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

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, em-
ploying 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 nonlo-
cal correlations in two-dimensional systems. We have, hence, carried out ladder dynamical vertex
approximation (lDΓA) simulations which allow for the inclusion of such nonlocal correlation effects
while retaining the local ones of DMFT. Our lDΓA 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 lDΓA 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 lDΓA kinetic and potential energies and a corresponding
reduction of the (absolute value of the) lDΓA Matsubara self-energy with respect to DMFT.