NewsAlert: Physicists Gain Insights Into New State of Matter
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March 29, 2012

NewsAlert: Physicists Gain Insights Into New State of Matter

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Physicists at the University of California, San Diego discovered the phenomena that underlie the formation of spontaneous coherence of excitons is certain to produce a better scientific understanding of this new state of matter. It will also add new insights into the quirky quantum properties of matter and, in time, lead to the development of novel computing devices and other applications in the field of optoelectronics where understanding of basic properties of light and matter is needed.

In a paper published in the March 29 issue of the journal Nature, the scientists describe the emergence of “spontaneous coherence,” “spin textures” and “phase singularities” when excitons—the bound pairs of electrons and holes that determine the optical properties of semiconductors and enable them to function as novel optoelectronic devices—are cooled to near absolute zero. This cooling leads to the spontaneous production of a new coherent state of matter which the physicists were finally able to measure in great detail in their basement laboratory at UC San Diego at a temperature of only one-tenth of a degree above absolute zero.

The research team was headed by Leonid Butov, a professor of physics at UC San Diego who in 2002 discovered that excitons, when made sufficiently cold, tend to self-organize into an ordered array of microscopic droplets, like a miniature pearl necklace.

Using a state of the art refrigeration system, the UC San Diego physicists were able to achieve temperatures ten times colder than that earlier effort, enabling them with an instrument called an interferometer to measure coherence and spin of each pearl or bead within this necklace.

What they discovered was that the exciton particles’ spin is not homogenous in space, but forms patterns around these beads, which they call “spin textures.” They also discovered that a pattern of spontaneous coherence is correlated with a pattern of spin polarization and with phase singularities in the coherent exciton gas.The polarization measurements showed that there was a strong correlation between the coherence and polarization.”

“We are working both on understanding the basic properties of excitons and on the development of excitonic signal processing,” said Butov. “The physics of excitons is interesting by itself. Furthermore, understanding the basic properties of excitons is needed to build excitonic devices in the future.”

The physicists created the excitons by shining a laser on cooled samples of gallium arsenide, the same semiconducting material used to make transistors in cell phones.

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, 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 control this annihilation, Butov and his team separate electrons and their holes in different nano-sized structures called quantum wells. This allows creation of excitons with the required lifetime, about 50 nanoseconds in the experiment.

“During that time, they cool down, form condensates and demonstrate interesting spin physics,” said High. “Eventually the electron and hole recombine and the light comes back out.”

In their experiments, the physicists sent that emission through a complex set of mirrors called an interferometer, which divides the light into two different paths. This allowed them to compare two spatially separated regions of the same sample, enabling them to see the fine details of spontaneous coherence in excitons that had never been seen before.

“Previous experiments required fiber optics to do any sort of optical measurements in a dilution refrigerator,” said High. “But with this equipment, we can actually take pictures of the excitons at extremely low temperatures.’

“This is a very interesting discovery,” he added. “There’s very rich physics involved.”

The Daily Galixy via University of San Diego 

The image at the top of the page shows Photoluminescence for indirect excitons, exhibiting macroscopic spacial ordering. Image credit courtesey of Prof. Leonid Butov, USC San Diago

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Comments

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

Could you, please, explain that part?
As i get it, a hole, like a hole in a semiconductor, is just an absence of electron, a free parking level if you please. It's not an any sort of particle. As a result, it is just impossible for it to annihilate with an electron, it's a positron's job.
So the process reminds me of a laser itself - electrons being provided with a piece of energy, making it to leave his, emm, low orbit and move to a higher one, consuming a photon. As a result, when an electron is switching back to a low one, he's emitting that extra energy with a photon, which is described as "annihilating with a hole in a flash of light".

Very interesting . exciton particles’ spin is not homogenous in space

Sometimes reading these articles make me feel stupid

"Sometimes reading these articles make me feel stupid"

know the feeling, really struggle to get my head around this stuff sometimes.

but hey, only human, averag iq,nothing wrong with it.
at least attempting to understand is laudable and respected :)

+this place doesnt have many people with ego issues so it's all good.

R-i-g-h-t........that's the same thing i was thinking.

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