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

Recent Scanning Tunneling Spectra(STS) measurements on underdoped cuprates have found that the maximum superconducting transition temperature T-c increases when the size of charge transfer gap (CTG) is reduced. Applying pressure is another well-known method to increase the maximum T-c. However, these pressure experiments also found that T-c is enhanced in underdoped and optimally-doped samples but suppressed in overdoped ones. Here we present a possible mechanism based on the charge fluctuation to explain both these observed effects. Starting from the 3-band Hubbard model, we retrieve the charge fluctuation (CF) between the oxygen 2p(6) band and copper 3d(10) band, which is ignored in the t - J model. This model is investigated using the variational Monte Carlo method(VMC).