Soft Lattice Modes and Pseudospin Condensates
The threshold of a continuous-phase transition is dominated by fluctuations of correlated pseudospins, which are responsible for critical anomalies in thermodynamical quantities measured in the transition region. According to neutron inelastic scattering and magnetic resonance results , , critical fluctuations are not random but sinusoidal in character at very low frequencies and long wavelengths. On the other hand, for instability of displacive crystals, Cochran  and Anderson  proposed to consider a lattice mode whose characteristic frequency is temperature-dependent, diminishing toward the transition temperature. While entirely different from ordering, such a lattice excitation known as a soft mode can interact with pseudospins under specific circumstances of the transition threshold. In an attempt to unify these views, we considered in Chapter 3 a collective mode of pseudospins that may arise from their negative correlation energy, while a soft mode is believed as always accompanying a continuous structural change. It seems logical to consider that these modes are coupled when they are in phase, resulting in transition anomalies. Although conceptually conceivable, such an interaction should concretely be substantiated.
KeywordsLattice Mode Soft Mode Ferroelectric Phase Transition Neutron Inelastic Scattering Phonon Dispersion Curve
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