PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 645  July 9, 2003   by Phillip F. Schewe, Ben Stein, and
James Riordon

ULTRA-INTENSE LIGHT FILAMENTS have successfully been sent through
laboratory "fog" that approximates atmospheric conditions.   This is
an important step which should benefit several laser applications,
such as free-space laser communication, monitoring of pollution, and
range finding (see figure at http://www.aip.org/mgr/png/2003/194.htm
).  Open-air laser light shows feature bright beams seemingly
traveling interminably through the sky.  But in general water
droplets are an avid absorber of laser light. Now a group of
physicists at the Universite Claude Bernard Lyon in France have used
ultra intense (10^14 watts/cm^2), ultrashort (120 femtosecond) laser
pulses to create "light filaments," streaks of light only 150
microns wide but hundreds of meters long, which can propagate
through an artificial cloud of water droplets without losing much
energy.  The filaments form up through two competing nonlinear
optical effects: the "Kerr effect" in which high intensity light
modifies the index of refraction in the transmission medium (in this
case air and water vapor) in such a way as to cause self-focusing;
and the creation of a defocusing plasma effect.  The French
researchers now plan to test their scheme in the open atmosphere
under controlled conditions.  (Courvoisier et al., Applied Physics
Letters, 14 July 2003; contact Jean-Pierre Wolf,
wolf{at}lasim.univ-lyon1.fr, 04072-43-13-01; text at
www.aip.org/physnews/select )

"MOTTNESS" MIGHT HELP TO EXPLAIN CUPRATE BEHAVIOR.   One of the
biggest problems in condensed matter physics is the effort to
understand the behavior of copper oxide (or cuprate for short)
superconductors.  One of the most studied materials in all of
science, cuprates are layer cakes consisting of copper-oxygen planes
alternating with planes in which other elements,  such as strontium
or lanthanum, are stocked in varying ratios. For instance, the
alternating layer might consist entirely of La, or it might contain
10%  Sr.  Like chefs looking for just the right recipe of spices,
physicists have tried different levels of doping in an effort both
to understand the underlying physics and to enhance the movement of
electrons through their  samples.  At moderate doping levels, the
cuprates are superconducting: moving electrons pair up and
constitute a resistance-less current of electricity.  Ironically,
the cuprates are much less hospitable to electricity at ultra-low
doping levels.  In fact, they are insulators when they are not
doped.  A material's conductivity is determined by the ease with
which electrons can move around.  In a conductor, there is an
abundance of free electrons.  (Hotel analogy: there are plenty of
guests and plenty of hotel rooms.)  In an ordinary insulator
electrons are bound two by two (the Pauli exclusion principle
insures that no two electrons, except those with opposite values of
spin, can occupy the same state) and there are very few if any free
electrons.  (In an insulating hotel all the rooms are filled with
two guests, with no room for more guests.)  In a Mott insulator
(named for Sir Neville Mott) conditions are even more inhospitable:
all electron energy states are filled with single electrons, and
these interact so strongly as to preclude even the arrival of a
second electron.  (In a Mott hotel all the rooms are single rooms,
and all are filled.).  Many scientists believe that one of the keys
to understanding why the cuprates are such good superconductors in
the cold regime is to learn why they are Mott insulators in the warm
regime and how such physics manifests itself when they are doped.
One more oddity about the cuprates is the issue of "pseudogaps."  In
a superconductor, the energy required to break up a pair of
electrons is termed the "energy gap."  But in the cuprates, a
partial gap still persists even when superconductivity is
destroyed.  Some have interpreted this as evidence that some pairs
can exist even when the material is warmed above its superconducting
transition temperature (see figure at
http://www.aip.org/mgr/png/2003/195.htm ). However, the pseudogap is
observed in Mott insulators that never became superconducting in the
first place, indicating that the pseudogap is of a more general
origin.   Maybe there is more to superconductivity than the pairing
of electrons. (See Nature, 4 January 2001 for background on this
topic.)
Now, a new theory addresses the problem of cuprate superconductivity
by suggesting that the existence of the curious pseudogap behavior
can be explained by the same physics that makes cuprates Mott
insulators.  Tudor Stanescu (Rutgers Univ) and Philip Phillips (Univ
Illinois) argue that "Mottness," involving the collective
interaction among many electrons,  is still present even when some
of the hotel rooms are empty, to use the hotel analogy.  They
propose that the pseudogap arises simply because transport of
electrons in a doped Mott insulator will still involve two electrons
temporarily occupying the same site (the same room in the hotel
analogy). Such events remind the doped state of its Mottness and
this produces a pseudogap.  They argue that such an effect
disappears when roughly 25% of the hotel rooms are vacant.  At such
an occupancy rate, an electron can move, on average, throughout a
layer without the inhospitability of Mottness.  (Physical Review
Letters, 4 July 2003; text at www.aip.org/physnews/select ; contact
Philip Phillips, 216-751-7348, philip{at}wirth.physics.uiuc.edu )

SEMICONDUCTORS ARE COOL.   One of the problems with electronic
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