Thursday, August 18, 2016

Signatures of strong electron correlations in the Hall coefficient of organic charge transfer salts

Superconducting organic charge transfer salts exhibit many signatures of strong electron correlations: Mott insulator, bad metal, renormalised Fermi liquid, ...

Several times recently I have been asked about the Hall coefficient. There really is little experimental data. More is needed. But, here is a sample of the data for the metallic phase.
Generally, increasing pressure reduces correlations and moves away from the Mott insulator. Almost all of these materials are at half filling and at high pressures there is well defined Fermi surface, clearly seen in angle dependent magnetoresistance and quantum oscillation experiments.

The figure below is taken from this paper. At low temperatures the Hall coefficient is weakly temperature dependent and has a value consistent with the charge carrier density, i.e., what one expects in a Fermi liquid. However, about 30 K, which is roughly the coherence temperature, corresponding to the crossover to a bad metal, R_H decreases significantly, and appears to change sign.

The next data is from this paper and shows measurements on two different samples of the same material.
Note how in the two samples for a pressure of 4 kbar the temperature dependence and magnitude is not the same. This should be a point of concern about the reliability of the measurements.
But, broadly one sees again a significant temperature dependence, particularly on the scale of the coherence temperature.

Finally, the data below is from a recent PRL, and is for a material that is argued to be away from half filling (doped with 0.11 holes per lattice site (dimer)).

At high pressures there are a large number of charge carriers and weak temperature dependence, consistent with a Fermi liquid with a "large" Fermi surface.
However, at low pressure (i.e. when the metal is more correlated) the Hall coefficient becomes large and temperature dependent.

I thank Jure Kokalj, Jernez Mravlje, Peter Prelovsek, and Andre-Marie Tremblay for stimulating discussions about the data.

I welcome any comments.
Later I will post about the theoretical issues.


  1. Regarding the sign change: these are measured at different pressures.
    (reproducing them is necessary, but) the way I see it now is that the sign change happens near 10 kbar.
    Hence it's not accessed in the left figure but is in the right.

    1. Thanks for pointing out my error. I have corrected the post. I now see that the differences between the samples are not as great as I thought.