Text 646, 102 rader
Skriven 2005-11-02 18:49:13 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 752
===============
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 752  November 2, 2005  by Phillip F. Schewe, Ben Stein

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 at
http://www2.avs.org/symposium/boston/pressroom/papers.html ).
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.  (Kalinin, Rodriguez
and Gruverman, meeting paper NS-WeM3)
                                                                
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
meeting, furthers the effort to employ nanotubes as a means for removing
unwanted heat from microcircuits.
Carbon nanotubes, nm-wide cylinders made from rolled up graphitic sheets, as a
vital conduit for removing  have a versatile array of mechanical, electrical,
and magnetic properties.  Its 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
C.  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.  (Paper NS-ThM4).

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: "http://www.aip.org/pnu/2004/split/703-1.html)
the force between the quarks will actually 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 the article:
http://www.aip.org/pt/vol-53/iss-8/p22.html).
    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, IL, USA) 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 10^17 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 at http://www.aip.org/png/2005/237.htm).
    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)

---
 * Origin: Big Bang (1:106/2000.7)