Tuesday, August 11, 2009

Frustration produces a valence bond state

Ever since Andersons seminal RVB Science paper in 1987, many theoretical papers on quantum magnetism have considered valence bond ground states for frustrated quantum antiferromagnets. However, there has been a paucity of experimental realisations.

But, a few years ago Tamura, Nakao, and Kato clearly observed such a state in the MOtt insulating phase of an organic charge transfer salt based on the Pd(dmit)2 molecule.

The definitive experimental signatures were:
  • a structural phase transition associated with dimerisation of the pairs of molecules associated with each spin
  • a gap in the magnetic susceptibility below the transition
The magnitude of the gap (between 30 and 50 K) can be compared to the exchange interaction J of about 250 K estimated from the temperature dependence of the magnetic susceptibility.

This all fits nicely with the results of calculations (using series expansion techniques) on the relevant Heisenberg model on an anisotropic triangular lattice.

If one is close to the isotopic triangle, i.e., J'/J is between 0.65 and 0.95, then the ground state has valence bond order and there is a spin gap with the magnitude shown below.
[ to compare notation J'/J=J1/J2]The magnitude of the gap, (0.1-0.2) J is consistent with the experiment.

Later I will write about how one can apply pressure and produce superconductivity. This really shows the interplay between frustration, quantum magnetism, and superconductivity anticipated by Anderson.

Some of the above figures were taken from a nice talk by Subir Sachdev.

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