Posted: September 28, 2007

Researchers make electron spin useful

(Nanowerk News) In the global race for possibilities for the use of electron spin, an international research team under the participation of Prof. Dr. Andreas Wieck (RUB - Chair for Solid State Physics) is right at the forefront: The researchers have managed to maintain the polarisation of nuclear spin and its associated electron spin for some ten minutes – a virtually endless amount of time for typical computer calculation steps. They polarised the electron spin by means of laser impulse, whereupon the decisive, uniform magnetic field of the electrons transferred itself to the surrounding atomic nuclei. These then function in this manner as memory for the spin polarisation, which the electrons alone normally ‘forget’ almost immediately. The researchers are report on their findings in the September 28th, 2007 issue of the scientific magazine Science.
Normally Chaotic
Along with their electrical charge, electrons transport their own rotation, know as ‘spin’, practically ‘for free’ as an additional property. “Each individual electron possesses both charge and spin”, explains Prof. Wieck. “In comparison, an ice skater has grace and rotation, for example – properties which do not necessarily have to have a direct connection with one another.” The spin creates a small magnetic field. Both this and the electrical charge could be used for information technology to the extent that the magnetic fields of electrons in a semiconductor, which are normally directed chaotically in all directions, could be aligned in the same direction and this state could be maintained long enough to enable the possibility of IT processes. Currently, spin is not in use at all in semiconductor electronics.
Tiny Magnetic Fields
The newly developing “spintronics” is to change all this by means of “spin polarisation”. And the Ruhr University in Bochum is at the forefront of the worldwide race for the most application-compatible spintronic components possible. Prof. Wieck’s workgroup has for years been producing the best “quantum dots”, in which spin phenomena are particularly suitable for study and being put into application. “These quantum dots are shaped something like the top part of a hamburger bun, but they are three million times smaller, and some five billion times five billion quantum dots would fit into an actual hamburger bun top”, explains Wieck in describing the tiny dimensions in which the researchers are experimenting.
Atomic Nuclei Are The Memory
The interesting thing about quantum dots for the researchers is that each one can be occupied by precisely one electron. Its spin is aligned in cooperative work with Dortmund-based physicists headed by Prof. Manfred Bayer by a laser pulse, and is then transferred to the surrounding nuclei in the quantum dot. These nuclei are then “spin polarised” and maintain their polarisation for some ten minutes, in contrast to the ‘more forgetful’ electrons. This is more than enough time to calmly conduct operations such as calculating steps or other normal computer programs. Afterwards, the nucleus polarisation can easily be read out by a second laser beam, or an additional pulse can be used to overwrite it or delete it as needed. “This process is somewhat similar to nuclear MRI, in which the spin of the nuclei in somatic cells are equally aligned by a strong magnetic field, only here it is nanoscopic quantum dots and not the human body that are being measured“, according to Wieck. “Their large quantity and packing density represents a significant step forward for the processing of information.”
Source: Ruhr-University Bochum