Thermal Properties and Electron Interactions in Semiconductor
Nanostructures
Introduction
We have attracted EPSRC funding (equipment, post-doc and travel money for
3 years) involves two collaborations: one with Cambridge for nanofabrication,
the other with Saclay (Paris) for shot-noise measurements. The main thrust
of the research, to be carried out at Royal Holloway, is to investigate the
thermal properties of one-dimensional (1D) quantum wires fabricated in GaAs;
due to electron-electron interactions such systems are theoretically expected
to show new states of matter. The project will involve quantum transport
measurements of low-dimensional electron systems at low temperatures and
high magnetic fields; there will also be an opportunity to learn semiconductor
nanofabrication techniques.
Background Information
The application of a negative voltage Vg to lithographically defined
gates over a GaAs-AlGaAs heterostructure allows the underlying two-dimensional
electron gas to be "electrostatically squeezed". This has allowed the study
of one-dimensional transport phenomena, where if the electron mean free path
is longer than both the channel length L and width W, plateaus
are observed in the conductance (G=dI/dV) characteristics. As shown in the
figure, the conductance steps are quantised in multiples of 2e2
/h, a result that can be understood as arising from a cancellation of the
1D density states and the group velocity. Using the world's cleanest one-dimensional
electron gases, we have strong evidence for many-body effects giving rise
to a spin polarisation in the last conducting 1D subband.[1,2]
Thermal Properties
Traditional studies of semiconductor nanostructures investigate
the motion of electrons due to an applied voltage. Thermopower looks at the
response of electrons to a temperature gradient, and provides complementary
information to the conventional resistance. From measurements of the thermopower
across a 1D wire we have developed a mesoscopic thermometer that can accurately
measure the electron temperature.[3] Future research will follow on from some
recent results[4] and will (i) compare thermopower and conductance measurements
to highlight many-body effects, and (ii), use thermopower-based electron
thermometry to investigate the heat transfer mechanisms in low-dimensional
systems.
Samples
Samples for our work are fabricated in collaboration with the
Semiconductor Physics
group at the Cavendish Laboratory (University of Cambridge), where there
are extensive growth and processing facilities.
Research Facilities
A 3He system and associated measuring equipment has been brought
to Royal Holloway from the Cavendish Laboratory and we are able to perform
measurements down to 0.3 K. In a recent SRIF (Science Research Infrastructure
Fund) bid there is now funding to purchase a new dilution fridge; this will
allow measurements down to a base temperature of 7 mK at magnetic fields
as high as 16 tesla.
Personnel
At the moment the subgroup consists of one post-graduate student, Olivio
Chiatti, and one post-doctoral associate, Stuart Nield. We have access to
the technical skills of Mass Venti.
Some Recent Publications
[1] "Possible Spin Polarization in a One-Dimensional Electron Gas",
K. J. Thomas, J. T. Nicholls, M. Y. Simmons, M. Pepper, D. R. Mace, and D.
A. Ritchie", Phys. Rev. Lett. 77, 135-138, (1996).
pdf
[2] "Interaction Effects in a One-Dimensional Constriction", K.
J. Thomas, J. T. Nicholls, N. J. Appleyard, M. Y. Simmons, M. Pepper, D.
R. Mace, W. R. Tribe, and D. A. Ritchie", Phys. Rev. B. 58, 4846-4852, (1998).
pdf
[3] "Thermometer for the 2D Electron Gas using 1D Thermopower",
N. J. Appleyard, J. T. Nicholls, M. Y. Simmons, W. R. Tribe, and M. Pepper",
Phys. Rev. Lett. 81, 3491-3494, (1998).
pdf
[4] "Direction-Resolved Transport and Possible Many-Body Effects in
One-Dimensional Thermopower", N. J. Appleyard, J. T. Nicholls, M. Pepper,
W. R. Tribe, and M. Y. Simmons", Phys. Rev. B 62, R16275-16278, (2000).
pdf
For further information Dr Nicholls can be contacted by email
James.Nicholls@rhul.ac.uk
or phone 01784 443444.