EFFECTS OF CYCLIC 4-SPIN EXCHANGE ON THE MAGNETIC-PROPERTIES OF THE CUO2 PLANE

For the CuO2 square lattice, as a parent system of high-T(c) supercond uctors, a numerical study has suggested that the effective-spin model contains a large cyclic four-spin exchange interaction J(c). This pape r investigates the effects of J(c) upon magnetic Raman scattering and upon properties of the ground state by an exact numerical method mainl y for a 16-site cluster. It is found that the main Raman peak with the B1g symmetry is shifted by 20% to the lower-energy side with a realis tic magnitude for J(c), which is about one fourth the size of the near est-neighbor exchange J. Accordingly a value of J extracted from analy ses of experimental data becomes larger by 10% than that estimated wit h the use of the Heisenberg model. Furthermore the four-spin exchange enhances a shoulder due to multimagnon states at an energy of about 4J , and this result compares favorably with the experimental line shape. Detailed discussion is given on the effect of the four-spin exchange upon the ground-state properties such as the staggered magnetization, the ground-state energy, weight of various spin configurations, and th e spin-spin correlation function.