Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

UC San Diego Physicists Observe New Property of Matter

03.11.2006
Physicists at UC San Diego have for the first time observed the spontaneous production of coherence within “excitons,” the bound pairs of electrons and holes that enable semiconductors to function as novel electronic devices.

Scientists working in the emerging field of nanotechnology, which is finding commercial applications for ultra-small material objects, believe that this newly discovered property could eventually help the development of novel computing devices and provide them with new insights into the quirky quantum properties of matter.

Details of the new finding appear in a paper published in the November 3 issue of the journal Physical Review Letters by a team of four physicists at UCSD working in collaboration with a materials scientist at UC Santa Barbara.

The effort was headed by Leonid Butov, a professor of physics at UCSD who in 2002 led a similar team at the Lawrence Berkeley National Laboratory to the discovery that excitons, when made sufficiently cold, tend to self-organize into an ordered array of microscopic droplets, like a miniature pearl necklace (shown in figure).

“What is coherence and why is it so important?” said Butov. “To start with, modern physics was born by the discovery that all particles in nature are also waves. Coherence means that such waves are all ‘in sync.’ The spontaneous coherence of the matter waves is the reason behind some of the most exciting phenomena in nature such as superconductivity and lasing.”

“A simple way to visualize coherence is to imagine cheering spectators at a stadium making ‘a wave’,” added Michael Fogler, an assistant professor of physics at UCSD and a co-author of the paper. “If the top rows get up and down at the same time as the bottom ones, the rows are mutually coherent. In turn, coherence is spontaneous when the cheering is done on the spectator’s own initiative and is not orchestrated by the directions of an external announcer.”

A famous example of spontaneous coherence of matter waves is the Bose-Einstein condensate, which is a state predicted by Einstein some 80 years ago. This new form of matter was eventually created in 1995 by University of Colorado physicists and regarded as so noteworthy the scientists were awarded the 2001 Nobel Prize in Physics. The Bose-Einstein condensate is a gas of atoms so dense and cold that their matter waves lose their individuality and condense into a “macroscopic coherent superatom wave.”

Atomic Bose-Einstein condensation occurs at temperatures near absolute zero. However, excitons are expected to exhibit the same phenomenon at temperatures that are million times higher (although admittedly still rather low on a common scale, some hundred times lower than the room temperature). Remarkably, this is a range of temperatures where Butov and his team have observed the onset of exciton coherence.

“Excitons are particles that can be created in semiconductors, in our case, gallium arsenide, the material used to make transistors in cell phones,” said Fogler. “One can make excitons, or excite them, by shining light on a semiconductor. The light kicks electrons out of the atomic orbitals they normally occupy inside of the material. And this creates a negatively charged ‘free’ electron and a positively charged ‘hole.’”

The force of electric attraction keeps these two objects close together, like an electron and proton in a hydrogen atom. It also enables the exciton to exist as a single particle rather than a non-interacting electron and hole. However, it can be the cause of the excitons’ demise. Since the electron and hole remain in close proximity, they sometimes annihilate one another in a flash of light, similar to annihilation of matter and antimatter.

To suppress this annihilation, Butov and his team separate electrons and their holes in different nano-sized structures called quantum wells.

“Excitons in such nano-structures can live a thousand or even a million times longer than in a regular bulk semiconductor,” said Butov. “These long-lived excitons can be prepared in large numbers and form a high density exciton gas. But whether excitons can cool down to low temperatures before they recombine and disappear has been a key question for scientists.”

“What we found was the emergence of spontaneous coherence in an exciton gas,” added Butov. “This is evidenced by the behavior of the coherence length we were able to extract from the light pattern (as shown in the figure) emitted by excitons as they recombine. Below the temperature of about five degrees Kelvin above absolute zero, the coherence length becomes clearly resolved and displays a steady and rapid growth as temperature decreases. This occurs in concert with the formation of the beads of the ‘pearl necklace.’ The coherence length reaches about two microns at the coldest point available in the experiment.”

Other members of the research team were UCSD students Sen Yang and Aaron Hammack and Arthur Gossard, a professor in UC Santa Barbara’s materials science department. The research project was supported by grants from the National Science Foundation, U.S. Army Research Office and the Hellman Fund.

Kim McDonald | EurekAlert!
Further information:
http://www.ucsd.edu
http://ucsdnews.ucsd.edu/newsrel/science/exciton.asp

More articles from Physics and Astronomy:

nachricht Hope to discover sure signs of life on Mars? New research says look for the element vanadium
22.09.2017 | University of Kansas

nachricht Calculating quietness
22.09.2017 | Forschungszentrum MATHEON ECMath

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: The pyrenoid is a carbon-fixing liquid droplet

Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.

A warming planet

Im Focus: Highly precise wiring in the Cerebral Cortex

Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.

The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...

Im Focus: Tiny lasers from a gallery of whispers

New technique promises tunable laser devices

Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...

Im Focus: Ultrafast snapshots of relaxing electrons in solids

Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!

When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...

Im Focus: Quantum Sensors Decipher Magnetic Ordering in a New Semiconducting Material

For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.

Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

“Lasers in Composites Symposium” in Aachen – from Science to Application

19.09.2017 | Event News

I-ESA 2018 – Call for Papers

12.09.2017 | Event News

EMBO at Basel Life, a new conference on current and emerging life science research

06.09.2017 | Event News

 
Latest News

Rainbow colors reveal cell history: Uncovering β-cell heterogeneity

22.09.2017 | Life Sciences

Penn first in world to treat patient with new radiation technology

22.09.2017 | Medical Engineering

Calculating quietness

22.09.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>