The Néel and the singlet RVB orders of the t-J model in a 2D square lattice are studied in the slave-boson mean-field approximation. It is shown that the Néel order parameter takes the maximum value at the finite temperature and disappear at the lower temperature for a certain range of doping. It is also shown that the Néel and the singlet RVB orders coexist at low temperature.This suggests the possibility of the coexistence of the Néel and the superconducting orders. I. INTRODUCTIONIn order to make clear the mechanism of high-T c superconductivity in cuprates, it is indispensable to understand the whole phase diagram on the temperature-doping plane. Recent theoretical studies by the slave-boson mean-field theory based on the t-J model show the validities of the method and the model for high-T c cuprates [ 1− 4]. These studies, however, do not take into account the Néel order explicitly. A few works based on the same theoretical scheme study the Néel order [ 5,6]. In ref. 5 the Néel order and the singlet RVB order parameters are considered, but the bond order parameters are not treated in the self-consistent way. In ref. 6 the Néel order and the bond order parameters of spinon and holon are included. It is shown that there is a phase transition between itinerant and localized antiferromagnetism in the Néel state. But the singlet RVB order is not studied there.

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