Thursday, May 21, 2009

Organic superconductors: questions and answers

I have been invited by F.C. Zhang to be the discussion leader for a session on Organic Superconductors at the forthcoming Gordon Research Conference on Superconductivity in Hong Kong. I will have to give a 5-10 minute talk introducing the field and identifying key issues. Speakers will be:

Kazushi Kanoda (University of Tokyo),
"Correlated electrons in quasi-2D organics with triangular lattice - from spin liquid to superconductivity"

Stuart Brown (UCLA)
"Role of magnetic resonance in probing organic superconductivity"

Nicolas Doiron-Leyraud (Sherbrooke)
"Transport properties and phase diagram of quasi-1D organic superconductors

Rolf Lortz
"Superconducting properties of carbon nanotubes"

I hope to get some discussion going on this blog
before the meeting. Here are a few key questions of the top of my head:
  • Is their a clear relationship between superconductivity in these materials and other strongly correlated electron systems?
  • Is the ground state of some materials, a spin liquid? If so, what is the relationship between the spin liquid and superconductivity?
  • In organics is there an anisotropic pseudogap, as in the cuprates?
  • Does RVB theory give the best possible theoretical description of the quasi-2-dimensional organics?
  • For systems close to the isotropic triangular lattice, does the superconducting state have time-reversal symmetry breaking (see Figure below)?
A couple of years ago, Ben Powell and I wrote a review that addresses some of these issues for the quasi-two-dimensional materials based on BEDT-TTF.

1 comment:

  1. Ross,

    Some questions I'd like to know the answers to include

    • Are there fractionalised excitations in any of these materials? One would think q1d materials and spin liquids are great places to look.

    • How much connection is between the physics of these materials? Physicists seem to lump all organic superconductors together because they have similar chemistry. That seems at little strange to me.
    ⁃ The q1d organics show some rather different physics from the q2d organics.
    ⁃ There seems to be quite good evidence for a pseudogap in the q2d materials - Nam et al [Nature 2007] saw a large Nearnst effect above Tc and the maximum in 1/T_1T is well above Tc. In contrast the maximum in 1/T_1T in the q1d superconductors seems to be at Tc [arXiv:0903.2881].

    • Having said that what are the advantages of looking at strong electronic correlations in organic systems?
    ⁃ These systems have small energy scales (compared to say the cuprates). So, while that does mean lower Tc's, it also means we can suppress superconductivity with experimentally accessible fields and pressures. This has allowed for some very nice experiments, such as Kanoda's measurements of the critical exponents of the Mott transition [Kagawa et al., Nature 534, 89 (2005)].
    ⁃ Transition metal oxides are usually tuned by doping them. This often leads, intrinsically, to disorder which can obscure the physics. Organic systems are typically stoichiometric and very clean.
    ⁃ Can we control the emergent physics by controlling the chemistry? One beautiful example is Taniguchi et al.'s work where they drive the Mott transition by deuterating an organic superconductor [PRB 67, 014510, (2003)]. But if we could really understand how the chemistry of these systems controlled the physics, we should be able to really tune a many-body system in much more subtle ways.