Commensurate and Incommensurate States of a Spin-Peierls System in a Magnetic Field

Commensurate and incommensurate states of a Heisenberg spin-Peierls system in a magnetic field are studied within the mean-field approximation. Free energy, order parameter and magnetization are calculated numerically by taking account of the effect of all the accompanying harmonics. In particular, it is found that at low temperatures the field lowers the free energy of the incommensurate states and results in the commensurate to incommensurate phase transition. The nature of this transition is discussed. It is also found that the incommensurate to normal phase transition temperature behaves anomalously in a strong field. The resultant phase diagram is compared with the experimental data on TTF-CuBDT.