Monday, June 1, 2009

Magnetic field induced superconductivity

Normally magnetic fields destroy superconductivity not help create it! However, in 2001 Uji and collaborators discovered that while the organic charge transfer salt (BETS)2FeCl4 had an insulating ground state applying a magnetic field parallel to the layers could create a metal, and for sufficiently high magnetic fields, superconductivity! This can be explained in terms of the exchange interaction between the magnetic Fe3+ ions and the pi electrons in the BETS molecules. When this exchange interaction is cancelled by the applied field (the Jaccarino-Peter effect, first proposed in the 1960's) the electron spins effectively see zero magnetic field.

I was very happy that in our paper Olivier Cepas, Jaime Merino, and I were able to predict the magnetic field range in which one should observe magnetic-field induced superconductivity for a specific material and this was subsequently observed.

One can tune between Mott insulating, metallic, and superconducting states by varying the magnetic field, temperature, or the relative concentration of magnetic Fe3+ ions and non-magnetic Ga3+ ions. This leads to a rich phase diagram such as that shown below. For a review see here.

Because the relevant field and temperature scales are so small these means that the superconducting and Mott insulating phase are very close in energy. i.e., within a few meV, which is orders of magnitude smaller than the energy scales in the underlying Hubbard Hamiltonian. Such a competition occurs naturally in RVB theory.

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