Phase transitions in a disordered extended Hubbard model and in the random field Blume-Capel model

The phase transitions in a half-filled random-site-energy extended Hubbard model (RSEHM) and in the random field Blume-Capel model (RFBCM) are studied with the mean-field approximation and with the renormalization group approach. The two-delta, the Gaussian and the square distribution of the random site energy are considered. The phase diagram includes the high charge order, the low charge order, the electron glass and the Mott phase. Depending on the form of distribution function, one may find lines of tricritical points, isolated critical points and triple points, as well as the heat charge order phenomenon. These results are then transformed in order to study the RFBCM, which exhibits a similar phase diagram but without heat magnetization. The effect of fluctuations on the mean-field phase diagram is briefly discussed using the renormalization group method. We conjecture that the tricritical point in a system with random field is different from that in a pure system.