Charge Mass Singularity in Two-Dimensional Hubbard Model

Two-dimensional Hubbard model with both nearest- and next-nearest-neighbor transfers is studied. A singularity in the charge susceptibility at the Mott transition point is observed, leading to the anomaly of the charge mass mx in the form mtoz\Sr\where S is the doping concentration. The singularity is similar to that in the case without the next-nearest-neighbor transfer. This indicates that the singularity in the charge mass is a universal nature of this model, irrespective of its band structure.