
doi: 10.5772/17136
Motivated by the progress of a multi-scale approach in magnetic materials the dynamics of the Ising model in a transverse field introduced by de Gennes (1963) as a basic model for a ferroelectric order-disorder phase transition is reformulated in terms of a mesoscopic model and inherent microscopic parameters. The statical and dynamical behavior of the Ising model in a transverse field is considered as classical field theory with fields obeying Poisson bracket relations. The related classical Hamiltonian is formulated in such a manner that the quantum equations of motion are reproduced. In contrast to the isotropic magnetic system, see Tserkovnyak et al. (2005), the ferroelectric one reveals no rotational invariance in the spin space and consequently, the driving field becomes anisotropic. A further conclusion is that the resulting excitation spectrum is characterized by a soft-mode behavior, studied by Blinc & Zeks (1974) instead of a Goldstone mode which appears when a continuous symmetry is broken, compare Tserkovnyak et al. (2005). Otherwise the underlying spin operators are characterized by a Lie algebra where the total antisymmetric tensor plays the role of the structural constants. Using symmetry arguments of the underlying spin fields and expanding the driving field in terms of spin operators and including terms which break the time reversal symmetry we are able to derive a generalized evolution equation for the moments. This equation is similar to the Landau Lifshitz equation suggested by Landau& Lifshitz (1935) with Gilbert damping, see Gilbert (2004). Alternatively, such dissipative effects can be included also in the Lagrangian written in terms of the spin moments and bath variables Bose & Trimper (2011). Due to the time reversal symmetry breaking coupling the resulting equation includes under these circumstances a dissipative equation of motion relevant for ferroelectric material. The deterministic equation is extended by stochastic fields analyzed by Trimper et al. (2007). The averaged time dependent polarization offers three modes below the phase transition temperature. The two transverse excitation energies are complex, where the real part corresponds to a propagating soft mode and the imaginary part is interpreted as the wave vector and temperature dependent damping. Further there exists a longitudinal diffusive mode. All modes are influenced by the noise strength. The solution offers scaling properties below and above the phase transition. The results are preferable and applicable for ferroelectric order-disorder systems. A further extension of the approach is achieved by a symmetry allowed coupling of the polarization to themagnetization. The coupling is related to a combined space-time symmetry due to the fact that the magnetization is an axial vector with m( x,−t) = − m( x, t) whereas 23
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