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Skriven 2005-01-28 21:29:50 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 717
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 717 January 27, 2005
by Phillip F. Schewe, Ben Stein

A PHASE CHANGE IN HIGH-DENSITY DATA STORAGE.  A new approach to storing bits of
information in a rewritable medium substitutes electron beams for optical
beams.  Scientists at Hewlett Packard create individual bits in the form of
tiny amorphous regions inside a thin indium-selenium layer.  That layer, along
with another layer beneath (gallium-selenium) and a silicon substrate, form the
principal parts of a pn-junction diode.  The read-write cycle goes like this:
short, high-power bursts from an electron beam are used to write a "1" by
melting a tiny portion of the InSe layer, turning it into a glassy blob. 
Alternatively the blob can be erased by the use of a longer, low-power beam
pulse, which recrystallizes the material.  With the help of an even lower-power
beam pulse the bit can be read out as either a 1 (the amorphous blob yields
little or no detectable current in the pn-junction diode ) or a 0 (the
crystalline material yields a high diode current).  Electron-beam storage can
potentially reach higher
 densities than optical storage due to the shorter wavelength of high-energy
electrons.  Ultimately, it may also enable faster data access through
electrostatic deflection of the electron-beams. The HP tests so far have used a
laser beam rather than an electron beam to do the writing part (their electron
beam isn't yet strong enough) but employ an e-beam (essentially a scanned
electron microscope) to do the reading.  The response of the diode storage
medium is fast enough to allow reading rates of at least a million bits per
second per electron-beam and more than 100 write/erase/rewrite cycles have been
carried out successfully.  The bit size right now is about 150 nm in lateral
extent (for an area density of about 29 gigabits per square inch), but this
will probably be made far smaller, maybe down to 10 nm.  (Gibson et al.,
Applied Physics Letters, 31 January 2005; contact Gary Gibson,
gary.gibson@hp.com, 650-857-2125 or Alison Chaiken, chaiken@hpl.hp.com, 650-23
6-2231)
        
ORGANIC MOLECULES ON THE REBOUND. Scientists at the International University of
Bremen and the University of Bonn have recently determined the precise
structure of a large organic molecule after its interaction with a metal
surface. The group of scientists also used the structure information to
decipher clues about the chemical bond between the molecule and the surface. 
The organic-metallic interface is very important in science, especially in the
fields of catalysis (chemical reactions between two species proceeding in the
presence of a third species), bio-sensing, and molecular electronics (where
signals are processed through circuit elements consisting, in some cases, of
single molecules or arrays of molecules). In this regard, larger molecules are
harder to study because of their size, their tortuous shape, and many internal
modes of vibration.  In the Bremen-Bonn experiment the starting point is a
super-clean silver surface in ultrahigh vacuum.  Next the molecule is allowed
to fall onto the surface where it reacts chemically with surface atoms and is
slightly distorted thereby.  Next, x rays from a synchrotron are brought to
bear on the adsorbed molecule.  By the scattering of the x rays the researchers
can deduce, in some cases atom for atom, where the component parts of the
molecule are relative to the nearby metal surface.  The worked-out structure of
the reacted molecule can then be compared to the structure for the same type of
molecule in the free (gaseous) state.  In this way the distortion of the
molecule, whose full name is perylene-tetracarboxylic-dianhydride (PTCDA), can
be worked out.  It is notable that the x-ray scattering technique used here was
not the normal Bragg scattering in use for decades.  Because the sample was so
thin, the approach employed here was based on standing x-ray waves.  The x rays
reflected from the silver crystal formed standing waves when they interfered
with incoming x rays.  The ensuing atomic-scale "ruler" can be used to map the
organic molecule by slightly grading the energy of the incoming x rays.  This
normal incidence x-ray standing wave technique has been used before but very
rarely on large organic adsorbates where it has great potential. What happened
as the normally planar molecule approached the surface?  Surprisingly, there
was some bending, mostly because of the readiness of some oxygen atoms (which
weren't supposed to play much of a chemical role) to form bonds with the
surface silver atoms.  Another discovery: the molecule forms not a single bond
but a hierarchy of two types of bonds.  (Hauschild et al., Physical Review
Letters, 28 January 2005; contact Stefan Tautz, 49-421-200-3223,
s.tautz@iu-bremen.de;
lab website http://imperia.iu-bremen.de/ses/physics/tautz/30797/ )

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