DYNAMICAL EFFECTIVE-MEDIUM THEORY FOR QUANTUM SPINS AND MULTIPOLES

A dynamical effective medium theory is presented for quantum spins and higher multipoles such as quadrupole moments. The theory is a general ization of the spherical model approximation for the Ising model, and is accurate up to O(1/z(n)) where z(n) is the number of interacting ne ighbors. The polarization function is optimized under the condition th at it be diagonal in site indices. With use of auxiliary fields and pa th integrals, the theory is flexibly applied to quantum spins and high er multipoles with many interacting neighbors. A Kondo-type screening of each spin is proposed for systems with extreme quantum fluctuations but without conduction electrons.