Negative thermal expansion in correlated electron system and Fermi liquid theory

We have performed powder X-ray diffraction measurement on many interesting materials from 300 K to below 1 K. From experimental results, we found that many of these materials, such as PrPtBi, Ce0.75La0.25B6, MgB2, Nb show negative thermal expansion (NTE) at low temperatures although these crystal structures are different. From this fact, we think that the NTE at low temperatures is common phenomenon. An explanation for the NTE in the correlated electron systems has been given by Misawa on the basis of Fermi liquid theory [S. Misawa, Solid State Commun., 11, 1972, 1019, S. Misawa, Solid State Commun., 58, 1986, 63, S. Misawa, H. Morita, Physica B, 165 and 166, 1990, 643]. We consider that the NTE we observed can also be explained by this Misawa model.