Frontiers
In brief
Molecules revealed in their full glory
Lasers probe teeth for minerals
Researchers in Australia and Taiwan have invented
a new technique for assessing dental health that
may no longer require dentists to prod and poke in
the mouths of patients. The technique involves
shining a laser at a tooth in order to trigger
ultrasonic surface acoustic waves (SAW) along the
tooth’s outer enamel coating. The velocity of a
SA W is governed by the elasticity of the material
through which it is propagating, and in healthy
teeth a high mineral content leads to high
elasticity. Therefore, measuring the speed of the
SA W vibration in a tooth should indicate how
healthy it is (Optics Express 17 15592).
Bespoke optical lattices created
Researchers in Germany have designed a
technique for creating customized optical lattices.
Standard optical lattices are periodic potentials
made by criss-crossing laser beams, in which
ultracold atoms can be held in a symmetric grid.
Because the atoms can be manipulated, optical
lattices are often used as an analogue of
quantum systems, which are otherwise difficult to
study directly. The new technique involves first
ionizing individual atoms with an electron beam
and then removing these ions from selected
lattice sites with an electric field (Phys. Rev. Lett.
103 080404).
Breath test for cancer
Researchers in Israel have invented a new type of
breath test for detecting lung cancer that uses
carbon-based sensors. When a patient breathes
into the device, particulates from the lungs are
attracted to the carbon layers. This makes the
sensor swell, thus altering the resistance of the
device. The presence of volatile organic
compounds linked with specific forms of cancer
has a characteristic effect on the resistance. So, by
applying a potential difference across the sensor,
the presence of the particles can be inferred from
fluctuations in the current (Nature Nanotech.
10.1038/nnano.2009.235).
IBM Research, Zurich
Top tip A single molecule of carbon monoxide brought
the five interlocking rings of benzene into view.
ical structure of a single molecule. The main
practical challenge was to find a way of edging the AFM tip close enough to the sample without it being laterally displaced or
even adsorbed owing to van der Waals
forces. Realizing that it is the atom or molecule at the very tip of the AFM probe that
governs the contrast and resolution of the
technique, Gross’s team replaced the metal
tip of a conventional AFM with a single
molecule of carbon monoxide (CO). CO is
chemically highly stable and is subject to significantly weaker van der Waals forces.
As a demonstration of their device, the
researchers turned their AFM tip to a well-studied hydrocarbon known as pentacene
(C22H14), which consists of five fused benzene
rings and measures just 1.4nm in length.
They produced an image showing all five carbon rings as well as the individual carbon and
hydrogen atoms within the molecule. The
observed spacing between individual atoms
was only 0.14 nm – the best resolution yet for
an AFM.
Gross and his team now intend to build up
a catalogue of chemical signatures so that
eventually the CO-tipped AFM could provide a quick and easy way of identifying molecules in chemical analysis. The researchers
believe that, in the longer term, the technique could be applied to the study of chemical reactions and catalysis at the atomic level
(Science 325 1110).
Physicists in the Netherlands and Switzerland have designed a new type of atomic
force microscope (AFM) that can identify
individual atoms within a molecule for the
first time. The technique could also shine
new light on the nature of chemical reactions.
Invented some 20 years ago, AFMs give
scientists the best view of atoms on the surfaces of both insulators and conductors. The
basic process involves scanning the sharp
metal tip of the AFM across a sample to generate images based on the balance of tiny
forces between the tip and the sample. Ongoing improvements to the technique have
revealed surfaces in unprecedented detail,
including the breakthrough in 2007 when
isolated atoms on a material’s surface were
imaged for the first time.
The new technique, developed by Leo
Gross of IBM Research in Zurich and colleagues, improves the resolution of AFMs
still further by revealing the detailed chem-
Magnetic ‘monopoles’ spotted in spin ices
Earth-like nature of exoplanet confirmed
Astronomers have obtained the best evidence yet
that a planet orbiting a star 400 light-years from
Earth is a solid, rocky world just like our own. The
extrasolar planet, known as CoRo T-7b, was
discovered earlier this year and found to have a
radius less than twice that of the Earth. By
observing the “wobble” it induces in its parent star,
CoRo T-7b has been found to have a mass about
five times that of the Earth, which implies a similar
density and rocky composition (Astron. Astrophys.
at press).
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Ever since magnetic monopoles were first
predicted by Paul Dirac in 1931, physicists
have searched in vain for these elusive entities in everything from particle accelerators
to Moon rocks. Now, two independent research groups claim to have finally caught
sight of monopoles – essentially magnets
with only one pole – in magnetic materials
called “spin ices”. Although these spin-ice
monopoles have very different origins from
those predicted by Dirac’s original work, further observation could still help with the
development of magnetic memories and
other spintronic devices.
One team of researchers, based at the
Helmholtz Centre in Berlin and working in
collaboration with scientists in Argentina,
Germany and the UK, studied the crystalline
material Dy2Ti2O7, which is termed a spin ice
because the arrangement of spins is similar
to that of hydrogen atoms in frozen water.
Dirac predicted that the spins in this kind of
magnetic material do not line up like a ferro-
magnet, but instead take the form of a knotted mess of flux lines, called “Dirac strings”,
connected by magnetic monopoles. The
team confirmed a prediction that at about 1 K
the heat capacity of a spin ice should resemble that of a gas of magnetic monopoles
(Science 10.1126/science.1178868).
Meanwhile, researchers at the Institute
Laue-Langevin in France, along with physicists in the UK, used a beam of spin-polar-ized neutrons to study a similar spin ice –
Ho2Ti2O7. They were particularly interested
in studying the ground states of the spin
ice to establish if these states can support
monopole excitations.
At low temperatures and zero magnetic
field, physicists had predicted that, in order
to have monopoles, this knotty mess of a state
must be a “magnetic coulomb phase” – which
the Anglo-French team was able to confirm
through the observation of “pinch points” in
its neutron-scattering data (Science 10.1126/
science.1177582).
