Symmetry characterization of electrons and lattice excitations

¹ Institut Laue Langevin, 6 rue Jules Horowitz, 38042 Grenoble Cedex 9, France
² Université Joseph Fourier, UFR de Physique, 38041 Grenoble Cedex 9, France

Abstract

Symmetry concerns all aspects of a physical system from the electronic orbitals to structural and magnetic excitations. In this article we will try to elaborate the fundamental connection between symmetry and excitations. As excitations are manyfold in physical systems it is impossible to treat them exhaustively. We thus concentrate on the two topics of Bloch electrons and phonons. These two examples are complementary in the sense that Bloch electrons describe single particles in an external periodic potential while phonons exemplify a decoupled system of interacting particles. The way we develop the argument gives as by-product a short account of molecular orbitals and molecular vibrations.