Text 638, 97 rader
Skriven 2005-10-18 16:44:03 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 750
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News Number 750
October 19, 2005  by Phillip F. Schewe, Ben Stein
        
THE PHONON HALL EFFECT, the acoustic equivalent of the electrical Hall effect,
has been observed by physicists at the Max Planck Institut fur
Festkorperforschung (MPI) and the Centre National de la Recherche Scientifique
(CNRS) in France.  In the electrical Hall effect, when an electrical current
(consisting of free electrons moving along a material sample) being driven by
an electric field is subjected to an external magnetic field, the charge
carriers will feel a force perpendicular to both the original current and the
magnetic force, causing the electrical current to be deflected somewhat to the
side.  Thermal transport is a bit more complicated than electrical transport. 
A "current" of heat can consist of free electrons carrying thermal energy or it
can consist of phonons, which are vibrations rippling through the lattice of
atoms of the sample.
Previously, some scientists believed that in the absence of free electrons, a
magnetically induced deflection of heat could not be possible.  The MPI-CNRS
researchers felt, however, that a magnetic deflection of phonons was possible,
and have now demonstrated it experimentally in insulating samples of Terbium
Gallium Garnet (a material often used for its magneto optical properties) where
no free charges are present. The sample was held at a temperature of 5 K and
was warmed at one side, creating the thermal equivalent of an applied voltage.
Application of a magnetic field of a few Tesla led to an extremely small
(smaller than one thousandth of a degree) yet
detectable temperature difference.   (Strohm et al., Physical Review
Letters, 7 October 2005; text at www.aip.org/physnews/select) The same team of
MPI-CNRS scientists earlier demonstrated a kind of "photon Hall effect"
(http://www.aip.org/pnu/1997/split/pnu349-2.htm).

DETECTING ALZHEIMER'S EARLY WITH NON-INVASIVE OPTICAL TOOLS.
Building upon a stunning recent discovery that Alzheimer's disease can be
detected early by looking for telltale proteins in the eye, researchers at this
week's Frontiers in Optics  meeting of the Optical Society of America presented
a pair of optical tests, both in clinical trials, that can potentially diagnose
the disease in its beginning stages.  Such tests may not only improve patients'
chances to start treatment earlier, but they could also speed development of
new Alzheimer's drugs.
Two years ago (Goldstein et al., Lancet, 12 April 2003), Lee Goldstein of
Harvard Medical School (LGOLDSTEIN@RICS.BWH.HARVARD.EDU) and his colleagues
showed that the exact same amyloid beta proteins which are a hallmark of
Alzheimer's disease are also found in the lens and its surrounding fluid.  In
those portions of the eye, the proteins form amyloid deposits similar to those
in the brain.  Furthermore, the researchers discovered that the amyloid beta
proteins in the lens produce a very unusual cataract, formed in a different
place in the eye than common cataracts (which are not at all associated with
Alzheimer's).  Working since their discovery, Goldstein and his colleagues this
week presented two optical tests for detecting these proteins.
Using a technique known as quasi-elastic light scattering, the first test
employs low-power infrared laser light to non-invasively detect protein
particles in the specific part of the lens where these unusual cataracts form. 
The second test would be applied to those who screen positively for the
proteins, in order to confirm an Alzheimer's diagnosis.  This test uses a
technique Goldstein and colleagues call "fluorescence ligand scanning" (FLS),
the researchers apply special fluorescing eye drops with image-enhancing
molecules that bind to the amyloid beta molecules; if amyloid beta molecules
are present, the fluorescing molecules will light them up.
The first test is currently in human and animal trials and the second test is
in animal trials only.  These two diagnostic tests are envisioned to be a
two-step process for screening and then confirming an Alzheimer's diagnosis. 
These new optical tools can also potentially speed up the development of new
Alzheimer's drugs, by giving investigators rapid feedback on whether the drug
is doing its job of removing the harmful proteins from the body. Moreover, the
researchers are using the same technologies to develop new tests for rapidly
detecting amyloid plaques resulting from prion diseases, including mad cow,
scrapie in sheep, and Creutzfeldt-Jacob disease in humans.
(http://www.osa.org/meetings/annual/; Paper FTuBB4 at meeting, October 18,
2005.)

SUPER LENSING IN THE MID INFRARED.  Physicists at the University of Texas have
made a "super lens," a plane-shaped lens that can image a point source of light
down to a focal spot only one-eighth of a wavelength wide; this is the first
time such super lensing has been accomplished in a functional device in the
mid-infrared range of the electromagnetic spectrum.  Historically lensing
required a lens-shaped (that is, lozenge-shaped) optical medium for bringing
the diverging rays coming from a point source into focus on the far side of the
lens.  But in recent years, researchers have found that in "negative
permittivity" materials (in which a material's response to an applied electric
field is opposite that of most normal materials), light rays can be refracted
in such a way as to focus planar waves into nearly a point, albeit over a very
truncated region, usually only a tenth or so of the wavelength of the light.
Such near-field optics are not suitable for such applications as reading
glasses or telescopes, but have become an important technique for certain kinds
of nanoscale imaging of large biological molecules than can be damaged by UV
light.  The micron-sized Texas lens, reported at the OSA meeting
(http://www.osa.org/meetings/annual), consists of a silicon carbide membrane in
between layers of silicon oxide.  It focuses 11-micron-wavelength light, but
the researchers hope to push on into the near-infrared range soon. 
Furthermore, the lensing effect seems to be highly sensitive to the imaging
wavelength and to the lens thickness.  Gennady Shvets (gena@physics.utexas.edu)
says that additional possible applications of the lens include direct laser
nanolithography and making tiny antennas for mid-IR-wavelength free-space
telecommunications.  (Paper fMG2 at meeting; Lab website:
www.ph.utexas.edu/~shvetsgr/)

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