The combination of advanced semiconductor growth and electron beam
lithography has allowed the fabrication of new types of structure for
investigating electron transport. In particular Molecular Beam Epitaxy
allows growth by atomic layer at a time and a consequently high degree
of order. This results in very high values of electron mobility
particularly at low temperatures where the use of the AlGaAa-GaAs
heterostructure gave rise to the discovery of the Fractional Quantum
Hall effect.
Formation of patterned gates allows the electron gas to be
"electrostatically shaped" into a desired configuration which can
possess either one or zero dimensionality, (1D, 0D).
In particular it has been possible to access the regime where both the
phase coherence and elastic scattering lengths exceed the sample
dimensions, this results in ballistic transport in 1D and single
electron behaviour in 0D. In 1D the conductance takes quantised values
given by 2ne2/h due to each of the n 1D subbands having a spin degenerate conductance value of 2e2/h,
and n can take values 1,2,3,4... Such behaviour is characteristic of a
spatially quantised system having freedom to change momentum in one
dimension only with a progressive filling of the subbands associated
with each quantised level. It will be shown that this is found in many
systems with restricted dimensionality and, in a surprising manner, at
room temperature in metallic contacts.
As the 1D confinement potential is weakened so the electron
wavefunctions relax in the second dimension and, in order to minimise
the electron-electron repulsion, an array is formed in which a two row
configuration is the ground state. This behaviour, which is the prelude
to formation of a Wigner Lattice, will be discussed.
Finally it will be shown that the use of strong 0D confinement allows
the operation of a quantum pump through which single electrons can be
transported giving a current I=ef, where e is the electron charge and f
is the frequency which can be in the Gigahertz range. At present this
type of pump provides an accuracy in the measurement of e of nearly 1
part in 106,
the prospects for improving this by the three orders of magnitude
necessary for a standard will be discussed.
The conclusion of this lecture is that by "engineering" parameters,
such as electron wavefunctions, it is possible to access a new range of
phenomena for basic physics and possible practical applications.
Faculty Host:
Coffee and cookies before the presentation at 3:15 pm, and refreshments after the presentation will both be served in Room 128.