Enhancement of the thermal expansion of organic charge transfer salts by strong electronic correlations
Our main results concerning the electronic contribution to the thermal expansion alpha are as follows.
(i) At low temperatures strong correlations can increase the thermal expansion by as much as an order of magnitude.
(ii) A non-monotonic temperature dependence of alpha is possible.
(iii) Significant orientational dependence is possible, including the expansion having the opposite sign in different directions.
(iv) In the metallic phase the crossover from a Fermi liquid to a bad metal may be reflected in a maximum in the temperature dependence of alpha.
(v) In the Mott insulating phase a maximum in the temperature dependence of alpha can occur, at a temperature comparable to that at which a maximum also occurs in the specific heat and the magnetic susceptibility.
(vi) All of the above results are sensitive to the proximity to the Mott metal-insulator transition and the amount of frustration, reflected in the parameter values (U/t and t'/t) in the Hubbard model.
Although, we can describe many of the unusual qualitative features of experimental data for organic charge transfer salts, the overall magnitude of the thermal expansion coefficients that we calculate are
up to an order of magnitude smaller than observed. This disagreement may arise from uncertainties in how uniaxial stress changes the Hubbard model parameters, and uncertainty in the compressibilities
including not taking into account the effect of softening of the lattice associated with proximity to the Mott transition.
We welcome any comments.