Superconducting and SDW condensation energies in the 2D Hubbard model and the meaning to the stripe phase

The variational Monte Carlo calculations give a finite bulk-limit value of superconducting (SC) condensation energy E-cond in the 2D Hubbard model wit h next-nearest-site transfer energy t' when electron density p greater than or equal to 0.84 with -0.25 less than or equal to t' less than or equal to -0.10: our energy unit is the transfer energy between the nearest-neighbor (n.n.) sites. However. the SDW E-cond computed by the same method is large r than the SC one in such a region. This suggests that the SC region is ver y restricted, if it exists. When eve improved the trial wave function takin g into account the n.n.-site correlation, the SC E-cond increased moderatel y while the SDW Econd decreased considerably. With this modified variationa l Monte Carlo method we have found a set of parameter values for which the bulk-limit SC E-cond is finite whereas that for the SDW vanishes. The SC E- cond Obtained in both ways around the optimal doping is in fair agreement w ith the experimental value similar to0.26 meV/site for YBCO estimated from the specific heat and the critical magnetic field, in contrast to the case of the t-J model which gives a value larger by almost two orders of magnitu de. In the low-doping region the SDW Econd is overwhelmingly larger than th e SC E-cond. Further, the striped SDW state stems more stable here. This is considered to be the energetic origin of the stripe features and presumabl y of the consequent pseudogap in the underdoped region. (C) 2000 Elsevier S cience Ltd. All rights reserved.