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 Applicability of dynamic facilitation theory to binary hard disk systems
08.12.2016 | Nagoya Institute of Technology

nachricht Will Earth still exist 5 billion years from now?
08.12.2016 | KU Leuven

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: Significantly more productivity in USP lasers

In recent years, lasers with ultrashort pulses (USP) down to the femtosecond range have become established on an industrial scale. They could advance some applications with the much-lauded “cold ablation” – if that meant they would then achieve more throughput. A new generation of process engineering that will address this issue in particular will be discussed at the “4th UKP Workshop – Ultrafast Laser Technology” in April 2017.

Even back in the 1990s, scientists were comparing materials processing with nanosecond, picosecond and femtosesecond pulses. The result was surprising:...

Im Focus: Shape matters when light meets atom

Mapping the interaction of a single atom with a single photon may inform design of quantum devices

Have you ever wondered how you see the world? Vision is about photons of light, which are packets of energy, interacting with the atoms or molecules in what...

Im Focus: Novel silicon etching technique crafts 3-D gradient refractive index micro-optics

A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.

Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...

Im Focus: Quantum Particles Form Droplets

In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.

“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...

Im Focus: MADMAX: Max Planck Institute for Physics takes up axion research

The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.

The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ICTM Conference 2017: Production technology for turbomachine manufacturing of the future

16.11.2016 | Event News

Innovation Day Laser Technology – Laser Additive Manufacturing

01.11.2016 | Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

 
Latest News

Closing the carbon loop

08.12.2016 | Life Sciences

Applicability of dynamic facilitation theory to binary hard disk systems

08.12.2016 | Physics and Astronomy

Scientists track chemical and structural evolution of catalytic nanoparticles in 3-D

08.12.2016 | Materials Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>