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A Thermal Harmonic Field Description of Phase Transition: The Alternative Approach to the Landau Theory

The study of critical phenomena and phase transitions is an important part of modern condensed matter physics. In this regard, the phenomenological Landau theory has been extraordinarily useful. Hereby we present an alternative theoretical description to the Landau theory for a system under phase transition, based on a priori assumption that the macroscopic system is made of the thermal mixing among multi harmonics each of them can be distinguished by crystal orientation, polar direction, magnetic direction, or even momentum etc. Our theory naturally gives rise to a long range field and is able to account for both the type of lattice and the spatial dimensionality, in addition to that the excess free energy is referenced to the low temperature structure together with the positive excess entropy. The improvements over the Landau theory are demonstrated using ferroelectric-paraelectric system of PbTiO3 on its phase transition and associated thermodynamic behaviors.