凝聚态物理导论-中科院课件-苏刚Chapter4.pdf

Chapter 4 Elementary Excitations §4.1 Prolog The ground-state wavefunction of the many-body interacting electrons in condensed matter is unimaginably complicated and rather interesting. It is also impossible to get an exact solution in absolutely most cases. Then, how to describe the responses of the solids (condensed matter) to experimental probes such as photons, beams of atoms or ions, the electrons emerging from an STM tip, or changes in temperature? What determines the electrical or thermal transport properties of solids (condensed matter) ? All these questions are beyond the scope of a ground-state theory. Facts: The thermal properties and elasticity in solids depend on the small vibrations of atoms on the lattice; The conducting behaviors in metals and semiconductors depend on the excitations of small portion of electrons or holes near the Fermi surface; The magnetic behaviors in magnets at low temperature depend on the excitations of magnetic moments; …. Elementary excitations: The low-lying excitations in the systems that can be usually viewed as a set of independent excited units. Sometimes they are also called as quasiparticles. Two types of elementary excitations: quasiparticles of collective excitations (for instance, phonon, plasmon, magnon, polaron, exciton, etc.), bosons; quasiparticles of individual excitations (e.g. quasielectron and quasiholes in metals, etc.) ,fermions . Electron-hole excitation Quasiparticle /wiki/Quasiparticle From Wikipedia, the free encyclopedia In the language of many-body quantum mechanics, a quasiparticle is a type of low- lying excited state of the system (a state possessing energy very close to the ground state energy) that is known as an elementary excitation. As a result of this closeness, most of the other low-lying excited s