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 with 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 larger than the SC one in such a region. This suggests that the SC region is very restricted, if it exists. When eve improved the trial wave function taking into account the n.n.-site correlation, the SC E-cond increased moderately while the SDW Econd decreased considerably. With this modified variational 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 with 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 magnitude. In the low-doping region the SDW Econd is overwhelmingly larger than the 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 presumably of the consequent pseudogap in the underdoped region. (C) 2000 Elsevier Science Ltd. All rights reserved.