Wednesday, May 8, 2013

Long live Fermi liquid theory!

There is a very nice preprint
Hidden Fermi Liquid, Scattering Rate Saturation and Nernst Effect: a DMFT Perspective
by Wenhu Xu, Kristjan Haule, and Gabriel Kotliar

I think it is original and important. I wish I had written it!

They consider the metallic phase of a two-dimensional Hubbard model at (close to optimal) hole doping 0.15 away from the Mott insulator, within Dynamical Mean Field Theory (DMFT). 

The surprising result (to me) is that one can talk about quasi-particles (i.e. poles in the one electron Green's function) up to much high temperatures than one might expect (specifically, far beyond the temperature T_FL, below which the scattering rate has a quadratic temperature dependence).
One just has to allow the quasi-particle weight Z to be temperature dependent, as shown in the Figure below.
This leads to a temperature dependent band structure.
Furthermore, most of the transport properties calculated within DMFT are quantitatively described by a quasi-particle approximation and Sommerfeld expansion, even into the bad metal region. The graph below shows the temperature dependence of the thermopower. Note the change of sign.
A few comments:

1. The authors suggest there may be a connection to Nigel Hussey's phenomenology of the cuprates, particularly with regard to saturation of the scattering rate at high temperatures.
These ideas are developed more in a recent PRB by Jure Kokalj, Nigel and I.
[But as the authors point out DMFT cannot capture the anisotropy observed in the cuprates].

2. I am not sure about calling this a "Hidden Fermi liquid" since that terminology is associated with a specific idea of Phil Anderson which I discussed here. It is not clear to me that these "Fermi liquids" are the same thing. In particular, Anderson's seems much more exotic.

3. The emergence of the different temperature scales and "strange metal" behaviour confirms my prejudice (and Anderson's) that the AdS/CFT approach is not relevant.

4. Minor quibble: It would be helpful to have units on the axes in Figure 2. I can see that the thermopower S is in units of k_B/e but have no idea about the Nernst signal. This would help in comparing the magnitude to experimental values for the cuprates.

I welcome more comments on this work.

No comments:

Post a Comment