Quantum fluctuations, pseudogap, and the T=0 superfluid density in   strongly correlated d-wave superconductors
  I study the effect of Coulomb interaction on superconducting order in a d-wave lattice superconductor at T=0 by considering the superconducting saddle point in the two dimensional t-J-U model with an on-site repulsion U. The theory of low energy phase fluctuations around this saddle point is derived in terms of the effective hard-core bosons (representing the density of spin-up electrons and the order parameter phase), interacting with the fluctuating density of spin-down electrons. Whereas the saddle point value of the gap is found to monotonically increase towards half filling, the phase stiffness at T=0 has a maximum, and then decreases with further underdoping. Right at half filling the stiffness vanishes for large U.   This argues that the pseudogap phenomenon of the type observed in cuprates is in principle possible without a development of any competing order, purely as a result of growing correlations in the superconducting state. The effects of finite temperature and disorder are discussed qualitatively. 
Igor F. Herbut