Thursday, October 18, 2012

How good metals turn bad

There is a really nice preprint
How bad metals turn good: spectroscopic signatures of resilient quasiparticles
by Xiaoyu Deng, Jernej Mravlje, Rok Zitko, Michel Ferrero, Gabriel Kotliar, and Antoine Georges

They study a doped Hubbard model using Dynamical Mean-Field Theory (DMFT). Although the ground state is a Fermi liquid this is only a good description at very low temperatures. In particular, the quadratic temperature dependence (characteristic of a Fermi liquid) only occurs below a temperature of about 0.05 delta D [where delta=doping and D=band width]. But, well-defined quasi-particles still exist all the way up to the "bad metal" region at which the mean-free path is comparable to the lattice constant.
I found this resilience of quasi-particles somewhat surprising [I am not quite sure why] and interesting. Furthermore, in this intermediate temperature regime there is large entropy and significant local magnetic moments, and Kelvin's formula gives a good description of the temperature dependence of the thermoelectric power.

The strong correlations lead to a significant particle-hole asymmetry (in the self energy and single-particle density of states) via the subtle interplay of the lower Hubbard band with the quasi-particle peak in the density of states.

To me this all illustrates how much (but obviously not all) of the rich physics seen in strongly correlated materials can be captured by DMFT.

Note added. I am told that the axis label on the left figure is in error and it should be 1/Z not Z, i.e. Z actually increases with temperature.


  1. That is a really interesting observation. Does it mean in simple words there is a state between "good metal" (fermi liquid) and "bad metal" state: "resilient quasiparticles" state? And very naive connection with resistivity plot for organic superconductors: a hump ~100K is bad metal-resilient quasiparticles transition, and starting rho~T^2 (~50K) is a resilient quasiparticles-fermi liquid transition, am I wrong?

    /Andrew Bardin

  2. Hi Andrew,
    This is a good question.
    For the organics I think the relevant temperature ranges are
    T < 20 K
    Fermi liquid with T^2 resistivity
    20 K < T < 50 K
    Resilient quasi-particles
    Drude peak in optical conductivity
    Thermopower increases monotonically with temperature
    Resistivity increases rapidly with temperature, faster than T^2
    50 K < T < 100 K
    bad metal
    No Drude peak in optical conductivity
    Thermopower DEcreases monotonically with temperature
    Resistivity is larger than Mott-Ioffe-Regel limit

    What causes the peak in resistivity at 100 K in the organics is controversial.
    Some sample dependence is observed.
    But DMFT calculations do give such a peak when close to the Mott transition