Suppression of the charge fluctuations by nonlocal correlations close to the Mott transition

Abstract

In this paper, we investigate the impact of nonlocal correlations on charge fluctuations in the two-dimensional single-band Hubbard model close to the Mott metal-to-insulator transition, employing the ladder dynamical vertex approximation. At half filling and for interaction strengths and temperatures where the system is in the Mott insulating phase, charge fluctuations are strongly suppressed. Under these conditions, dynamical mean-field theory (DMFT) calculations predict a strong enhancement of the charge susceptibility at small (electron or hole) doping. However, these DMFT results include only the effects of purely local correlations despite the importance of nonlocal correlations in two-dimensional systems. We have, hence, carried out ladder dynamical vertex approximation (lD$\Gamma$A) simulations which allow for the inclusion of such nonlocal correlation effects while retaining the local ones of DMFT. Our lD$\Gamma$A numerical data show that close to half filling the large uniform charge susceptibility of DMFT is strongly suppressed by nonlocal fluctuations but gradually increases with (electron) doping. At a certain doping value, charge fluctuations eventually become larger in lD$\Gamma$A with respect to DMFT, indicating that the absence of nonlocal correlations underestimates the mobility of the charge carriers in this parameter regime. This metallization effect is also reflected in an enhancement of the lD$\Gamma$A kinetic and potential energies and a corresponding reduction of the (absolute value of the) lD$\Gamma$A Matsubara self-energy with respect to DMFT.