Wednesday, May 29, 2013

Write your abstract for your audience

Next week I am giving the Quantum science seminar at UQ. This is attended by people with diverse interests and backgrounds: cold atoms, condensed matter, quantum information, and quantum optics.

Hence, I have written a talk abstract that is hopefully attractive and interesting enough to motivate people to come to the talk.

Comments welcome.

When good metals turn bad: from organic superconductors to ultracold atomic gases

Key properties usually associated with metals are that they are shiny and excellent conductors of electricity and heat. Hence, one might think that the best strategy to find a good superconductor (e.g. one that works at room temperature) is to study good metals. Actually, the opposite is true. The past few decades have shown that the most interesting and important metals are "bad metals". They often occur in proximity to a Mott insulating phase. Bad metals are characterised by a large electrical resistance of the order of the quantum of resistance h/e^2 and their theoretical description is an outstanding problem.

I will discuss the distinct experimental and theoretical signatures of bad metals [1]. They occur in a wide range of correlated electron materials, including high-Tc cuprate superconductors, heavy fermion compounds, and superconducting organic charge transfer salts [2]. A key feature is that with increasing temperature good metals turn bad, at a temperature corresponding to the loss of quantum coherence.

The simplest possible effective Hamiltonian for organic charge transfer salts is a one-band Hubbard model on an anisotropic triangular lattice at half-filling [2]. The model exhibits a transition from a Mott insulator to a bad metal as the interaction (U/t) is reduced or the frustration (t'/t) is varied.
A recent numerical study of the model [3] showed that near the Mott insulator the calculated quantum coherence temperature was much less than the non-interacting Fermi temperature, consistent with experiment. Furthermore, the bad metal is characterised by a small charge compressibility, a large spin susceptibility, and fluctuating local magnetic moments.

Finally, I will discuss the connection with the viscosity of perfect fluids, including experiments on ultracold atomic gases, and calculations based on string theory techniques!

[1] J. Merino and R.H. McKenzie, Phys. Rev. B 61, 7996 (2000).
[2] B.J. Powell and R.H. McKenzie, Rep. Prog. Phys. 74, 056501 (2011).
[3] J. Kokalj and R.H. McKenzie, Phys. Rev. Lett. 110, 206402 (2013).

1 comment:

  1. Silly comment but you state that "Bad metals are characterised by a large electrical resistance of the order of the quantum of resistance h/e^2 and their theoretical description is an outstanding problem."
    We usually don't refer to the resistance but to the resistivity of compounds (Ohms cm) which makes your comparison to h/e^2 (Ohms only) a bit strange. I suspect that you are referring to the sheet resistance.