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Appendix B. Site Reports—Europe 201
Delft Institute of Microelectronics and Submicron
Technology (DIMES)
Dr. Cees Dekker (Faculty of Applied Physics) made a short presentation
on research activities in the area of quantum transport through
nanostructures. Prof. Mooij (who was touring the United States at the time
of our visit) heads the research program supported by DIMES. Four projects
were discussed briefly:
1. Junction Arrays (Project Director, Hans Mooij) and Single
Electronics (Project Director, Peter Hadly). The quantum behaviors of
small circuits of superconducting tunnel junctions were studied
experimentally and theoretically. A quality sample was designed and
fabricated, in which quantum superpositions of charge states, as well as
vortex states, have been experimentally observed. Quantum vortices were
studied in one-dimensional arrays. Disorder was seen to lead to localization,
while in periodic superlattices the vortices maintained their mobility.
Fluctuations were studied in normal metals near a tunnel barrier.
In single electronics, there are three main efforts: (1) fabrication of small
junctions for the study of charging effects, (2) understanding of highfrequency
behavior of single-electron tunneling (SET) transistors, and
(3) characterization of single-electron circuits. Figure B.5 shows an RS
flip-flop consisting of four SET transistors, each with three gates fabricated
by the group.
2. Quantum Dots (Project Director, Leo Kouwenhoven). Transport
experiments on quantum dots were performed on a gated device, as shown in
Figure B.6. Measurements of gate voltages vs. source-drain voltages show a
shell structure corresponding to a 2-D harmonic confinement potential in
normal atoms. Staff observed that the filling of a shell occurs according to
Hund’s rule: electrons occupying degenerate states prefer to have parallel
spins, which lowers the total energy due to an increased exchange
interaction.
3. Single Molecular Wires (Project Director, Cees Dekker). Singlewall
carbon nanotubes were obtained from R.E. Smalley at Rice University
for transport measurements. The nanotubes behave as coherent quantum
wires at the single-molecular scale. The density of states appears to consist
of well-separated discrete electron states. The approximate 0.4 meV energy
separation corresponds to estimated 1-D quantum box where a 3 µm long
nanotube constitutes the electron box. Electrical conduction through these
discrete electron states appears to occur quantum coherently over micronlength
distance.