Posted on 05/04/2008 9:17:11 PM PDT by neverdem
These images collected by Princeton University scientists show (top) the first direct image of the dancing pattern of electrons on the edge of the bismuth-antimony bulk crystal, which is a quantum Hall insulator; (center) a schematic and another image showing the electron distribution in three dimensions; and (bottom) a schematic and an image conveying the distribution of edge-electrons in two dimensions. Images: Zahid Hasan
A team of scientists from Princeton University has found that one of the most intriguing phenomena in condensed-matter physics -- known as the quantum Hall effect -- can occur in nature in a way that no one has ever before seen.
Writing in the April 24 issue of Nature, the scientists report that they have recorded this exotic behavior of electrons in a bulk crystal of bismuth-antimony without any external magnetic field being present. The work, while significant in a fundamental way, could also lead to advances in new kinds of fast quantum or "spintronic" computing devices, of potential use in future electronic technologies, the authors said.
"We had the right tool and the right set of ideas," said Zahid Hasan, an assistant professor of physics who led the research and propelled X-ray photons at the surface of the crystal to find the effect. The team used a high-energy, accelerator-based technique called "synchrotron photo-electron spectroscopy."
And, Hasan added, "We had the right material."
The quantum Hall effect has only been seen previously in atomically thin layers of semiconductors in the presence of a very high applied magnetic field. In exploring new realms and subjecting materials to extreme conditions, the scientists are seeking to enrich the basis for understanding how electrons move.
Robert Cava, the Russell Wellman Moore Professor of Chemistry and a co-author on the paper, worked with members of his team to produce the crystal in his lab over many months of trial-and-error. "This is one of those wonderful examples in science of an intense, extended collaboration between scientists in different fields," said Cava, also chair of the Department of Chemistry.
"This remarkable experiment is a major home run for the Princeton team," said Phuan Ong, a Princeton professor of physics who was not involved in the research. Ong, who also serves as assistant director of the Princeton Center for Complex Materials, added that the experiment "will spark a worldwide scramble to understand the new states and a major program to manipulate them for new electronic applications."
Electrons, which are electrically charged particles, behave in a magnetic field, as some scientists have put it, like a cloud of mosquitoes in a crosswind. In a material that conducts electricity, like copper, the magnetic "wind" pushes the electrons to the edges. An electrical voltage rises in the direction of this wind -- at right angles to the direction of the current flow. Edwin Hall discovered this unexpected phenomenon, which came to be known as the Hall effect, in 1879. The Hall effect has become a standard tool for assessing charge in electrical materials in physics labs worldwide.
In 1980, the German physicist Klaus von Klitzing studied the Hall effect with new tools. He enclosed the electrons in an atom-thin layer, and cooled them to near absolute zero in very powerful magnetic fields. With the electrons forced to move in a plane, the Hall effect, he found, changed in discrete steps, meaning that the voltage increased in chunks, rather than increasing bit by bit as it was expected to. Electrons, he found, act unpredictably when grouped together. His work won him the Nobel Prize in physics in 1985.
Daniel Tsui (now at Princeton) and Horst Stormer of Bell Laboratories did similar experiments, shortly after von Klitzing's. They used extremely pure semiconductor layers cooled to near absolute zero and subjected the material to the world's strongest magnet. In 1982, they suddenly saw something new. The electrons in the atom-thin layer seemed to "cooperate" and work together to form what scientists call a "quantum fluid," an extremely rare situation where electrons act identically, in lock-step, more like soup than as individually spinning units.
After a year of thinking, Robert Laughlin, now at Stanford University, devised a model that resembled a storm at sea in which the force of the magnetic wind and the electrons of this "quantum fluid" created new phenomena -- eddies and waves -- without being changed themselves. Simply put, he showed that the electrons in a powerful magnetic field condensed to form this quantum fluid related to the quantum fluids that occur in superconductivity and in liquid helium.
For their efforts, Tsui, Stormer and Laughlin won the Nobel Prize in physics in 1998.
Recently, theorist Charles Kane and his team at the University of Pennsylvania, building upon a model proposed by Duncan Haldane of Princeton, predicted that electrons should be able to form a Hall-like quantum fluid even in the absence of an externally applied magnetic field, in special materials where certain conditions of the electron orbit and the spinning direction are met. The electrons in these special materials are expected to generate their own internal magnetic field when they are traveling near the speed of light and are subject to the laws of relativity.
In search of that exotic electron behavior, Hasan's team decided to go beyond the conventional tools for measuring quantum Hall effects. They took the bulk three-dimensional crystal of bismuth-antimony, zapped it with ultra-fast X-ray photons and watched as the electrons jumped out. By fine-tuning the X-rays, they could directly take pictures of the dancing patterns of the electrons on the edges of the sample. The nature of the quantum Hall behavior in the bulk of the material was then identified by analyzing the unique dancing patterns observed on the surface of the material in their experiments.
Kane, the Penn theorist, views the Princeton work as extremely significant. "This experiment opens the door to a wide range of further studies," he said.
The images observed by the Princeton group provide the first direct evidence for quantum Hall-like behavior without external magnetic fields.
"What is exciting about this new method of looking at the quantum Hall-like behavior is that one can directly image the electrons on the edges of the sample, which was never done before," said Hasan. "This very direct look opens up a wide range of future possibilities for fundamental research opportunities into the quantum Hall behavior of matter."
uhhh, ummmm, ok. Seriously, what are some future applications or implications of this discovery? Seems to me like the crystal development aspects are the bigger story......
Great name. I wish my name was Horst Stormer.
Well, I'm just an idiot when it comes to things like this - but I think this sentence says something important. I'm guessing that whenever something is orderly rather than random is a good thing - especially when it comes to electrons.
How about thin, ultra-efficient batteries or something?
bump
Your username is a Rush Album?
Soup marches in lock-step? Wow! Well, there was that soup Nazi guy.
That was Greek to me.
I’d like it in layman’s terms.
Yeah. I tried 2112 but that didn’t work. Probably just as well as that is a little too obvious and not much fun.
Now I’m listening to Passage to Bangkok.
A quantum leap forward
In time and in space
The universe learned to expand
The mess and the magic
Triumphant and tragic
A mechanized world out of hand
RUSH: Natural Science - II. Hyperspace, verse 1
Fitting lyrics for a geeky thread with some Rush fans on it!
Nice
:)
This is standard heuristics to talk about certain excited quantum states of solid matter. Conduction electrons in metal are such an excited state, but the single electron approximation works well, so the heuristics for that are just in terms of electrons whizzing around. When there are strong enough interactions between the crystal lattice ( say ) and the electrons, then the excited quantum states involve motion not just of one electron, but two or more electrons and motion of the lattice as well. Superconductivity is the most famous example.
This all follows standard quantum theory, but the theory is a lot more difficult to work out, and the experimental conditions often involve low temperatures to allow these interactions to come into play.
It's all very interesting, but nothing profoundly new. My favorite of these is the Mossbauer effect, or "zero phonon line". In this case the cooperation is total, and the theory is simple. A nucleus in a crystal lattice absorbs or emits a particle, and the entire lattice absorbs the recoil momentum. It doesn't sound so exciting, but you have to realize this is complete quantum weirdness. Classically, if you ping one element of the lattice, you excite a pulse composed of a variety of plane waves, which then propagates and dissipates. In QM, this pulse is probabilistically separated into components, so the same thing doesn't happen every time, and in fact you get this zero phonon line as one component of the spectrum. The effect was used in some of the Mars Sojourner instrumentation to examine the surface of martian rocks.
I read your entire post and found it 100% very interesting, while understanding very little! (That’s why it is so interesting/amazing). Especially interesting with respect to the Mar’s rocks.
I always liked the Moody Blues, In Search of the Lost Chord:
This garden universe vibrates complete.
Some we get a sound so sweet.
Vibrations reach on up to become light,
And then thru gamma, out of sight.
Between the eyes and ears there lay,
The sounds of colour and the light of a sigh.
And to hear the sun, what a thing to believe.
But it’s all around if we could but perceive.
To know ultra-violet, infra-red and X-rays,
Beauty to find in so many ways.
Two notes of the chord, that’s our fluoroscope.
But to reach the chord is our lifes hope.
And to name the chord is important to some.
So they give a word, and the word is OM.
( That track is The Word, with credit to Graeme Edge )
About the only example of simple science appreciation in rock music that I can think of. ... and something I never thought of until this moment, one small stroke converts OM to QM !
The rain is on the roof
Hurry high, butterfly
... Oh yeah! Oh yeah!
Yup...just another late night discussion on FR!
I like Klaus von Klitzing.
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