Phase transition induced by pairing of pi -kinks in weakly coupled magnetic sine-Gordon chains

A phase transition appearing in systems of weakly coupled magnetic sine-Gordon chains with pi -kink solitons is analysed by the use of the transfer operator method and the pseudo-spin approach. It is shown that the crucial role for this phase transition is played by solitons. A phenomenological picture is proposed in which the phase transition considered is driven by a pairing of kinks below Tc. This picture is confirmed by the transfer operator result for the free energy of the system analysed below Tc.