Frontier Physics with Correlated Electrons

摘要

In the mid-1970s, many solid-state physicists, including ourselves, felt that our field was reaching maturity. The independent electron approximation worked marvelously well for most semiconductors and metals; the phase transition problem was solved; and the fundamentals of magnetism, ferroelectricity, and superconductivity appeared to be well in hand. There were some niggling concerns; for example, we still did not know why nickel oxide was an insulator rather than a metal. However, many of us suspected that this was a detail that would be worked out in due course. As this issue of Science dramatically exemplifies, our self-satisfaction was completely misplaced. A remarkable variety of new (and in some cases, old) materials have been discovered that cannot be understood at all with traditional ideas. These have in common the dominant role played by electron-electron interaction effects, and hence such systems are collected together under the general rubric of "highly correlated electron systems." Examples are transition metal oxides, including, most spectacularly, the copper oxide high-temperature superconductors; heavy fermion metals, which typically involve rare earths or actinides; organic charge transfer compounds; and one- and two-dimensional electron gas systems. The possibilities for technological applications of some of these compounds are tantalizing, but they also present some of the deepest intellectual challenges in physics.