  We use scanning tunneling microscopy to study the temperature evolution of
electronic structure in Ca3Cu2O4Cl2 parent Mott insulator of cuprates. We find
that the upper Hubbard band moves towards the Fermi energy with increasing
temperature, while the charge transfer band remains basically unchanged. This
leads to a reduction of the charge transfer gap size at high temperatures, and
the rate of reduction is much faster than that of conventional semiconductors.
Across the Neel temperature for antiferromagnetic order, there is no sudden
change in the electronic structure. These results shed new light on the
theoretical models about the parent Mott insulator of cuprates.
