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Andrew Fefferman
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Research -- Low
Temperature Properties of Glass
It was initially thought that glasses should behave the same
way that crystals do at low temperatures because long wavelength phonons should
be insensitive to disorder in the microscopic structure of glasses. However, in
1971 Zeller and Pohl discovered that the properties of glasses and crystals are
very different at low temperatures. Whereas the heat capacity and thermal
conductivity of a crystal both vary as T^3, the heat capacity of a glass is
linear in T and much higher than in a crystal, and the thermal conductivity of
a glass is quadratic in T and much lower than in a crystal. The surprising
properties of glass were explained by the tunneling model in terms of phonon
scattering by two level systems. However, some puzzles remain. What is the
origin of the quantitative universality of the ratio of the phonon wavelength
to the phonon mean free path among nearly all glasses (except for certain thin
films) [R. O. Pohl, X. Liu and E. Thompson, Rev. Mod. Phys., 74, 991 (2002)]?
And what are these tunneling entities that supposedly exist in glasses and
account for their characteristic behavior?
Research --
Superfluidity of He-3 in Aerogel
Superfluidity
occurs in several different systems including He-4 and the electrons in a
superconductor. In 1972, Osheroff, Richardson and Lee discovered superfluidity
of He-3. Bulk He-3 is extremely
pure because elements other than helium freeze out on the cell walls, and the solubility
of He-4 in He-3 is tiny. The density of impurities in bulk He-3 is well below
the density in interstellar space. However, impurities can be introduced with
aerogels, which are solids with porosities up to 99%. The strands of the
aerogel permeate the He-3 and act as an impurity. Aerogel does not destroy the
superfluidity, but suppresses the superfluid transition temperature by an
amount depending on the correlation length of the aerogel. It is thus possible
to vary the aerogel porosity and correlation length and observe the effect on
the superfluid phase diagram and the superfluid fraction.