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