Science > Physics > PHYSICS NEWS UPDATE -- Number 836 August 21, 2007 by Phillip F. Schewe
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21 Aug 2007 09:02:00 AM |
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PHYSICS NEWS UPDATE -- Number 836 August 21, 2007 by Phillip F. Schewe |
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 836 August 21, 2007 by Phillip F. Schewe www.aip.org/pnu
LIGHT-DRIVEN FEMTOSECOND ELECTRICITY. Scientists in Canada foresee the
use of electromagnetic fields of laser light for inducing and reversing
tiny electrical currents along molecular wires without the use of a
voltage applied across leads. They would accomplish this feat by
shining special laser pulses containing light waves at two different
frequencies onto a polyacetylene molecule which acts like a junction
between two metallic leads on either side (see figure at
http://www.aip.org/png/2007/286.htm). Depending on the exact
frequencies used, the time duration of the pulse, and the relative
phase relation between the two components of light, the induced pulse
of electric flow could consist of as little as a single electron or
many. For the case of one electron set in motion by the
400-femtosecond pulse of laser light the resulting electrical *current*
would be about 0.4 microamps. Why use light rather than voltage to
drive electricity? Because the whole thing can be done on a
femtosecond scale with lasers. Ignacio Franco
(ifranco@chem.utoronto.ca, 416-978-4422) says that a potential use of
laser-driven electricity would be in future optoelectronic devices such
as ultrafast nanoswitches. (Franco, Shapiro and Brumer, Physical
Review Letters, upcoming article)
OBSERVING MAGNETIC POLARIZATION IN SINGLE ATOMS. Physicists from UC
Berkeley and the Naval Research Lab have measured the spin properties
of individual atoms added to a metal surface. They do this by first
forming nm-sized triangular islands of cobalt on top of a copper
crystal. The cobalt is ferromagnetic, which means that the spins of
the cobalt atoms in the islands all line up together (half of the
islands have their collective spins pointing up, while the other half
point down). Additional magnetic atoms sprinkled on top of the islands
(adatoms) have spins that interact magnetically with the underlying
cobalt, causing the adatom spins to either align or anti-align with the
underlying island spins. Thus when a small amount of iron atoms
(chromium atoms were also used) are dropped onto the islands they
immediately become oriented (polarized) by contact with a cobalt
island. In this way isolated atoms (up to 5 nm apart) were prepared
with a definite spin polarization state (see figure at
http://www.aip.org/png/2007/285.htm). Next the quantum energy levels
of the magnetic adatoms were studied using the tip of a scanning
tunneling microscope (STM) which itself had been magnetized. The
quantum energy levels of the iron and chromium adatoms were sampled by
observing currents flowing from the adatoms into the STM tip. Current
measured in this way will be larger or smaller depending on whether the
spin polarization of the tip is aligned with or against thepolarization
of the individual magnetic adatoms being probed. The adatom energy
states are seen to differ for spin-up and spin-down states, indicating
that iron and chromium atoms couple magnetically to cobalt with
opposite polarity. One of the researchers, Michael Crommie of UCB
(crommie@berkeley.edu, 510-642-9392), says that it is still too early
to try to store data in the form of individual polarized atoms. Rather
they are seeking to understand how the spin of a single atom is
influenced by its environment, with an eye toward future spintronics
and quantum information applications. (Yayon et al., Physical Review
Letters, 10 August 2007; lab website,
http://physics.berkeley.edu/research/crommie/)
BEN STEIN, co-editor of Physics News Update almost from its inception
17 years ago, has left AIP to take up an important public-information
job at NIST-Gaithersburg, where his email address is bstein@nist.gov.
During his AIP years, Ben was an important figure in operating the
pressroom at large APS meetings, in establishing innovative outreach
techniques such as *virtual pressrooms* (e.g.,
http://www.acoustics.org/press/index.html#t01), and in publicizing the
research results of AIP*s member societies in general.
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.
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