One of the fundamental properties of bulk metals is the cancellation of electric
fields. The free charges inside of a metal will move until they find an arrangement where
the internal electric field is zero. This implies that the electric dipole moment of a metal
particle should be exactly zero, because an electric dipole moment requires a net separation
of charge and thus a nonzero internal electric field.
This thesis is an experimental study to see if this property continues to hold for tiny sub-
nanometer metal particles called clusters (2 - 200 atom, R < 1 nm). We have measured the
electric dipole moments of metal clusters made from 15 pure elements using a molecular
beam electric deflection technique. We find that the observed dipole moments vary a great
deal across the periodic table. Alkali metals have zero dipole moments, while transition
metals and lanthanides all have dipole moments which are highly size dependent. In most
cases, the measured dipole moments are independent of temperature (T = 20 - 50 K), and
when there is a strong temperature dependence this suggests that there is a new state of
matter present. Our interpretation of these results are that those clusters which have a non-
zero dipole moment are non-metallic, in the sense that their electrons must be localized
and prevented from moving to screen the internal field associated with a permanent dipole
moment.
This interpretation gives insight to several related phenomena and applications. We
briefly discuss an example cluster system RhN where the measured electric dipole moments
appear to be correlated with a the N2O reactivity.
Finally, we discuss a series of magnetic deflection experiments on lanthanide clusters
(Pr, Ho, Tb, and Tm). The magnetic response of these clusters is very complex and highly
sensitive to size and temperature. We find that PrN (which is non-magnetic in the bulk) becomes magnetic in clusters and TmN clusters have magnetic moments lower than the atomic value as well as the bulk saturation value implying that the magnetic order in the cluster involves non-collinear or antiferromagnetic order. HoN and TbN show very similar size dependent trends suggesting that these clusters have similar structures.
| Identifer | oai:union.ndltd.org:GATECH/oai:smartech.gatech.edu:1853/34751 |
| Date | 06 July 2010 |
| Creators | Bowlan, John |
| Publisher | Georgia Institute of Technology |
| Source Sets | Georgia Tech Electronic Thesis and Dissertation Archive |
| Detected Language | English |
| Type | Dissertation |
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