The pseudo-spin model and its application to KDP-type materials

Abstract

The applicability of the pseudo-spin model is discussed in relation to the existence of constituents with a restricted number of low-lying energy states. The situation certainly occurs in materials with low-lying electronic levels which undergo co-operative Jahn-Teller phase transitions. The Slater-Takagi model of KDP is analysed to show that under certain approximations it leads to a double well in the collective co-ordinates with two levels in each unit cell and hence to a pseudo-spin model of one S = 1/2 per cell. The spin hamiltonian is the Ising model in a transverse field together with four spin interactions. The thermodynamic properties of this model are discussed and best values of the parameters obtained. The elastic constant anomalies are interpreted in terms of cs', in the adiabatic regime. The spin dynamics is discussed in terms of random phase approximation for spin waves, and more complete treatments are reviewed. Raman scattering data is analysed in terms of a coupled spin-phonon model both above and below T_c .