Abstract
Physicists have realized during the last two decades that the physical properties of many condensed-matter systems are often best described in terms of lumps of charge, known as quasiparticles, rather than by electrons and ions. In a crystal under equlibrium conditions, for example, the mutual repulsion of the electrons leads to a cloud of positive charge surrounding each individual electron. This "dressing" of electrons leads to charge screening. In other words, a test charge placed far from an electron will feel the influence of a new entity with a smaller charge, rather than the charge on the "bare" electron.