Quantum solids

Article By:

Goodkind, John M. Department of Physics, University of California-San Diego, La Jolla, California.

Last reviewed:August 2020

DOI:https://doi.org/10.1036/1097-8542.563100

Solids in which the quantum-mechanical wave functions of individual atoms overlap the wave functions of neighboring atoms in the quantum ground state of the system. The spatial extension of the wave functions is called zero-point motion since the location of the atoms cannot be determined within the width of the wave function even at the absolute zero of temperature. The zero-point motion becomes large when the constituent atoms have small mass and the attractive force between them is small. The isotopes of helium have the weakest interaction of any atom or molecule (and the smallest mass except for the much more strongly interacting hydrogen molecules). The root-mean-square zero-point motion in those solids is approximately 25% of the mean distance between atoms. This zero-point motion results in some very unusual properties that are manifestations of the quantum statistical mechanics of many-particle systems. The properties of solid helium are currently an active subject of research with some fundamental questions remaining unanswered. See also: Helium; Intermolecular forces; Nonrelativistic quantum theory; Quantum mechanics; Quantum statistics

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