Journal of Physics C, Solid State Physics vol:17 issue:36 pages:6811-6818
The reaction-field approximation is applied to the d-dimensional q-state quantum Potts model in an external transverse field. With the exception of d >= 3, the tricritical value qc(d), below which the system has a second-order phase transition, decreases with increasing external field strength. From the ground state of the quantum d-dimensional model, an improved value of qc is obtained for the classical model in d+1 dimensions. A direct explanation is also obtained for the expected tricritical point in finite-thickness slabs of cubic ferromagnets (like DyAl2) with a field in the (111) direction.