Electron correlations in narrow-band systems at finite temperatures: Gutzwiller-type variational approach

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Abstract
A theory is presented for finite-temperature band magnetism which takes into account the effect of electron correlations using the Gutzwiller-type variational approach. The functional-integral method recently proposed by Kotliar and Ruckenstein (1986), is combined with the alloy-analogy approximation to include the effect of local spin fluctuations at finite temperatures. The theory at T=0 K is equivalent to Gutzwiller's approximation for the correlated ground state, while in the high-temperature limit it reduces to the local spin-fluctuation theory developed previously by Hasegawa and Hubbard. Numerical calculations for the half-filled simple cubic Hubbard model demonstrate that both electron correlations and local spin fluctuations play important roles at finite temperatures. The Brinkman-Rice-type metal-insulator transition is critically discussed on the basis of the model calculation.