Ballistic transport and tunneling in small systems
Author(s)
Advisor
Çıracı, SalimDate
1990Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
Ballistic transport and tunneling of electrons in mesoscopic systems have become
one of the most important subjects of condensed matter physics. The quantum
point contacts and scanning tunneling microscope form the basic experimental
tools in this area and have been used for understanding many features of small
systems. In this work ballistic transport and tunneling in small systems are
investigated theoretically.
Ballistic transport through narrow constrictions is investigated for a variety
of configurations. It is found that for a uniform constriction the conductance
is quantized in units of the quantum of conductance (2e^/A) for long channels.
The interference of waves in the constriction gives rise to the resonance structure
superimposed on the quantized steps. The lack of the resonance structure in
the experimental results are attributed to temperature effects and/or adiabatic
transport due to tapering of the constriction. It is shown that elastic scattering by
an impurity distorts the quantization of conductance. Novel resonant tunneling
effects due to formation of bound states are predicted for an attractive impurity
or a local widening at the center of the constriction.
It is shown that the probing in scanning tunneling microscopy have very
much in common with narrow constrictions. The transition from tunneling to
point contact regime is explained by the vanishing effective potential barrier as
a result of tip-sample interaction. For noble and simple metals it is conjectured
that lateral position dependent interaction between the tip and sample leads to
corrugation of the potential barrier and in turn to atomic corrugation observed
by scanning tunneling microscopy. The focused field emission of electrons from
point sources is analyzed in a systematical way. The effective barrier due to the
lateral confinement and nonadiabatic transport through the horn-like opening are
found to be responsible for focusing.
The nonequilibrium nature of transport is investigated by use of Keldysh
Green’s function technique. The effects of elastic and inelastic scattering
are analyzed in a strictly one-dimensional geometry. The features of voltage
and current probes are studied and the Landauer formulae are examined for
multiprobe measurements.
Keywords
Mesoscopics,Landauer formulae.
Keldysh technique
nonequilibrium quantum transport
focused field emission
scanning tunneling microscopy
resonant tunneling
adiabatic transport
quantized conductance
ballistic transport
quantum point contacts
tunneling