Electron Clouds Can Freeze Into an "Orbital Glass"

Electron clouds can freeze into an "Orbital Glass" at low temperatures. In the modern picture of quantum mechanics, electrons take the form of "clouds" within the atoms and molecules in which they inhabit. The clouds, which have various shapes such as spheres or dumbbells, represent the general boundaries within which one may find an electron at any one measurement in time. Typically, processes involving electron clouds (more formally known as "orbitals") are blazingly fast. In the order of a femtosecond (10^-15 s), for example, an electron orbital can make transitions between degenerate states (those containing the same amount of energy), transforming from a vertical dumbbell to a horizontal one with respect to some axis.

Now, scientists have found evidence that these and other orbital processes can slow down dramatically--to as long as 0.1 seconds, a slowing by 14 orders of magnitude--for electrons in low-temperature FeCr2S4, a spinel (class of mineral) with a relatively simple crystalline structure. The researchers, who hail from the Center for Electronic Correlations and Magnetism at the University of Augsburg in Germany (Peter Lunkenheimer, Peter.Lunkenheimer@Physik.Uni-Augsburg.de) and the Academy of Sciences of Moldova (a former Soviet republic), consider these frozen electron orbitals in spinels to constitute a new class of material which they have dubbed an orbital glass. By measuring the response of the material to alternating-current electric fields in the audio- to radio-frequency range, they found that processes involving non-spherical orbitals dramatically slow down at low temperatures to form a glass-like state, in a manner very similar to the arrest of molecular motion that occurs when glass blowers perform their craft.

It's not just the orbitals that slow down; the neighboring atomic nuclei that surround the electrons also distort more slowly in response to the glacially changing orbitals. In contrast to conventional glasses, a complete "freeze" of the electron clouds does not occur at the lowest temperatures. Completely frozen orbitals are prevented by quantum-mechanical tunneling: the clouds keep themselves moving by making transitions between different low-energy cloud configurations even without the energy they normally require. (Fichtl et al., Physical Review Letters, 21 January 2005