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abstract, mathematical ‘envelope’ function.
This envelope function describes the period
of the oscillatory coupling between parallel
and anti-parallel spin alignment of the magnetic layers. Without going into too much
detail, it is worth noting that this ingenious
experiment provides a hint of what to expect
from the new generation of bright X-ray
sources such as the synchrotron at Berkeley.
The confluence of artful sample preparation
and probing techniques in this work provides
a way of seeing quantum size effects in magnetic films. The basic knowledge gained can
help unravel the physics of new materials for
the information revolution.
S. D. Bader is at the Argonne National Laboratory,
MSD-223, Argonne, Illinois 60439, USA.
e-mail: [email protected]
1.
2.
3.
4.
Kawakami, R. K. et al. Nature 398, 132–134 (1999).
Baibich, M. N. et al. Phys. Rev. Lett. 61, 2472–2475 (1988).
Ortega, J. E. & Himpsel, F. J. Phys. Rev. Lett. 69, 844–847 (1992).
Garrison, K. Chang, Y. & Johnson, P. D. Phys. Rev. Lett. 71,
2801–2804 (1993).
5. Carbone, C., Vescovo, E., Rader, O., Gudat, W. & Eberhardt, W.
Phys. Rev. Lett. 71, 2805–2808 (1993).
6. Paggel, J. J., Miller, T. & Chiang, T.-C. Phys. Rev. Lett. 81,
5632–5635 (1998).
Carbon cycling
The mysterious missing sink
David W. Schindler
can remember discussing the implications of early data1 on atmospheric CO2
almost 30 years ago. Geochemists were
confident that uptake of carbon at the
ocean’s surface could explain the imbalance
between carbon released by man’s activities
(then considered to be almost entirely the
burning of fossil fuels) and measured concentrations in the atmosphere. The rather
large seasonal oscillation was regarded as a
terrestrial signal, but one that remained in
balance from year to year. Studies in the
1970s and 1980s, however, showed that
both of these assumptions were incorrect.
Detailed surface carbon-flux measurements
were made at many sites in the world’s oceans
and, as fluxes for different regions became
more precisely known, estimated average
fluxes dwindled. In 1990, Tans et al.2
announced that the annual flux to the oceans
was likely to be less than 0.5 2 1015 g of carbon per year — a quarter the amount necessary to balance the global carbon budgets.
So where is the ‘missing sink’ for atmospheric CO2? On page 145 of this issue, Nadelhoffer et al.3 provide convincing evidence
that, just as it cannot be explained by the
oceans, neither can the missing carbon
be rationalized by uptake in the northern
temperate forests — until now, the favoured
theory.
The conclusion that the missing sink
was terrestrial forests in the northern hemisphere4 came about because, after several
decades of measuring atmospheric CO2, the
I
NATURE | VOL 398 | 11 MARCH 1999 | www.nature.com
amplitude of the annual fluctuation was
found to be increasing. The increased flux to
the atmosphere was calculated to be largely
from biomass burning5. But the increased
CO2 stimulation
to the continents
0.4×1015
release of carbon to the atmosphere was
more or less balanced by increased annual
uptake. Comparisons of the 13C/12C ratio in
various parts of the upper ocean with carbon
emitted by fossil-fuel burning reinforced the
conclusion6.
It was assumed that either increasing
atmospheric CO2 or increasing nitrogen
would drive carbon uptake in the forests.
Circumstantial evidence indicated that
nitrogen was involved. Not only is it the
nutrient that most limits growth in the
northern forests, but human releases of
nitrogen to the atmosphere have more than
doubled natural sources this century. Over
5 2 1012 g of nitrogen fall annually on the
world’s forests, and wind patterns carry this
airborne fertilizer over large forested areas of
eastern North America and northern Eurasia7. Studies in Europe documented a 25%
increase in carbon uptake by forests during
the period of nitrogen increase8. And calculations based on plausible carbon:nitrogen
ratios indicated that nitrogen-stimulated
‘greening’ in northern forests could account
for the missing sink9. Finally, many largescale studies have shown that increased
atmospheric CO2 alone is unlikely to be
enough to explain the increased carbon
sequestration10.
Nadelhoffer et al.3 now show that nitrogen is not stimulating forest carbon uptake
in temperate forests — the very areas where it
seemed most likely. In 18 large-scale controlled experiments at nine sites in Europe
and North America, the authors added 15N
Slight but widespread
increase in N deposition ?
MOUNTAIN HIGH MAPS/DIGITAL WISDOM, INC.
the Pauli exclusion principle, one of the fundamental tenets of quantum mechanics).
When the magnetic layer and copper are
brought into proximity, bonding (or electronic hybridization) can occur if the energy
levels match across the interface. The unique
feature is that, because of the magnetic energy splitting, if one of the copper spin states
can hybridize with its magnetic counterpart,
then the other is mismatched in energy, and
hence quantum confined. The properties
of such a ‘spin-polarized’ quantum-well
system govern the strength of the magnetic
coupling3–6, which is relevant in determining
device characteristics in magnetic recording
applications.
These quantum wells are novel in that
although they are analogous to their semiconductor counterparts, they differ in one
fundamental way — they are spin-polarized,
or magnetic. So, this work provides a fresh
way of looking at a rather abstract concept.
The data are sufficiently rich that the researchers are able to go full circle, and summarize the underlying physics in terms of an
Longer growing season ?
High N deposition
to northern temperate
forests 0.25×1015 (ref. 3)
Ocean uptake
0.5-2.1×1015
(ref. 10)
Inter-hemisphere transfer of CO2 out
atmospheric CO2 via circulation
in the deep ocean 0.6×1015
(ref. 10)
CO2 in
South
North
Figure 1 The global carbon whodunnit — where is the missing sink? The possibility of a single culprit
has been all but dismissed by Nadelhoffer et al.3. Estimated annual uptake by various processes is
shown, but 1015 g of carbon cannot account for all of the missing carbon. Question marks indicate
that no estimates have been made.
© 1999 Macmillan Magazines Ltd
105
news and views
and traced it through various forest compartments. Some catchments were exposed
to regional nitrogen deposition; others were
fertilized with nitrogen. In still other areas,
nitrogen input was reduced by catching precipitation falling through the forest canopy
on transparent roofs and removing most of
the nitrogen. An average of only 20% of the
added 15N was retained by plant tissues. Just
5% entered woody tissues with a high carbon:nitrogen ratio, most going to relatively
low carbon:nitrogen soils. Nadelhoffer et al.
estimate the maximum contribution to the
missing sink for atmospheric CO2 to be a disappointing 0.25 2 1015 g of carbon per year.
So where in the biosphere is the missing
carbon hiding (Fig. 1)? There are several possibilities. First, perhaps simple surface fluxes
of CO2 underestimate the role of the oceans.
Broecker and Peng10 suggest that CO2 is
transported, via the deep ocean, from the
North Atlantic to the southern oceans, where
it is vented to the atmosphere. Second, it is
possible that northern temperate forests that
were stimulated by nitrogen in the past are
now saturated with this element. Deposition
of nitrogen in these regions has increased
several-fold in the past few decades7. But if
this picture is correct, it does not bode well
for future carbon uptake in the terrestrial
biosphere. Third, maybe other northern
ecosystems are important in carbon uptake.
Boreal forests approach the size of tropical
forests — both in area and as a carbon reservoir — if peat deposits are considered. Large
tracts of northern wetlands are probably very
nitrogen-limited, if the few experimental
studies are typical11, although not much of
the nitrogen released to the atmosphere currently falls in such areas.
Finally, perhaps there has been a small
but widespread increase in forest growth
stimulated by continuous warming in
northern regions. Growing seasons in many
boreal areas have increased by 2–3 weeks
over the past few decades, and warming in
central and western boreal regions of North
America has ranged from 1–2 degrees since
the early 1970s. But the most likely possibility is that the missing sink will turn out to be
several small sinks — perhaps a combination
of some of the above. Unfortunately, modern
methods are still inadequate to identify global sinks of less than 0.5 2 1015 g carbon per
year with confidence.
To a patient scientist, the unfolding
greenhouse mystery is far more exciting than
the plot of the best mystery novel. But it is
slow reading, with new clues sometimes not
appearing for several years. Impatience
increases when one realizes that it is not the
fate of some fictional character, but of our
planet and species, which hangs in the
balance as the great carbon mystery unfolds
at a seemingly glacial pace.
David W. Schindler is in the Department of
Biological Sciences, University of Alberta,
NATURE | VOL 398 | 11 MARCH 1999 | www.nature.com
Edmonton, Alberta T6G 2E9, Canada.
e-mail: [email protected]
1. Keeling, C. D. Tellus 25, 174–198 (1973).
2. Tans, P. P., Fung, I. Y. & Takahashi, T. Science 247, 1431–1438
(1990).
3. Nadelhoffer, K. J. et al. Nature 398, 145–148 (1999).
4. IPCC Climate Change (Cambridge Univ. Press, 1995).
5. Bolin, B. Science 196, 613–615 (1977).
6. Ciais, C. et al. Science 269, 1098–1100 (1995).
7. Vitousek, P. M. et al. Ecol. Appl. 7, 737–750 (1997).
8. Kauppi, P. E., Mielikäinen, K. & Kuusela, K. Science 256,
70 (1992).
9. Schindler, D. W. & Bayley, S. E. Glob. Biogeochem. Cycles 7,
717–733 (1993).
10. Broecker, W. S. & Peng, T.-S. Greenhouse Puzzles 2nd edn
(Eldigio Press, Palisades, New York, 1998).
11. Rochefort, L., Vitt, D. H. & Bayley, S. E. Ecology 71, 1986–2000
(1990).
Materials science
Asymmetry the easy way
Samuel P. Gido
rom the atomic scale to the macroscopic, symmetry pervades nature. Crystalline materials, for example, can be
described by any one of 230 space filling
symmetry groups, which optimize energetic
interactions between atoms or molecules.
Self-assembled systems generally form with
a point, line or plane of symmetry. This natural tendency towards symmetric structures
has been the bane of self-assembled systems
in applications ranging from liquid-crystal
displays to frequency modulators to optical
switches. On page 137 of this issue, Stadler
and colleagues1 demonstrate a novel
approach to producing a material that selfassembles to form a non-centrosymmetric
layered nanostructure.
Materials with bulk electric polarization
can display many properties, such as nonlinear optical effects and switchability.
Whereas individual molecules can have a
permanent dipole or magnetic moment, in a
large assembly of such dipoles, the orientations of consecutive molecules oppose one
another, cancelling out any permanent
moment in the material. External magnetic
or electric fields can be used to overcome the
energetics underpinning the alternating or
random orientation of dipoles in a material,
but slow, inexorable relaxations tend to drive
the system to its lowest energy state — that is,
with no permanent net dipole. In addition,
such poling processes increase fabrication
complexity and thus cost, reducing the
likelihood of practical application.
Much effort has gone into using surfaces
or interfaces to force a local asymmetry due
to the abrupt termination of a material2–4.
However, these effects are too small to produce orientation of a bulk material, and
remain, for the most part, of academic interest. Alternatively, extensive synthetic procedures have been used to prepare rod–coil
polymers, which form macroscopic arrays
of nanostructures, arranged in a noncentrosymmetric manner5. The advantage of
the approach taken by Stadler and co-workers2 is that subtle macromolecular interactions are tailored to produce a self-assembling asymmetric structure from a blend of
two simple and potentially inexpensive
block copolymer materials. Block copoly-
F
© 1999 Macmillan Magazines Ltd
a
Diblock copolymer
A’ C’C’ A’A’ C’C’ A’A’ C’
b
Triblock copolymer
A B CC B A A B C
Figure 1 Diblock and triblock copolymer
morphologies. Chemically different polymers
are generally incompatible, so low-energy AA
and CC contacts are preferred to AC interfaces.
This results in the formation of
centrosymmetric structures for both the
diblock, a, and triblock, b. The orientations of
the polymer chains associated with adjacent
parallel interfaces are rotated by 180° with
respect to one another in order to maximize the
association of similar block materials.
mers are produced by covalently linking
together two or more chemically distinct
polymer blocks, each composed of a single
type of monomer unit. By carefully selecting
different types of materials, block copolymers can be tailored for applications ranging
from the exotic to such everyday materials as
removable adhesive pads and the soles of
running shoes.
Imagine a polarized system that spontaneously orders into a non-centrosymmetric
structure after simply being heated to a temperature where the molecules can flow. One
strategy for doing this, which has achieved
some success, is to marry an electrically
anisotropic dipole to a molecular structure
that wants to align itself owing to its shape
(liquid crystallinity) or amphiphilic character (block copolymers)6–9. The diblock or
triblock copolymers in Fig. 1 are themselves
chemical dipoles. These chemical dipoles
point from one block material to the other
along the trajectories of the polymer chains.
The energetic (enthalpic) driving force that
associates A blocks with A blocks and B
blocks with B blocks leads these materials to
self-assemble into morphologies in which
the net chemical dipole cancels out. If one
could persuade the chemical dipoles of a
block copolymer system to self-assemble
into an asymmetric structure they could
potentially carry an optically active group
along with them, producing an optically
107