Thermodynamic studies of electron correlation effects on organic salts based on BEDT-TTF and DCNQI molecules

A thermodynamic investigation on the electronic properties of several organ ic salts based o \ hskip24pt \ hbox{(1)}n BEDT-TTF and DCNQI molecules is p erformed by means of high-precision specific-heat measurements in the tempe rature range between 0.9 and 6 K. The electronic specific-heat coefficient gamma for dimeric insulators is almost 0 mJ/mol K-2, which is explained by the picture of Mott-Hubbard gap in the effectively half-filled band. The no n-dimeric salt of (DI-DCNQI)(2)Ag, on the other hand, shows an unexpectedly large gamma value of about 30-40 mJ mol(-1) K-2 representing the existence of large low-energy excitations from the charge-ordered insulating ground state. The difference of the low-temperature electronic specific-heat behav ior between the dimeric and non-dimeric systems demonstrates that manifesta tion of electron correlation effects is qualitatively different in these sy stems. (C) 2000 Elsevier Science Ltd. All rights reserved.