Elementary excitations in quantum liquids

By 1931 it was already clear that condensed matter was to be a marvelous proving ground for the then newly developed quantum mechanics. The Sommerfeld free electron gas model for a metal; Bloch's theorem; the description of energy bands in solids; the theoretical explanation of why some solids are metals, some insulators, some semiconductors; all these provide eloquent testimony to the ingenuity of the theorist in developing a quantum description of independent electron motion that explained or illuminated experiment. During the succeeding decade, theorists began to examine some consequences of electron–electron interactions. Thus in 1934, when Frederick Seitz and Eugene Wigner developed the first theory of energy bands and cohesion in the alkali metals, they drew upon the pioneering study by Wigner of Coulomb correlations in a quantum plasma (interacting electrons moving in a uniform background of positive charge). In 1937, John Bardeen took into account the screening of ion motions by electrons in his seminal investigation of electron–phonon interactions in metals, while the concept of a pseudopotential to describe the way in which strong short‐range electron correlations influence the electron–ion interaction in solids was put forward by H. Hellman in 1936 and independently by Conyers Herring in 1939.