THIS WEEK IN

P ERSPECTIVES
observed by STM for single atoms at surfaces
in various configurations (2–6). Such
Kondo physics play a role whenever localized
spins interact with conduction electrons
(11, 12).
In their work with CoPc, Zhao et al.
interpret the onset of the narrow resonance
at E F for their pruned (that is, dehydrogenated)
molecules as a sign of the Kondo
effect, and thus proof that the dehydrogenated
molecules have a well-defined magnetic
moment. This is in contrast to their
pristine CoPc molecules, which show no
Kondo resonance and are believed to be nonmagnetic.
This central result is interesting
because it demonstrates an ability to change
the magnetic state of a molecule by directly
modifying its structure via single-molecule
manipulation. Mechanical (13) and electronic
(14) properties of individual molecules
have been manipulated previously, but
the modification of single-molecule spin
properties by Zhao et al. takes such manipulations
to a new level. Their results are also
somewhat surprising because they observe a
Kondo temperature for the dehydrogenated
molecule that is even higher than the Kondo
temperature observed previously for bare
cobalt atoms sitting on a similar substrate
(2). Typically the Kondo temperature
increases when an ion is more strongly contacted
to a substrate (that is, more strongly
electronically screened and/or hybridized
with the substrate’s continuum states). The
results imply that the modified phthalocyanine
molecular cage connects the interior
cobalt atom more strongly to substrate electrons
than if the atom were sitting on the substrate
unadorned.
This counterintuitive result highlights
how STM single-molecule studies help us to
understand how molecules might be used to
connect the electrodes of future electronic
and spintronic devices. Currently, one of the
greatest questions in molecular electronics
is what happens at the contact between a
molecule and a metal electrode. Many interesting
effects have been observed in molecular
transport experiments, including the
Kondo effect (15–17), but the microscopic
basis of much of this behavior remains a
mystery. Experiments such as that reported
by Zhao et al. form a beautiful complement
to transport measurements because they
provide direct microscopic evidence of how
specific, well-characterized molecular contact
configurations lead to different electronic
and spin behaviors.
There are many exciting future possibilities
in this area, including the exploration
of other classes of magnetic molecules that
show different spin behaviors. One example
is molecules having high magnetic
anisotropy energy [“single molecule magnets”
(7)]. These molecules exhibit welldefined
spin-up and spin-down states and
have been suggested for numerous applications
ranging from quantum information
processing to data storage (7). Controlling
spin at the single-molecule scale in these
and related systems promises a new level of
control in magnetic nanostructures.
References
1. F. J. Himpsel, J. E. Ortega, G. J. Mankey, R. F. Willis, Adv.
Phys. 47, 511 (1998).
2. V. Madhavan,W. Chen,T. Jamneala, M. F. Crommie, N. S.
Wingreen, Science 280, 567 (1998).
3. J. Li,W.-D. Schneider, R. Berndt, B. Delley, Phys. Rev. Lett.
80, 2893 (1998).
4. H. C. Manoharan, C. P. Lutz, D. M. Eigler, Nature 403,
512 (2000).
5. N. Knorr et al., Phys. Rev. Lett. 88, 096804 (2002).
6. A. J. Heinrich, J.A. Gupta, C. P. Lutz, D. M. Eigler, Science
306, 466 (2004).
7. J. R. Long, in Chemistry of Nanostructured Materials,P.
Yang, Ed. (World Scientific, Hong Kong, 2003), pp.
291–315.
8. I. Zutic, Rev. Mod. Phys. 76, 323 (2004).
9. A. Zhao et al., Science 309, 1542 (2005).
10. A. C. Hewson, The Kondo Problem to Heavy Fermions
(Cambridge Univ. Press, Cambridge, 1993).
11. D. Goldhaber-Gordon et al., Nature 391, 156 (1998).
12. S. M. Cronenwett,T. H. Oosterkamp, L. P. Kouwenhoven,
Science 281, 540 (1998).
13. F. Moresco et al., Phys. Rev. Lett. 86, 672 (2001).
14. R.Yamachika, M. Grobis, A.Wachowiak, M. F. Crommie,
Science 304, 281 (2004).
15. J. Park et al., Nature 417, 722 (2002).
16. W. Liang et al., Nature 417, 725 (2002).
17. L. H.Yu, D. Natelson, Nano Lett. 4, 79 (2004).
10.1126/science.1117039
PHYSICS
Reduced Turbulence and
New Opportunities for Fusion
The authors are in the Department of Physics,
Imperial College London, London SW7 2AZ, UK.
S. Cowley is also in the Department of Physics
and Astronomy, University of California–Los
Angeles, Los Angeles, CA 90095, USA. E-mail:
kmkr@imperial.ac.uk, steve.cowley@imperial.ac.uk
Karl Krushelnick and Steve Cowley
Fusion has long been considered the
energy source of the future—since its
fuel supply (deuterium and lithium) is
virtually limitless and the environmental
impact is minimal. However, although
fusion is a spectacularly
successful
Enhanced online at
www.sciencemag.org/cgi/
energy source for
content/full/309/5740/1502
the Sun, the practicalities
of producing useful amounts of
fusion energy in a laboratory on Earth are
technically challenging—primarily
because of the difficulty of confining a
plasma (an ionized gas) heated to the hundred
million degree Celsius temperatures
necessary to induce nuclear fusion reactions.
Recent findings about plasma behavior
in such conditions, however, have led to
new hope that the control of fusion plasmas
may become much easier.
The use of magnetic “bottles” to confine
thermonuclear plasmas for fusion has been
an active area of research since 1946 when
Thomson and Blackman obtained a British
patent for this concept (1). Enormous
progress has been made since that time. The
critical parameter, termed the energy confinement
time, measures the time taken for
the plasma energy to leak out of the magnetic
bottle. In a fusion reactor this energy
must be replaced by heat produced in the
fusion reactions. The energy confinement
time achieved in experiments has increased
by six orders of magnitude since the 1960s.
Today scientists are confining plasmas with
temperatures around a hundred million
degrees Celsius for many seconds in a
toroidal (donut-shaped) magnetic field
configuration called a tokamak. Despite
such progress, the theoretical understanding
of the physical causes of the leakage—
so-called anomalous transport—is incomplete,
and experimental techniques to
reduce it are still being developed.
In an idealized situation, the motion of
charged particles in a strong magnetic field
is restricted to a tight spiral around the field
lines. In a plasma, these particles will
escape in the direction perpendicular to the
field lines via a random walk as a result of
infrequent collisions with one another (with
the step size being the radius of the spiral).
However, such “classical” diffusion of particles
and thermal energy across magnetic
fields lines should be about a thousand
times slower than that actually observed in
experiments. Indeed, if plasma confinement
were as efficient as classical theory
suggests, it is likely that fusion power stations
would now dot the landscape and climate
change would not be a particularly
important issue.
Consequently, the experimentally
observed diffusion rate was famously
termed “anomalous transport” due to the
lack of understanding of the physics behind
this effect. Anomalous transport is a ubiquitous
phenomenon in astrophysical, geophysical,
and laboratory plasmas since it
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2 SEPTEMBER 2005 VOL 309 SCIENCE www.sciencemag.org