ZPEnergy.com - LIGHT-DRIVEN FEMTOSECOND ELECTRICITY; MAGNETIC POLARIZATION IN SINGLE ATOMS

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)
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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/)

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 836 August 21, 2007 by Phillip F. Schewe