Order-Disorder Transition in the Spin 1/2 Heisenberg Antiferromagnet on the 4-8 Lattice

Abstract

By means of the modified spin wave method, we study the ground state properties of the spin 1/2 Heisenberg antiferromagnet on the 4-8 lattice, which is composed of a combination of squares and octagons, as a function of the exchange interaction within the square I (>0) and that between the squares J (>0). This model relates to the magnetism of CaV 4 O 9 . 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 exponentially 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 originate in purely quantum mechanical nature since this system contains no frustration. At I / J =1, the ground state energy is -0.370(2 I + J ) and the long-range order parameter is 0.210.