.8t or so) we predict A_2 symmetry with nodes required by symmetry.
The picture below shows the phase of the superconducting order parameter over the Brillouin zone for parameter values in a comparable regime to the new kappa-BETS material. The solid line is the Fermi surface (and the diagonals correspond to the Y and Z axes shown above.

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