Tuesday, April 13, 2010

Organic superconductors at the nanoscale

I just read through the paper (and the Supplementary information), Superconductivity in just four pairs of (BETS)2GaCl4molecules which just appeared in Nature Nanotechnology.

This is certainly an impressive achievement and goes beyond anything I have seen before in these materials.

However, I always ask myself, "have the authors actually seen what they claim to have seen?".
Some of my experimental colleagues tell me that STM measurements, particularly on organic charge transfer salts, are fraught with problems.

First it should be stressed that was has been observed is NOT superconductivity, but rather a temperature dependent "pseudogap" in the tunneling spectrum.
The authors claim to not just see a superconducting gap but also to determine the direction of nodes in the gap, based on curve fitting. I did not follow this. The nodal direction is claimed to be in the direction parallel to the crystal a axis, which is parallel to the layers. The anti-nodal direction is claimed to be in between the a and b direction, where b is the interlayer direction. All theories of superconductivity in
the bulk materials predict that both the nodal and anti-nodal direction are within the layers, but here the c axis is perpendicular to the layer on the Ag(111) surface, and so truncated after one (or two molecules).

A key Figure from the paper is below. The right side shows the dI/dV curves for the chain lengths shown on the right. The graph is the energy gap as a function of chain length.

The energy gaps observed (about 10 meV) are actually about 5 times larger than that observed in a different STM study (ref. 23). This difference is not discussed.

Here is an alternative explanation for the experiments. The systems under study consists of short weakly coupled chains of BETS dimers, which can be described by a one-dimensional Hubbard model, in contrast to the two-dimensional Hubbard model relevant to the bulk materials. At low temperatures this one-dimensional model will always be a Mott insulator and so the energy gap observed may actually be a Mott gap, rather than a superconducting gap.
I welcome comments, particularly from experimentalists familiar with the subtleties of doing STM measurements.

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