Münster 1999 – wissenschaftliches Programm
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DY: Dynamik und Statistische Physik
DY 50: Gitter- und Spindynamik
DY 50.3: Vortrag
Freitag, 26. März 1999, 10:00–10:15, R4
Symmetry-adapted microscopic Hamiltonian for the phase transition sequence in A2BX4 compounds — •Boris Neubert1, Jorge Íñiguez González2, Iñigo Etxebarria2, Mois Aroyo1, and Juan Manuel Pérez Mato1 — 1Universidad del País Vasco, Facultad de Ciencias, Física de la Materia Condensada, Apartado 644, E-48080 Bilbao — 2Universidad del País Vasco, Facultad de Ciencias, Física Aplicada II, Apartado 644, E-48080 Bilbao
The phase transition sequence (P63/mmc→) Pnam → IC → Pna21 observed in the class of A2BX4 compounds (e.g. Rb2ZnCl4) is used to explain the basic concepts for the derivation of an effective symmetry-adapted microscopic Hamiltonian. The latter is a function of pseudo spins and local modes which are centered at the 2c Wyckoff positions of the hexagonal phase and transform according to irreducible representations of the site symmetry group 6m2. The local modes induce the symmetry of the unstable normal modes (negative eigenenergies of the force constant matrix) and thus are expected to expand the subspace of atomic displacements relevant for the transitions. Lowest-order couplings of the model variables to themselves and to elastic strains are rigourously derived by group theoretical methods. The values of all parameters as well as the inner structure of the local modes are determined making use of a semiempiric force model with atom-atom potentials. Possible treatments of the Hamiltonian are sketched. Supported by the DAAD (Deutscher Akademischer Austauschdienst, Grant D/98/14573) and the Government of the Basque Country (Project PI97/71).