Tuesday, May 26, 2009

Quantifying antiferromagnetic spin fluctuations

At the Gordon Research Conference on superconductivity, Nicolas Dorion-Leyraud is going to talk about his work described in the very nice preprint, Correlation between linear resistivity and Tc in organic and pnictide superconductors. It contains very detailed measurements of the temperature and pressure dependence of the resistivity of two Bechgaard salts, (TMTSF)2X where X=PF6,ClO4. These materials have a quasi-one-dimensional electronic structure. The resistivity is fit to a quadratic temperature dependence with A the co-efficient of the linear term [not to be confused with the quadratic coefficient associated with the Kadowaki-Woods ratio, and usually also denoted A!]. The Figure below shows how both A and the superconducting transition temperature Tc decrease with increasing pressure, as one moves away from the spin-density-wave phase which occurs below about 5 kbar.

The next Figure below shows how one also observes a similar correlation between A and Tc, for the X=ClO4 material, new pnictide superconductors, and overdoped cuprates. The paper discusses these results in the theoretical framework of recent calculations from two of the authors, Bourbonnais and Sedeki, who have a preprint, Link between antiferromagnetism and superconductivity probed by nuclear spin relaxation in organic conductors. The corresponding theory emphasizes the importance of the interference between superconducting and spin-density-wave fluctuations.

One of my first thoughts is:
how does this compare to what the antiferromagnetic spin fluctuation theory of Moriya and Ueda would predict?
Near an antiferromagnetic quantum critical point in two-dimensions they also predict the temperature dependence of the resistivity will be linear and the nmr T1 relaxation rate will be Curie-Weiss like. They also find that Tc is correlated with the energy scale T0, of the spin fluctuations. It is not clear to me what this theory predicts for the coefficient A as one moves away from the quantum critical point. It looks like at the QCP the resistivity slope A, scales with 1/T0.

It would be really nice to see an analysis that compares the two theories to both nmr, resistivity, and Tc data with a single set of parameters for each pressure.

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