Rethinking the Structure of Water
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Tetrahedral, ring, and chain structures
in liquid water (courtesy of H. Ogasawara). |
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Water is the key to our existence
on this planet and it is involved in nearly all biological, geological,
and chemical processes. Knowledge about how its molecules bond
with each other is essential for understanding its unusual chemical
and physical properties. In its condensed phase (i.e., ice), each
water molecule bonds loosely to four others in a tetrahedral arrangement.
For 20 years, it has been commonly accepted that liquid water also
forms a semi-tetrahedral structure. Previous studies, relying largely
on neutron and x-ray diffraction data, could not provide a unique
experimental determination of local molecular arrangements. A different
approach, using soft x-ray absorption spectroscopy (XAS), probes
how chemical bonding perturbs the local valence electronic structure.
From the data obtained, the researchers concluded that liquid water
consists mainly of structures with two strong hydrogen bonds, in
contrast to the four bonds found in the tetrahedral structure of
ice. This new result resurrects models that were previously discarded,
such as the possibility that water molecules form chains or closed
rings. Eventually, the outcome could be a better understanding
of the chemistry of the cell — notoriously hard to imitate using
different liquids — and perhaps a clearer answer to why water
is essential for life.
Ph. Wernet, D. Nordlund, U. Bergmann,
M. Cavalleri, M. Odelius, H. Ogasawara, L.Å. Näslund,
T.K. Hirsch, L. Ojamäe, P. Glatzel, L.G.M. Pettersson,
and A. Nilsson, “The structure of the first coordination
shell in liquid water,” Science 304,
995 (2004).
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Nanodiamonds Show Buckyball Surfaces
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Ball-and-stick representation of bucky diamond
cluster, showing diamond
core (yellow) and a fullerenelike reconstructed surface (red). |
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Scientists have found that diamonds made of
up to a few hundred carbon atoms (“nanodiamonds”) do not exhibit the smooth,
faceted surfaces commonly associated with crystals. Instead, at this
scale, portions of the diamond’s surface will spontaneously buckle
into the curved, geodesic-dome structure found in buckyballs. The researchers
came to this surprising conclusion after performing soft x-ray absorption
and emission spectroscopy experiments on nanodiamonds synthesized in
detonation waves from high explosives. Diamond, like silicon and germanium,
is a semiconductor whose behavior depends on the size of its optical
gap — the energy difference between its valence and conduction
bands. Emission and absorption spectroscopy together reveal the optical
gap in semiconductors, with emission revealing the valence band maximum
and absorption revealing the conduction band minimum. The techniques
also reflect the density of states around the band gap — a sensitive
fingerprint of atomic bonding configurations. The nanodiamond absorption
spectra showed features not observed in bulk diamond samples. Comparison
to theoretical models suggests that the feature is the signature of
the pentagonal and hexagonal bonding configurations found on buckyball-like
surfaces. The discovery of this new family of carbon clusters, dubbed “bucky
diamonds,” may have implications for a wide range of areas,
from astronomy, where diamonds are studied as a constituent of
meteorites and interplanetary dust, to optoelectronics, where nanodiamonds
might be used as photonic switches and tunable lasers.
J.-Y. Raty, G. Galli,
C. Bostedt, T.W. van Buuren, and L.J. Terminello, “Quantum
confinement and fullerenelike surface reconstructions in nanodiamonds,” Phys.
Rev. Lett. 90, 037401 (2003).
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Creation of an Antiferromagnetic Exchange Spring
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Antiferromagnetic exchange spring.
A magnetic field (purple arrow) applied to
an FM/AFM bilayer rotates the magnetization. |
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In the ongoing quest for faster and more efficient magnetic data
storage, designs for devices such as read heads in computer hard
drives are mostly produced through a trial-and-error process, combining
thin magnetic films with different properties. To speed up this search
for better materials, researchers are striving for a better understanding
of the microscopic structure and interactions between ferromagnetic
(FM) and antiferromagnetic (AFM) layers. Researchers have now solved
a piece of this puzzle using x-ray magnetic linear dichroism (XMLD)
spectroscopy and an x-ray magnetometer that allows the rotation of
a strong magnetic field in any direction in space. When a ferromagnet
and an antiferromagnet are combined in a layered structure — such
structures are part of the read heads in computer hard drives — the
hard ferromagnet pins and holds the magnetization of the ferromagnet
across the interface in the presence of an applied magnetic field,
up to a certain field threshold. This pinning, known as exchange bias,
results from atomic exchange forces across ferromagnet–antiferromagnet
interfaces, which tend to align the magnetization of nearby atoms.
When a stronger magnetic field above the threshold is applied, abrupt
movement of the ferromagnet is expected, leaving the hard antiferromagnet
relatively unaffected. In reality, as the experiments showed, the
magnetization of the soft layer dragged the magnetization of the
antiferromagnet, winding it like a clock spring. The result is creation
of a domain wall between the rotated region at the surface of the
sample and the unrotated region below. This behavior is common with
ferromagnets but was unknown for antiferromagnets.
A. Scholl,
M. Liberati, E. Arenholz,
H. Ohldag, and J. Stöhr, “Creation of an antiferromagnetic
exchange spring,” Phys. Rev. Lett. 92,
247201 (2004). |
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