Project title: Strongly Correlated Phonons in Ferroelectrics and Perovskite Superlattices
Supervisors: Dr. Paul Tangney and Dr. Johannes Lischner
Phonon theory is the most common theory used to describe lattice vibrations. In phonon theory, atoms are assumed to move altogether in the form of a plane wave; with this hypothesis, the motion of atoms can be viewed as the combination of a set of discrete independent modes, thus leading to the concept of quantisation of the vibrations. Phonon theory assumes that materials are homogeneous and that phonons are quasi-independent entities. When these assumptions break down, phonons themselves are poorly defined.
Phonon theory breaks down when atoms routinely traverse local energy barriers. By analogy with electrons, we may view this as phonons becoming strongly correlated. Strong electron correlation, which causes exotic phenomena such as high-temperature superconductivity, has been studied extensively for decades but strong phonon correlation has received much less attention, despite the central role that it can play in the physics of materials and devices. An understanding of this correlation may improve our ability to design materials, particularly at high temperatures (e.g. thermal barrier coatings).
The project is concerned with the study of the breakdown of phonon band theory in barium titanate (BaTiO3) and aims at developing a methodology to calculate the dispersion relation of phonons at finite temperatures.