Enhancement of maximum superconducting temperature by applying pressure and reducing the charge transfer gap

Abstract: Recent Scanning Tunneling Spectra(STS) measurement on underdoped cuprate discovers the increase of the maximum superconducting transition temperature $T_c$ when the size of charge transfer gap (CTG) is reduced. Applying pressure is another well known method to increase maximum $T_c$. However, these pressure experiments also found another puzzle that $T_c$ is enhanced in underdoped and optimal doped samples but suppressed in overdoped. Here we present a possible mechanism based on the charge fluctuation to explain both these two effects simultaneously. Starting from 3-band Hubbard model, we retrieve the charge fluctuation(CF) between oxygen 2$p<sup>6$ band and copper 3$d<sup>{10}$ band which is ignored in the $t-J$ model. This model is studied via variational Monte Carlo method(VMC).