Materials Science Research Lecture

Abstract:  Electrons and phonons are the "quasiparticles" that determine the behavior of crystalline solids.  Quasiparticles are excitations.  They can often be visualized by their close analogy to free particles.  Electrons in crystals, for example, have the same charge as free electrons, but altered mass.  They are accelerated by electric fields, obey Fermi statistics, and have hybrid particle/wave features like free electrons.  But they are also very different from free electrons.  In pure semiconductors, they "disappear" in equilibrium at low temperatures.  They emerge, at high T or under external excitation, as electron-hole pairs.  These have a close analogy to electron-positron pairs.  In metals at low temperature, electrons are constrained by a sharp Fermi surface, form Cooper pairs, and carry supercurrents.  Phonons are vibrational-wave particles that do not exist outside their crystal host.  They are a lot like photons in vacuum, but more complicated.  We are still learning how to think about quasiparticles in crystals.  We want to understand how they evolve in time, temperature, and as their interaction strength varies.  The incredible diversity of crystalline matter gives us new surprises, challenges, and opportunities to deepen our understanding.

Refreshments served at 3:30pm in Spalding Laboratory Lobby.