Number 752, November 2, 2005
by Phil Schewe and Ben Stein
A Nanoscale Galvani Experiment
A nanoscale galvani experiment provides a new way to obtain images
of biological tissue. Applying state-of-the-art technology to a
seldom-exploited electromechanical property in biomolecules, Sergei
Kalinin of Oak Ridge National Laboratory (sv9@ornl.gov) and his
colleagues have demonstrated a nanometer-scale version of Galvani's
experiment, in which 18th-century Italian physician Luigi Galvani
caused a frog's muscle to contract when he touched it with an
electrically charged metal scalpel. Described at this week's
AVS
Science & Technology meeting in Boston, the new, 21st-century
demonstration promises to yield a host of previously unknown
information in a variety of biological structures including
cartilage, teeth, and even butterfly wings.
Employing a technique named Piezoresponse Force Microscopy (PFM),
Kalinin and colleagues sent an electrical voltage through a tiny,
nanometer-sized tip to induce mechanical motion along various points
in a biological sample, such as a single fibril of the protein
collagen. The electromechanical response at various points of the
sample, as measured by the probe tip, enabled the researchers to
build up images of the collagen fibrils, with details less than 10
nanometers in size. This resolution surpasses the level of detail
that can be gleaned on those biostructures by ordinary
scanning-probe and electron microscopes (get a lengthier description
here).
The PFM technique exploits the well-known but infrequently used fact
that many biomolecules, especially those that are made of groups of
proteins, are piezoelectric, or undergo mechanical deformations in
the presence of an external electric field. The researchers have
used the PFM technique to produce images of cartilage as well as
enamel and dentin (found inside teeth). Besides providing images of
biostructures on a nanometer scale, the new technique yields
information about the electromechanical properties and molecular
orientation of biological tissue. In recent work, the researchers
even found unexpected piezoelectric properties in butterfly wings
which enabled them to yield molecular-level images of wing
structures.
The first observation of digital heat flow
in a nanostructure at
ambient conditions has been made using carbon nanotubes suspended
between two electrodes. A new experiment carried out at Caltech,
and reported at the
AVS
Science & Technology meeting in Boston,
furthers the effort to employ
nanotubes as a means for removing unwanted heat from microcircuits.
Carbon nanotubes, nanometer-wide cylinders made from rolled up graphitic
sheets have a versatile array of
mechanical, electrical, and magnetic properties. Their thermal
properties should be just as valuable. Because phonons (the
particle manifestations of heat flow) can move so freely in
nanotubes, even ballistically (meaning that they refrain from
scattering and travel in straight lines), the flow of heat in
nanotubes should have quantum properties.
Indeed, Caltech scientist
Marc Bockrath (mwb@caltech.edu) and his colleagues have observed
that heat conductivity in nanotubes can readily reach
quantum-mechanical limits; heat conduction occurs in multiples of a
quantum unit of heat flow. Phonons seem to move nearly as far as a
micron (a long distance for nanoscopically sized objects) even at
temperatures of 900 degrees Celsius. The mean free path between
scattering for the phonons should be even larger at room
temperature. This, says Bockrath, underscores the fantastic
potential of nanotubes as thermal conduits.
Meeting paper NS-ThM4
Color Superconductivity
Color superconductivity, the hypothetical condensation of quark
pairs at the cores of super-dense collapsed stars, might represent a
unique example of superconductivity being made stronger, not weaker,
by the presence of magnetism.
In ordinary electrical
superconductivity, in a metallic lattice of atoms, free electrons
can pair up through the agency of a very weak coupling force
mediated by the subtle vibrations in the lattice itself,
establishing a weakly attractive force between pairs of electrons.
An external magnetic field is either repelled from such a
superconducting environment (the Meissner effect) or can serve to
undo the fragile superconducting state. On the other hand, if
quark matter is realized inside the core of neutron stars -- with
densities about 10 times the density of ordinary atomic nuclei -- or
within the still hypothetical quark stars -- objects ranking
somewhere between neutron stars and black holes in terms of matter
density -- quarks will be pressed together so firmly that by the
rules of asymptotic freedom (see the description of last year's
physics Nobel prize in
PNU 703)
the force between the quarks will be quite weak and
attractive.
This weakly interactive highly dense quark matter is
expected to behave similarly to ordinary superconductors in
condensed matter, and the quarks will form pairs as do the electrons
in metallic superconductivity. Since quarks possess "color charge"
("colors" like red, green, or yellow are just another way of
referring to a special type of charge, analogous to electric charge,
carried by quarks) the quark-quark pair carries a net color charge;
hence the phenomenon is called color superconductivity (for a
detailed explanation of color superconductivity see this article in the
August 2000
issue of Physics Today).
One might think that an applied magnetic field will produce in the
color superconductor the same kind of counteracting effect that it
does in ordinary superconductivity. However, a new study by Vivian
de la Incera and Efrain Ferrer of Western Illinois University
(Macomb, Ill., U.S.) and Cristina Manuel of the Instituto de Fisica
Corpuscular (Valencia, Spain) shows theoretically that the powerful
magnetic fields inside some super-compressed stars can actually
enhance color superconductivity.
The authors say that, in the core
of compact stars, the coming together of very high nuclear density,
an enfeebled color nuclear force, and very strong magnetic fields
(as high as 1017 gauss in some collapsed stars), enables the
formation of a new phase of low-temperature color superconducting
quark matter, one in which superconductivity and magnetism are on
good terms (see figure).
Right now, the authors admit, testing this hypothesis will be
difficult, as more investigations are still needed to identify
signatures that can connect the inner phase of the star to its
observable properties, such as the mass-to-radius ratio.
Ferrer et
al., Physical Review Letters, 7 October 2005
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