Sunday, August 1, 2010

Getting frustrated about frustrated systems

In a previous post When a little makes a big difference I wrote how in frustrated quantum antiferromagnets small perturbations can have a big effect.
A recent PRB (and an Editors Choice) by Starykh, Katsura, and Balents makes a similar point in great detail, for the specific case of Cs2CuCl4, which as a first approximation can be modelled by a Heisenberg model on an anisotropic triangular lattice. Here is some of their abstract:
First, we find that when the magnetic field is oriented within the triangular layer, spins are actually most strongly correlated within planes perpendicular to the triangular layers. This is despite the fact that the interlayer exchange coupling in Cs2CuCl4 is about an order of magnitude smaller than the weakest (diagonal) exchange in the triangular planes themselves.

Second, the phase diagram in such orientations is exquisitely sensitive to tiny interactions, heretofore neglected, of order a few percent or less of the largest exchange couplings. These interactions, ....induce entirely new phases
What are the general implications of this?
There is both very good and bad news for theory and modelling such materials.

The good news is there is lots of rich and beautiful physics to understand.

The bad news is that most materials (and their residual interactions) are not as well characterised as Cs2CuCl4 it is going to be extremely difficult to model and describe such materials in a definitive way.

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