The spin-Peierls transition in CuGe0 3

Abstract

Structural phase transition are an important area of research in solid state physics. They are characterized by a spontaneous breaking of the lattice symmetry. Below the critical temperature the lattice undergoes a distortion enlarging the unit cell. Consequently such transitions are not only experimentally accessible through thermodynamic methods probing anomalies due to the critical behavior at the phase transition but also by spectroscopic methods such as X-ray or neutron diffraction. The interesting fact about CuGeO 3 is that a large number of different experimental methods have been applied. As I try to show with this work the number of data available made it possible to achieve a consistent understanding of the spin-Peierls transition in CuGeO 3. The overall structure of the thesis is to first derive an appropriate model for the description of the spin-Peierls phenomenon using the example of CuGeO 3. I verify the applicability of the model regarding static effects using simple standard many-particle methods. I then turn to the formulation of a field-theoretical approach to the partition function. The partition function contains all the information of the system and is used to discuss a series of approximations common for the description of spin- and electron-phonon coupled systems. Next the effect of the spin-phonon coupling on the phonon dynamics is studied. Finally the topic of thermodynamics in the ordered phase is addressed via a phenomenological model.