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.
“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!
Astronomers find unexpected, dust-obscured star formation in distant galaxy
24.03.2017 | University of Massachusetts at Amherst
Gravitational wave kicks monster black hole out of galactic core
24.03.2017 | NASA/Goddard Space Flight Center
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to simulate these confined natural conditions in artificial vesicles for the first time. As reported in the academic journal Small, the results are offering better insight into the development of nanoreactors and artificial organelles.
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
24.03.2017 | Materials Sciences
24.03.2017 | Physics and Astronomy
24.03.2017 | Physics and Astronomy