ORDER-DISORDER TRANSITION IN THE SPIN-1 2 HEISENBERG-ANTIFERROMAGNET ON THE 4-5-LATTICE/

By means of the modified spin wave method, we study the ground state p roperties of the spin 1/2 Heisenberg antiferromagnet on the 4-8 lattic e, which is composed of a combination of squares and octagons, as a fu nction of the exchange interaction within the square I(> 0) and that b etween the squares J(> 0). This model relates to the magnetism of CaV4 O9. It is found that the system is in the ordered state for 0.401 < I/ J < 1.803, in the disordered state with an energy gap and exponentiall y decaying correlation functions for 0.395 < I/J < 0.401 and 1.803 < I /J < 1.836, and in the singlet state for I/J < 0.395 and 1.836 < I/J. The ground state order-disorder transitions at I/J = 0.401 and 1.803 o riginate in purely quantum mechanical nature since this system contain s no frustration. At I/J = 1, the ground state energy is -0.370(2I + J ) and the long-range order parameter is 0.210.