Physicists find patterns in new state of matter

Mar 29, 2012
Physicists find patterns in new state of matter
Excitons self organize into an ordered array that looks like a miniature pearl necklace.

(PhysOrg.com) -- Physicists at the University of California, San Diego have discovered patterns which underlie the properties of a new state of matter.

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 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 discovery of 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 commercial applications in the field of optoelectronics where understanding of basic properties of light and matter is needed.

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.

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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.

“It was a surprise to see this pattern,” said Alex High, a graduate student and the first author of the paper. “And it was even more surprising that 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.”

Physicists find patterns in new state of matter
The physicists discovered a correlation between the coherence (at left) and spin polarization (at right) of the excitons.

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 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.”

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Graeme
4.5 / 5 (2) Mar 29, 2012
As absolute zero is approached there should be more and more sorts of unfamiliar effects that only show up at lower energies or weaker forces. This is one example of this.
DarkHorse66
3 / 5 (4) Mar 29, 2012
"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."

My knowledge of physics phenomena doesn't stretch to the concept of a particle and a hole annihilating each other. I would have thought that an electron would have sought to FILL that hole. As for matter and antimatter, they are both forms of matter. A hole is not. (So, how does such an analogy make sense?) Would somebody be willing to have a go at explaining how this kind of thing might be feasible (conceptually)?

NO AETHER, OR AETHER-LIKE EXPLANATIONS, PLEASE!!!

Thanks in advance, DH66.
Llewellian
5 / 5 (3) Mar 29, 2012
@ DarkHorse66: As many Electrons orbit an Atom, until the positive charge is neutralized with the negative charge of the electrons.

en.wikipedia.org/wiki/Atomic_orbital

If now one of the electrons is missing, you could say that this is a hole. Electrons orbiting a nucleus have a lower energy state than free electrons. If now a free electron falls into that "hole" (aka Potential well), the energy difference is released as a photon. That is why you could describe holes like a particle. And about "Holes" wandering through material: Think of the whole thing a little bit like the "Musical Chairs" Game (en.wikipedia.org/wiki/Musical_chairs).
Callippo
1 / 5 (10) Mar 29, 2012
The spin excitons are essentially a tiny magnets and the formation of linear textures is just what every magnetized particles do. http://www.wired..../snakes/

I do consider it interesting - but really surprising? Nope, until you're not a physical idiot.
Callippo
1 / 5 (17) Mar 29, 2012
.. As for matter and antimatter, they are both forms of matter. A hole is not. So, how does such an analogy make sense? No Aether, or Aether-like explanations, please...
In dense aether model of vacuum the antiparticles are bubbles in vacuum particle material in the same way, like the semiconductor holes are bubbles in electron sea. Sorry, I didn't invented this model - Mr. Dirac did... Are you feel smarter than him? I seriously doubt it...
rowbyme
4.4 / 5 (13) Mar 29, 2012
Stop it Callippo! Stop polluting this site with your idiotic comments.
And you might try learning English one of these days...
DarkHorse66
2 / 5 (3) Mar 29, 2012
@Llewellian: I appreciate your willingness to answer seriously. I do understand the concept of electrons filling 'spaces'/holes and losing energy when they fill them in a lower orbit. My understanding breaks down where the above text describes ANNIHILATION of hole AND particle taking place. This would appear to describe a physical DESTRUCTION. An object=electron being destroyed. No problem there. But the only way to destroy a 'hole'=space, is to fill it. Yet if the electron is destroyed, it hasn't filled it. This doesn't appear to be describing the same process. It is this distinction that I am having problems with. How would you differentiate between 'filling, a hole with an intact electron, and making them both disappear by mutual destruction? Thanks, DH66 :)
DarkHorse66
4.1 / 5 (9) Mar 29, 2012
@Callippo: Are you feeling intimidated?? I'm not the one making the assertion that I am smarter than anyone. On the contrary. I am deliberately airing my ignorance so that I might actually learn something & am not ashamed of it. But I guess that that kind of a concept is a sheer novelty for you. You don't even seem to respect anybody or what they have to contribute on this site, except your own views. You have repeatedly demonstrated that you are too fixated and closed-minded to adapt an explanation for the sake of genuinely clarifying something without bringing aether into it, let alone for any real communication. Besides, I did specify that I didn't wish for aether to be included. You couldn't stop yourself. Which bit didn't you get???DH66
Llewellian
5 / 5 (3) Mar 29, 2012
Hi DarkHorse66, in a way you could see it as a kind of "destruction":

The free electron and the hole "anhillate" themselves, as neither the hole nor the "free" electron exist anymore. Only a hull electron remains. Which is still an electron, but now behaving in another way than a "free" electron. You could - sort of - say, that that is another particle.
Callippo
1 / 5 (12) Mar 29, 2012
And you might try learning English one of these days
My apology goes especially for the using of "Are you feel smarter" question, which I had no time to correct.
I did specify that I didn't wish for aether to be included
You should wish such things at the discussion bellow private blog, not at public forum - don't you think? Anyway, as you can see, it's difficult to invent working analogy of these things without having aether model on mind. At any case, many people before you already did it in most straightforward way possible.
Callippo
1 / 5 (12) Mar 29, 2012
..you have repeatedly demonstrated that you are too fixated and closed-minded to adapt an explanation for the sake of genuinely clarifying something without bringing aether into it, let alone for any real communication..
This is your subjective interpretation of the fact, I'm just trying to use the simplest and least ad-hoced explanations possible. I indeed know about mainstream formal theories in the same way, like you (if not better) - but why I should use them, if they're counterintuitive and unnecessary complex? You're forcing the readers into religious thinking in such way. And this forum is not social club: you're not supposed to deal with subjective aspects of personality of both yours, both other commenters.
TrinityComplex
5 / 5 (6) Mar 29, 2012
From Llewellian
You could - sort of - say, that that is another particle.
This is probably the closest explanation, as they say 'It also enables the exciton to exist as a single particle rather than a non-interacting electron and hole.', so the exciton consists of the free electron, and the hole, in such close proximity to each other that they act as a single entity. "Annihilating" each other means that specific state that qualifies it as an exciton no longer exists. Fill a hole with extra dirt and it's no longer a hole, and the dirt isn't 'extra' dirt, I suppose. Similar to higher dimensional math, microphysics that cause particles to act in a way we've never perceived before, and can't perceive 'en-macro' is really tough to describe.
Callippo
1 / 5 (10) Mar 29, 2012
In dense aether model the particles are bubbles filled stuffed with longitudinal waves, whereas the surface is formed with transverse waves. The antiparticles are thingies arranged in opposite way.

The surface and longitudinal waves do resonate mutually like inside of every soliton. The important aspect of anti-particles is the opposite chirality of these surface vibrations. Therefore their vorticity compensates mutually during contact like during collision of vortex rings.
Callippo
1 / 5 (7) Mar 29, 2012
Inside of superconductor the holes would play an analogy of normal particles rather than antiparticles: they're heavier and of lower mobility. Whereas the antiparticles would correspond to electrons itself. Analogously, mens are human analogy of bosons and antiparticles, women play the role of particles and fermions: they're heavier, more social and less mobile. The solitons at the water surface do play another analogy of matter and antimatter: the normal particles are Russel's surface solitons, whereas the antiparticles do correspond the Falaco solitons. Note that the Falaco solitons bump the water surface in the opposite way, than the Russel's solitons: they're of negative gravitational charge.
Urgelt
not rated yet Mar 29, 2012
I'm having difficulty grasping the idea of a positively-charged "hole." Doesn't the positive charge come from protons in the nucleus of the atom?

Take the nucleus out of the picture, and you don't have a "hole" into which an electron can fit, do you?

I'm not sure if the terminology used in this article is just strangely vague, or if there's an actual physics principle I'm missing.
Eikka
5 / 5 (1) Mar 30, 2012
Take the nucleus out of the picture, and you don't have a "hole" into which an electron can fit, do you?


Correct. The hole exists as a vacant spot in an atom's electron shell, so the only way to have holes is to have atoms. But the hole can travel around like a bubble in water, hopping into other orbitals or other atoms entirely, so it is abstracted into a particle to make things conceptually simpler.

Makes things like semiconductors easier to understand when you don't have to mind the atoms.
Callippo
1 / 5 (3) Mar 30, 2012
Makes things like semiconductors easier to understand when you don't have to mind the atoms.
The "wet sponge" model of semiconductor junction. I find somewhat surprising, nobody did come with it already.
Urgelt
not rated yet Mar 31, 2012
Thanks, Eikka. I was defeated by terminology there for a bit.
DarkHorse66
1 / 5 (1) Apr 02, 2012
Hi, Llewellian, TrinityComplex. Just got back to a computer at uni after the weekend (don't actually have one at home). Your answers (each in slightly different wording, but essentially saying the same thing)complemented each other beautifully. I think that I get it now. It refers more to a transformation of state, rather than 'simple' destruction; based on the concept that when you transform/change something, you automatically DESTROY/DELETE the original state. (I REALLY wish that this editor allowed italics!)This is regardless of whether you can take a separate action to reverse it again. Thanks guys, DH66
DarkHorse66
1 / 5 (1) Apr 02, 2012
Additional note to Llewellian: You gave an address to a site in your first post, but didn't hyperlink it. Usual practice (for the sake of accommodation) is to do so. The quickest way to do that is to find, then open the site that you are posting as your reference, then click on the address in the 'address bar', click 'copy', then 'paste' in comment box. The editor will do the rest, when it uploads. Oh, and in case you lose what you have already typed, save it by 'select all', then 'copy' with right-clicking your mouse. If it does happen, you need only 'paste'. This editor can be quite fickle that way and it can happen. Welcome to the crew :) Best Regards, DH66
DarkHorse66
2.3 / 5 (3) Apr 02, 2012
My apology goes especially for the using of "Are you feel smarter" question, which I had no time to correct.

Apology accepted - as long as it does not become a habit.
I did specify that I didn't wish for aether to be included
You should wish such things at the discussion bellow private blog, not at public forum - don't you think?

That is one point on which I must disagree.A public forum is precisely the place to discuss Standard Physics - without having to have the expectation that someone will insist on bringing in deviations from the 'standard' into play, especially on something this basic.One must first learn the Standard Basics, before being in a position to interpret non-standard stuff that gets extrapolated out of it. To do it the other way around, would be a case of putting the cart before the horse, or in your case, the donkey.I would just LOVE to see my professors' faces, if I even tried to bring aether theory into a classroom. I'd be a laughing stock.
DarkHorse66
2 / 5 (3) Apr 02, 2012
"This is your subjective interpretation of the fact, I'm just trying to use the simplest and least ad-hoced explanations possible. I indeed know about mainstream formal theories in the same way, like you (if not better) - but why I should use them, if they're counterintuitive and unnecessary complex? You're forcing the readers into religious thinking in such way. And this forum is not social club: you're not supposed to deal with subjective aspects of personality of both yours, both other commenters."
DarkHorse66
1 / 5 (1) Apr 02, 2012
Bloody Editor! It's got a mind of its own!
DarkHorse66
2.8 / 5 (4) Apr 02, 2012
I did specify that I didn't wish for aether to be included
You should wish such things at the discussion bellow private blog, not at public forum - don't you think? Anyway, as you can see, it's difficult to invent working analogy of these things without having aether model on mind. At any case, many people before you already did it in most straightforward way possible. That is one point on which I must disagree. This is a forum aimed at discussing physics from a STANDARD point of view. We have a right to be able to discuss it, WITHOUT the expectation that someone will insist on bringing deviant theories into it -and insisting that they be treated as if they were accepted by the mainstream. THAT is the kind of thing that belongs on a private blog!!! My question and the kind of answer that I was seeking was in accordance with the physorg guidelines. Yours was not. Your comment indicates a major ignorance of the rules of this site. So please, don't chastise ME for...cont
DarkHorse66
3 / 5 (4) Apr 02, 2012
cont...for doing precisely what the these codes of conduct/rules proscribe. Check your own behaviours first. You've been told often enough. As for subjective aspects of personality; since when is aether in any way a personality trait!!! As for the rest of it: Hey pot, this is kettle!! (Reference to the pot calling the kettle black.)You keep imposing your convictions on others. Every one has told you that what you are doing is unacceptable. We all agree on that point. You can't stick to your own advice! You think that this is a social club? You certainly make the mistake of treating this place as your personal soap box!
I do agree with you on one point though, the others were clear, concise, to the point -and they didn't need to invoke aether. It seems that they (nor I) didn't find the explanations either counter-intuitive, ad hoc OR complex, especially at such a basic level. You seem to be the only one. That shows lack of flexibility. Pity. DH66
Higgums
not rated yet Apr 03, 2012
A couple of straight forward questions regarding the statisics... Do excitons obey boson or fermion statistics? Also, is there an observed quantum entanglement of these exciton states? Thanks JC