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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