Properties of the isothermal compressibility in the 2D t-J model

The isothermal compressibility for the 2D t-J model has been calculated for the full range of densities at J/t = 0.4 by using a 12th order high temperature expansion.  The compressibility is found to have a subtantial peak for n ≈ 0.8, where the dominant fluctuations in the model change from antiferromagnetic fluctuations for n ≥ 0.8 to d-wave pair fluctuations for n ≤ 0.8.  A smaller peak is found near n ≈ 0.5.  For densities below this smaller peak it is likely d-wave pair fluctuations are no longer present.  Results from the temperature derivative of the pressure and the d-wave pair correlation length show changes in behavior where the compressibility has peaks.