Tuesday, March 22, 2011

Strongly correlated electron systems in high magnetic fields

It is surprising to me how little theoretical attention has been given to this important question. The development of new high magnetic field facilities (50 to 100 Tesla) means that there will be a new generation of experimental data available. A few basic questions are the following:
  • What is the magnetic field scale that is required to significantly modify the metallic state of a strongly correlated material?
  • What is the relative importance of coupling of the field to orbital and spin degrees of freedom?
Several interesting experiments that are relevant are:
  • In heavy fermion metals a large magnetic field can suppress the effective mass enhancement.
  • In an organic charge transfer salt a magnetic field can be used to drive the system from the metallic state into the Mott insulating state [see this PRL].
  • In cuprates when one measures quantum oscillations is the role of the magnetic field just to suppress the superconducting state or does it also change the character of the metallic state (e.g., pseudogap or marginal Fermi liquid)?
There is a nice theory paper, Field-dependent quasiparticles in the infinite-dimensional Hubbard model by Bauer and Hewson. They find that a magnetic field [which just couples to the spins via the Zeeman effect] can drive the metallic phase into the Mott phase.

However, it is important to appreciate the magnetic field scales involved here. If U/t is large enough that the quasi-particle weight Z=0.1 then it requires a field h=g mu_B B/2t =0.2 to be driven into the Mott phase. Moreoever, for h<0.1 there is only a very small change in Z. But, how big are these fields in Tesla?
If the tight binding parameter t ~ 30 meV, as it is in some organic charge transfer salts then h =0.1 for a field of 50 Tesla. Note, that in many transition metal oxides t may be orders of magnitude larger. Hence, I feel there must be different physics going on in some of these materials that are sensitive to smaller fields.

Similar considerations and concerns apply to papers by Lai and Motrunich on the effect of a magnetic field on frustrated ladders with Bose liquid ground states that consider the coupling of a field to orbital and spin degrees of freedom. The phase diagrams are interesting but the magnetic field scales required are on the scale of at least hundreds of Tesla.

1 comment:

  1. Dear Colleagues,

    “In heavy fermion metals a large magnetic field can suppress the effective mass enhancement”. Let me note that such a research has been carried out: “High-field phase diagram of the heavy-fermion metal YbRh2Si2”, New Journal of Physics 8 (2006) 171, P. Gegenwart et al. A theory explaining the observed experimental facts was presented in: EPL 93, 17008 (2011). The obtained results are in good agreement with the experimental facts.

    With warm regards,
    Vasily Shaginyan