[FPSPACE] FW: Physics News Update 780

[LARRY KLAES]

>From: physnews at aip.org
>Reply-To: physnews at aip.org
>To: ljk4 at MSN.COM
>Subject: Physics News Update 780
>Date: Fri, 9 Jun 2006 11:10:02 -0400
>
>PHYSICS NEWS UPDATE
>The American Institute of Physics Bulletin of Physics News
>Number 780 June 9, 2006  by Phillip F. Schewe, Ben Stein,
>and Davide Castelvecchi        www.aip.org/pnu
>
>A HINT OF NEGATIVE ELECTRICAL RESISTANCE emerges from a new
>experiment in which microwaves of two different frequencies are
>directed at a
>2-dimensional electron gas. The electrons, moving at the interface
>between two semiconductor crystals, are subjected to an electric
>field in the forward (longitudinal) direction and a faint magnetic
>field in the direction perpendicular to the plane. In such
>conditions the electrons execute closed-loop trajectories which
>will, in addition, drift forward depending on the strength of the
>applied voltage. A few years ago, two experimental groups observed
>that when, furthermore, the electrons were exposed to microwaves,
>the overall longitudinal
>resistance could vary widely---for example, by increasing by an
>order of magnitude or extending down to zero, forming a
>zero-resistance state, depending on the relation between microwave
>frequency and the strength of the applied magnetic field (for
>background, see Physics Today, April 2003).
>Some theorists proposed that in such zero-resistance state, the
>resistance would actually have been less than zero: the swirling
>electrons would have drifted backwards against the applied voltage.
>However, this rearwards motion would be difficult to observe because
>of an instability in the current flow---that is, the current
>distribution becomes inhomogeneous so as to yield a vanishing
>voltage drop.  A Utah/Minnesota/Rice/Bell Labs group has by now
>tested this hypothesis in a clever bichromatic experiment using
>microwaves at the two frequencies. Michael Zudov (now at the
>University of Minnesota, zudov at physics.umn.edu, 612-626-0364) and
>Rui-Rui Du (now at Rice University) sent microwaves of two different
>frequencies at the electrons, observing that for nonzero-resistance
>states the resultant resistance was the average of the values
>corresponding to the two frequencies separately. On the other hand,
>when the measurements included frequencies that had yielded a zero
>resistance, the researchers observed a dramatic reduction of the
>signal.  Judging from the average resistance observed for non-zero
>measurements, they deduce that whenever zero resistance was
>detected, the true microscopic resistance had actually been less
>than zero. In other words, an observed zero resistance was masking
>what was in fact an unstable negative- resistance state.  (Zudov et
>al., Physical Review Letters, 16 June 2006)
>
>ON MARS, NO ONE COULD HEAR A LAWN MOWER'S SOUND farther than a
>couple of hundred feet, compared to the several miles it can travel
>on Earth, according to a new computer simulation of sound
>propagation on our next-door planetary neighbor. In general, what do
>things sound like on Mars?  At this week's meeting of the Acoustical
>Society of America in Providence, Amanda Hanford (ald227 at psu.edu)
>and Lyle Long of Penn State presented detailed computer calculations
>that simulate how sound travels through the Martian atmosphere,
>which is much thinner than Earth's (exerting only 0.7% of the
>pressure of our atmosphere on the surface) and has a very different
>composition (containing 95.3% carbon dioxide, compared to about
>0.33% on our planet).  The loss of 1999's Mars Polar Lander, which
>was to record sounds directly on the planet, has compelled
>researchers to find other means to study how sound travels there.
>To determine the behavior of sound on Mars, the researchers analyzed
>how gas molecules move and collide in its atmosphere.   The
>researchers took into account the gas molecules' mean free path, the
>average distance a molecule travels before it collides with a
>neighbor (6 microns, compared to 50 nm on Earth). They also
>considered the different ways in which gas molecules could exchange
>energy when colliding with each other.  In their computational
>approach, known as direct simulation Monte Carlo, collisions
>occurred randomly, though at a statistically accurate rate.
>Accounting for the different combinations of molecule species that
>could collide along with the many different ways in which they could
>lose or gain energy required a huge amount of computation---over 60
>hours---even for simulating a small patch of atmosphere for every
>sound frequency they considered, using a 32 processor "Beowulf"
>computer cluster that was one of the most powerful computers in the
>world.  With their approach, the researchers could determine all
>physical properties of interest in the propagation of sound on Mars.
>The researchers' results show that the absorption of sound on Mars
>is 100 times greater than it is on Earth, because of the differences
>in molecular composition and lower atmospheric pressure. Owing to
>computational considerations (they could only analyze collisions
>over a relatively small region of space), the researchers only
>simulated the propagation of lower-wavelength sounds (with
>frequencies in the ultrasound regime) but extrapolated the results
>down to audible frequencies.  (Meeting paper 2aPA3; more information
>at http://www.acoustics.org/press/151st/Hanford.html)
>
>***********
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