Text 604, 86 rader
Skriven 2005-09-21 15:36:06 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 746
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 746   September 21, 2005  by Phillip F. Schewe, Ben Stein

WEIGHING THE AMAZON RIVER has been accomplished by watching the rise and fall
of the Earth's crust with a Global Positioning Service (GPS) unit over several
years as the river floods and drains during its seasonal cycles.  GPS, through
its network of satellites and carefully staged series of signals timed with
exquisite precision by atomic clocks, can provide information about the
position at the Earth's surface with horizontal uncertainty of about 1 mm and a
vertical uncertainty of about 9 mm.  Repeated measurements made over several
years yield velocity measurements for any spot to an accuracy of about 1
mm/year.  Around the wide world, a typical land movement up or down will be
about 2 to 10 mm/year.  But in large tropical drainage areas, with huge volumes
of water pressing down on a river channel and floodplain, the oscillation can
be bigger.  Indeed, the peak-to-peak amplitude reported in this present
measurement amounts to 50-75 mm/year.  When the river is heavy, the land sinks
down.  Later, when the river lessens, the land rebounds.Scientists from the
Instituto Brasileiro de Geografia e Estatistica and the Instituto Nacional de
Pesquisas da Amazonas (Brazil), and from Ohio State University, the University
of Memphis, and University of Hawaii (US), saw the biggest displacement in
Manaus, Brazil.  One of the researchers, Michael Bevis of Ohio State, said that
they were surprised by the size of the oscillation.  (Bevis et al., Geophysical
Research Letters, 15 September 2005; contact Mike Bevis at mbevis@osu.edu or
Doug Alsdorf at alsdorf@geology.ohio-state.edu; see also www.mps.ohio-state.edu
; article at http://www.agu.org/pubs/crossref/2005.../2005GL023491.shtml )

FIRST BEC IN A SOLID.  A Bose-Einstein condensate (BEC) has been observed in a
solid material for the first time.  The BEC in this case is not a collection of
atoms but rather a collection of particle-like excitations in the solid called
"magnons." In the presence of extremely high magnetic fields, atoms with an
intrinsic magnetism of their own (as represented by a spin vector) can be
oriented all in one direction if the field strength is larger than a certain
value.  In this configuration a small input of energy can tilt some of the
spins out of the general formation.  The successive tilting of spins can take
the form of a wave moving through the sample.  If also the temperature of the
sample is extremely low, then the moving wave can be considered as a
particle-like (or
quasiparticle) entity, much as mechanical vibrations in a solid can be
construed as sound waves or as phonons.  A magnon is such a moving
magnetic-spin disturbance.  What the present experiment observes is a
condensation of magnons if the magnetic field is lower than the critical
strength and the temperature is below a characteristic value.  The work was
carried out by a group of scientists from these institutions: Max Planck
Institute for Chemical Physics of Solids (MPI, CPfS), Dresden; JINR Lab, Dubna;
Oxford University; and Adam Mickiewicz University, Poznan.  They used a
antiferromagnetic material (in which the spins of neighboring atoms tend to be
alternately aligned up and down) with a chemical composition of Cs2CuCl4.  The
temperatures were in the mK range and the external magnetic field used was at
high as 12 T (120,000 gauss).  In an atomic BEC, dilute vapors of atoms
(typically a million or so at a time) are chilled until they enter into a
single quantum state, as if all the atoms were one atom. In a magnon BEC what
is formed is a monolithic static magnetic alignment in the solid.  About 10^23
magnons participate in the condensation.  A magnon BEC had been predicted
several years ago but not realized unambiguously until this work.  The evidence
for condensation is that the material undergoes a phase transition at a
critical temperature dependent on the size of the external field used.  What
the researchers look for is a significant change in the measured heat capacity
(the energy needed to raise the material's temperature by a certain amount). 
(Radu et al., Physical Review Letters, 16 September 2005; contact Heribert
Wilhelm, wilhelm@cpfs.mpg.de )

SOLID-STATE SUPERCAPACITORS.  A new type of solid state device, prepared by
scientists at UCLA, may provide a better method for backing up memory
information on a computer in the case of a power failure.  A capacitor is an
electrical component for storing electrical energy in the form of negative and
positively charged opposing electrodes.  Its ability to do this is measured in
units of farads.  So called supercapacitors are perhaps a thousand times better
than ordinary capacitors by being much smaller in size and by bringing the two
electrodes closer together.  As a quick energy storage platform, a
supercapacitor can charge or discharge in a time of mere microseconds to
seconds, whereas batteries take minutes to hours.  However, the energy density
for batteries is much higher.  Hence many believe that the ideal backup energy
storage device would be a hybrid of battery and supercapacitor.  To be useful
in that role, however, supercapacitors must be easily made and integrated onto
chips.  Here's where the UCLA model proves itself: its fabrication process is
simple (a simple dielectric layer of lithium fluoride sandwiched between Au,
Cu, or Al electrodes), it doesn't need an electrolyte (many other
supercapacitors are halfway toward being miniature batteries in that they need
electrolytes), and it can be integrated for device applications.  It features a
capacitance of tens of microfarad/cm^2 and charging rates of 10 kHz.  (Ma and
Yang, Applied Physics Letters, 19 September 2005; contact Yang Yang, UCLA,
310-825-4052, yangy@ucla.edu ; website,  http://www.seas.ucla.edu/yylabs)

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