Variational tests of current order parameters in the Hubbard model

Variational tests are performed for current order parameters as probable so urces of the pseudogap normal state of cuprates. The calculations are carri ed out based on the states with correlations of the valence bond type whose formation can induce in principle both the superconducting order of the d symmetry and current phases. It is shown for the t-t'-U Hubbard models with a large value of U(similar to 8t) and the Hubbard splitting of the conduct ion band that (1) phases of alternating charge and longitudinal spin curren ts cannot be realized and (2) transverse spin currents are not compatible w ith the superconducting order and they could exist against the normal-state background only within a very narrow doping region near the optimal one. T his region does not correspond to the region of existence of a pseudogap in cuprates, which refutes the above-mentioned hypothesis of the pseudogap or igin. The requirements to the parameters of models for which the considerat ion of correlations of the valence bond type yields a reasonable phase curv e. The existence of current phases in the t-t'-U-V Hubbard models with a st rong interaction (V > 0.25t) of particles in neighboring sites is predicted when the d-superconductivity is completely suppressed. (C) 2001 MAIK "Nauk a/Interperiodica".