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Applications of Normal Modes Physics 213 Lecture 20 What are normal modes? • ``Characteristic’’ motions of the system • All parts of the system move with same frequency • As many normal modes as number of oscillators in the system. • But are masses coupled by springs really that interesting? • All matter is made of atoms, and our first model for an atom is: M M = Atomic mass x 1.66053886 × 10-27 kg • But atoms group together to form molecules by forming ``Atomic Bonds’’ H H O • What are ``Atomic Bonds’’? • An atomic bond means there is a stable equilbrium of the two atoms: Energy Position of atoms • We know that around a stable equilibrium you can have oscillations. Vibrational motions of molecules • See Wikipedia • The frequencies of the normal mode motions of molecules are in the infrared. • This means they absorb thermal radiation very well • Especially certain molecules like CO2 What else? • If we put lots of molecules together we get a solid. • Solids have a lattice structure: Lattice vibrations: normal modes • Solids can be metals, insulators, semiconductors, superconductors • What decides which is which? • The lattice structure will have normal modes of oscillation (like the beads on the string in this weeks assignment) • These normal modes tell us EVERYTHING about the lattice vibrations Lattice vibration - phonons • Lattice vibrations interact with the electrons in the solid • This interaction determines whether the material is a metal, insulator or semiconductor • We call the normal modes ``phonons’’ as if they were a new kind of particle • If the electrons keep crashing into phonons - we expect bad conduction • But it gets weirder…….. Superconductors • Electron A and Electron B can ``talk’’ to each other by A exciting a phonon, and sending it off to B. • This can make the electrons interact • In some materials the electrons pair up, and the materials become superconducting.