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Research Description
University of Virginia
November 17, 2006
Studies of atomic clusters containing tens or hundreds of atoms have gained much interest
in recent decades because of their potential to bridge the gap between isolated atoms and
bulk systems. Notable results include the observation of a shell structure, similar to that
found in electronic shells of single atoms. Theoretical calculations show that certain levels
within this shell structure allow for strong Cooper pairing. These calculations also show
that these particular shell levels, which are realistically attainable, could show substantially
higher values of the superconducting transition temperature TC than are observed in the
bulk material.
The number of electrons in the familiar electronic shell structure is analogous to the
number of atoms N in the cluster. Certain clusters sizes are energetically favorable; these
\magic clusters" of size Nm have geometries resulting in lower total energies. Magic clusters
are of interest to supercondictivity applications because of their high density states in the
HOS and LUS levels. If the energy gap between these two levels is smaller than or comparable
to the pairing energy gap, the magic cluster will be in the superconducting phase.
Our group at the University of Virginia is capable of mass-selecting metallic clusters and
probing their energy. Clusters of various sizes are created by a pulsed ND:YAG laser, then
pushed through the a narrow temperature-controlled canal by an appropriately timed pulse
of He gas. After passing through the canal long enough to thermalize, the clusters emerge
between the plates of a mass spectrometer. The voltage between the plates is then switched
on, causing ionized clusters to accelerate through a long tube toward a microchannel plate
detector. The purpose of this time-of-癷ght mass spectrometer is to identify the masses of
the clusters by their arrival time. Once the arrival time of a particular cluster of interest
is identi¯ed, a pulse from a UV laser is triggered to photodetach the extra electron from
the cluster. The energy of this electron is measured and, upon repeating this process many
times, an energy spectrum for the cluster is obtained.
The calculations which motivate this work predict that at temperatures near TC, strong
pairing will have a substantial impact on the cluster energy spectrum. The onset of pairing
should increase the minimum excitation energy. Our group will ¯rst look for this energy
increase in Al clusters at around 90K, the predicted TC for \magic" Al clusters. Future
research will include the search for diamagnetism in superconducting Al clusters.

 

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