Some results of a cluster variation method (C.V.M.) study on the B.C.C. lattice

Abstract

Some results of cluster variation method (CVM) calculations on the BCC lattice, with an Ising hamiltonian, with first and second neighbour interactions, are presented. Apparent discrepancies, between our results and the previous ones, lead to some essential discussions. We find, (in the case J 2 J 1 = 1 ), that the transition between DO 3 and the disordered phase A2 is of second order, in contrast with previous results by other authors, who base their opinion on the Landau theory. In fact, the Landau theory establishes the possibility that this transition is of the second order type. We present this proof. An other problem appears in our CVM calculation: the presence and the stability of a new phase (always in the case J 2 J 1 = 1 ), that we call a ‘four sub-lattice phase’. The surprising stability of this phase can be explained using the concept of super-degenerate point at zero temperature, between B32 and DO 3. We also present this argument.