The dirt on spin liquids

The organic charge transfer salt -(ET)₂Cu₂(CN)₃ has attracted a lot of attention the past few years because there is significant experimental evidence that the ground state of the Mott insulating phase is a spin liquid.

One important and puzzling observation is that the NMR lines are much more broadened than those of comparable materials undergo antiferromagnetic ordering. Furthermore, this broadening increases significantly with decreasing temperature.

A nice paper by Gregor and Motrunich performs several model calculations to see if they can explain this large broadening by taking into account the role of disorder. They find they can only explain the experimental data above about 5 K, if there is much larger disorder than expected and that it is strongly temperature dependent.

It is interesting that the authors have done comparable calculations for a kagome antiferromagnetic and they can explain the experimental data for that.

Gregor and Motrunich mention that it is hard to estimate the strength of the disorder and the role of temperature dependent screening.

A recent paper by two of my UQ colleagues may help a:

Toward the parametrization of the Hubbard model for salts of bis(ethylenedithio)tetrathiafulvalene: A density functional study of isolated molecules

J. Chem. Phys. 130, 104508 (2009)

http://link.aip.org/link/?JCPSA6/130/104508/1

They find that the difference in the site energies for BEDT-TTF molecules in the staggered and eclipsed conformations is 0.12 eV.

I note that this is comparable to the single dimer Hubbard U ~ 0.2 eV estimated from the optical conductivity. This could have a big effect on the exchange interaction between the localised spins in the Mott insulating phase (see equation B2 in Gregor & Motrunich).

Hopefully, the findings in these two papers can be combined to pin down just how large the disorder is in this material, which is such a promising candidate for a spin liquid.

The two really hard questions that remain are:

Is the large line broadening a definitive signature of a gapless spin liquid?

Is disorder essential to understand this line broadening?