Thursday, October 25, 2012

Searching for the nodes in an unconventional superconductor

In the cuprate high-Tc superconductors it is now well established experimentally that the superconductivity has d_x^2-y^2 symmetry with nodes in the energy gap along the Brillouin zone diagonals. This is what is predicted by spin fluctuation and RVB theories.

In the organic charge transfer salts the experimental situation is not as clear. There are clear theoretical predictions of unconventional superconductivity with nodes in the gap.
Hence, I was interested to read the paper:

Location of gap nodes in the organic superconductors kappa-(ET)2Cu(NCS)2 and kappa-(ET)2Cu[N(CN)2]Br determined by magnetocalorimetry
L. Malone, O.J. Taylor, J.A. Schlueter, and A. Carrington

This experiment is difficult [measuring the specific heat at low temperatures while rotating a magnetic field within the layers] and its interpretation is subtle. The conclusion is that the nodes are as shown below. This is where they are predicted to be by both spin fluctuation and RVB theory. (A nice article by Ben Powell discusses the relevant theory).
The authors (helpfully and honestly) point out that these results are inconsistent with some thermal conductivity measurements. 
However, Tony Carrington pointed out to me:

The point is that the results could be consistent if they are in different parts of the B/T phase diagram.  The problem is that in both thermal conductivity and specfic heat measurements the sign of the oscillations changes as a function of B and T.  In the low B, low T limit both give a maximum when B is in the antinodal direction, but this becomes a maximum at higher T and higher B.  For thermal conductivity this occurs at a different temperature compared to the specific heat (according to Vekhter's model).  So both could be consistent with the same nodal structure.

Something that would be particularly interesting would be to do similar measurements on  a different organic charge transfer described in an earlier post because RVB theory predicts the nodes would be in a different location.


  1. Ross, I a bit misunderstood. Do you imply k-ET and k-BETS salts could have different symmetry of SC gap? Say dx2-y2 vs. dxy?

    PS Long ago you dreamed somebody has Raman data on organics. So I have. But the results are difficult to understand. And we can discuss only pseudogap (all measurements >13K)

    1. Hi Andrey
      Thanks for the question.
      No, I am not implying that in general BEDT-TTF and BETS have different pairing symmetries.
      Rather I am claiming that for the specific materials named the pairing symmetries will be different.

    2. Thanks Ross,
      It is coming clearer. Could you name, please, the possible pairing symmetry for that BETS material then?