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Heiner Linke
MSc TU Munich (Germany)
PhD Lund University (Sweden)
ARC Postdoctoral Fellow
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NOTE:
From September 2001 I will be at the Physics
Department of the University of Oregon, Eugene, USA.
Department
Condensed
Matter Physics
Selected publications
Contact details
Popular reviews of
my research
Research Interests
1. Quantum Ratchets and Molecular Motors
What is a ratchet?
Ratchets are devices that can make particles
flow in one direction without any macroscopic forces. The "flashing
ratchet" shown below is an example for how this can work. Some molecular
motors in biological systems are also thought to make use of ratchet
effects. In general, one means by a 'ratchet' any sort of asymmetric
potential.
A flashing ratchet: The
random diffusion when the potential is off (t = 0.5) is converted
into net motion to the left when the ratchet is switched on. A discussion
of this Brownian motor, and a nice on-line simulation can be found
at
http://www.chaos.gwdg.de/java_gallery/brownian_motor/bm.html
The second law of thermodynamics says
that, as long as the system is left alone and remains in thermal
equilibrium, particles in a ratchet can not diffuse in any preferential
direction, in spite of the spatial asymmetry. When the system is
driven away from thermal equilibrium, however, for instance if the
ratchet potential changes with time, then the particles in the ratchet
will in general start to move in one direction. The 'flashing ratchet'
above is only one example for how this can work.
A very good introduction to ratchets
and their possible relevance to biology is the review by Dean Astumian
in Science 276, 917 (1997) (pdf)
Quantum ratchets
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 My
own research on ratchets is concerned with experimental studies
of ratchets that make use of quantum effects, such as tunneling
or wave interference. Two different types of quantum ratchets
for electrons are shown in the images. Both are made from
extremely pure semiconducting material and have a lateral
size of about one micrometer. In both cases, the spatial asymmetry
of the device makes it more difficult to drive a current through
the device in one direction compared to the opposite direction.
When an AC voltage is applied, this asymmetry leads then to
partial rectification of the induced current.
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 Such
quantum rectifiers have quite unusal properties: In the tunneling
ratchet (above), the direction of the rectified current depends
on the temperature. In the quantum dot ratchet (right) wave
interference leads to rectification whose sign depends on
the AC voltage and the electron energy.
(For a review of this work, see
"Quantum Clockwork" by Michael Brooks in New
Scientist, 22 January 2000, p. 28-31) (pdf)
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Ratchets and molecular
motors
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 Molecular
motors are biological machines of only about 0.01 µm
size which, for instance, perform transport tasks, generate
force, and play a role in in cell mitosis. One example is
kinesin, a protein molecule that can "walk" along microtubules
in living cells and transports material, another is myosin
which is active when a muscle contracts. Some nice videos
and animations of molecular motors can be found at
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Similar to large, man-made machines,
molecular motors need fuel. The fuel used in cells is called ATP
and provides the chemical energy that is then transduced to mechanical
energy. However, in spite of this similarity, molecular motors operate
under very different conditions compared to man-made machines. Due
to frequent collisions with other molecules in the surrounding water,
motor proteins are contineously subject to substantial Brownian
motion. This makes it impossible for a molecular motor to move ahead
smoothly and deterministically like, for instance, a car on the
road.
How do molecular motors deal with such
a noisy environment? One model suggests that the motors actually
use the random Brownian motion to do work. A physical model for
how such a Brownian motor may work is the 'ratchet'.
Semiconductor devices, such as the asymmetric
electron cavities shown above, are used to carry out controlled
experiments in structures less complicated than the biological original,
and to study what role quantum effects may play in ratchet physics.
2. Electron Wave
Billiards
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Small electron cavities such as
the "quantum dot ratchet" shown above are also called electron
billiards: flat structures in which particles move on ballistic
trajectories unless they bounce off one of the boundaries.
Electron billiards allow experimental access to a fascinating
regime: At subkelvin temperatures, the electrons behave in
certain aspects similar to classical particles, while other
properties of the electrons must be explained using quantum
mechanics.
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Experiments combined with theoretical
modelling have allowed us to develop a very detailed understanding
of the classical trajectories of electrons in triangular billiards,
and how these classical orbits relate to quantum interference
effects. The classical orbits that turn out to be most important
for the electronic properties are those that are most stable,
or least chaotic. These same orbits can then semiclassically
be related to the quantum transport properties - creating
a link between classical chaos and quantum behaviour.
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Selected Publications
- Pumping Heat with Quantum Ratchets
T. Humphrey, H. Linke, and R. Newbury
to appear in Physica E (2001)
cond-mat/0103552 (abstract)
(pdf)
- The Evolution of Fractal Patterns
during a Classical-Quantum Transition
A. Micolich, R.P.
Taylor , A.G. Davies, J.P. Bird, A. Ehlert, T.M. Fromhold,
R. Newbury, H. Linke, L.D. Macks, W.R. Tribe, E.H. Linfield, D.A.
Ritchie
to appear in Phys. Rev. Lett. (2001)
- Chaos in Quantum Ratchets
H. Linke, T. Humphrey, R.P. Taylor, R. Newbury
Nobelsymposium on Quantum Chaos, Bäckaskog, Sweden (2000)
Physica Scripta T90, 54 (2001)
- Asymmetric nonlinear conduction
in quantum dots with broken inversion symmetry
H. Linke, W. D. Sheng, A. Svensson, A. Löfgren, L. Christensson,
H. Q. Xu, P. Omling
Phys. Rev. B 61, 15914 (2000) (pdf)
- Experimental Tunneling Ratchets.
H. Linke, T.E. Humphrey, A. Löfgren, A.O. Sushkov, R. Newbury,
R.P. Taylor, and P. Omling
Science 286, 2314 (1999) (pdf)
This work has been reviewed
by Michael Brooks in New
Scientist, 22 January 2000, p. 28-31 (pdf)
- A quantum dot ratchet: Experiment
and theory.
H. Linke, W. Sheng, A. Löfgren, Hongqi Xu, P. Omling, P.E.
Lindelof.
Europhys. Lett. 44, 341 (1998) (pdf)
This work has been reviewed by P. Hänggi and P. Reimann
in Physics World, March 1999 p 21. (pdf)
- Classical and quantum dynamics
of electrons in open, equilateral triangular billiards.
L. Christensson, H. Linke, P. Omling, P.E.Lindelof, I. Zozoulenko
and K.F. Berggren.
Phys. Rev. B. 57, 12306 (1998).(pdf)
- Stability of classical electron
orbits in triangular electron billiards.
H. Linke, L. Christensson, P. Omling and P.E. Lindelof.
Phys. Rev. B., 56, 1440 (1997) (pdf)
Popular reviews
of my research
- Quantum Clockworks
M. Brooks
New Scientist , 22 January 2000, p. 28-31
(pdf)
- Quantum ratchets reroute electrons
P. Hänggi and P. Reimann
Physics World 12 (3), 21 (1999)
(abstract)
(pdf)
- Einbahnstraße Quantenpunkt
M. Rauner
Phys. Bl. 55 (1), 16 (1999) (in German)
(pdf)
- Novel Semiconductor Quantum Ratchet
Pumps Electrons
Paul Mortenson
Semiconductor
Online News 11 May 2000
- Die Kanalisierung des Zufalls
Christian Speicher
Neue Zürcher Zeitung, 9 May 2001 (in German) (pdf)
(html)
Contact Details
Mail Address
School of Physics
The University of New South Wales
SYDNEY 2052
Australia
From September 2001:
Assistant Professor
Physics Department
University of Oregon
Eugene, OR 97403-1274
USA
FAX +1 (541) 346-5861
email: linke@darkwing.uoregon.edu
http://physics.uoregon.edu/physics/faculty/linke.html
Email Address
hl@phys.unsw.edu.au
Phone Number
61 2 9385 5928
Facsimile Number
61 2 9385 6060
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