Scientists have designed a simple system to study how electrons travel through energy barriers instead of over them.
This unusual behavior, called tunneling, is the particle equivalent of a person being able to walk through rather than over a mountain. The particle behavior is also one of the most common signs that everyday physics has broken down and quantum mechanics now controls the system.
Duke physicist Gleb Finkelstein and his collaborators were manipulating the environment of tunneling electrons' using electrical leads and carbon nanotubes when they unexpectedly discovered they could create a quantum phase transition -- an abrupt change in the quantum state of the system. The team reports its findings Aug. 2 in the journal Nature.
"There are very few examples where you can see quantum phase transitions in a direct, controllable way. That is what is exciting here," said Duke theoretical physicist Harold Baranger, a co-author on the paper.
Quantum phases are similar to phases of matter in the everyday world, such as ice being a phase of water. But quantum phases occur at or near zero temperature, -459.67 degrees Fahrenheit, and usually happen when large groups of electrons and other particles change their characteristics collectively. Quantum phase transitions are ubiquitous in modern physics, but are hard to study. One of the most recognizable examples is found in superconducting materials, where electrons overcome their negative repulsion of each other and flow with little resistance.
In their experiment, the scientists were looking for signs of resonant tunneling, where the electrons hop onto the carbon nanotube on their way between the two electrical leads in the system. "Tunneling is like jumping across a creek," he said, adding that resonant tunneling is where "you have a small island, the resonant level inside the nanotube, to briefly plant your foot."
The team created an energy-draining environment in the leads and then measured how easily the electrons moved through the resonant level in the nanotube at ultra-low temperature. At the leads, "it's like the banks of the creek are swampy, so it takes energy to push yourself for a jump," said Finkelstein, who led the study.
If the resonant 'island' is positioned right between the two 'banks', then the electrons can easily hop between the banks. But if the island is closer to one of the banks, the electrons stay tied to either one of the leads. This difference in behavior, which was unexpected, signals a quantum phase transition, Finkelstein said.
The discovery might not make it into technology any time soon, but the lead experimenter, former Duke physics graduate student Henok Mebrahtu, does now work at Intel, Baranger said. The results also give scientists a simple system to begin testing a range of environments where quantum phase transitions can occur, he added.
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More information:
Quantum Phase Transition in a Resonant Level Coupled to Interacting Leads. Mebrahtu, H., et. al. Nature. 2012. 488:7409, 61-64. DOI: 10.1038/nature11265
HannesAlfven
1 / 5 (2) Aug 01, 2012If Gerald Pollack is right, then it would seem that the easiest way to build a quantum computer would be to mimic, with polymers, the way in which biological organisms deploy gel phase transitions using structured water.
But, of course, since biologists tend to refuse to consider that there might be a big mistake lurking in the sodium pump hypothesis, this line of investigation remains largely abandoned. Those of us who have read his book, "Cells, Gels and the Engines of Life" can see that conventional wisdom might actually turn out to be wrong this time around.
Alexander Riccio
not rated yet Aug 01, 2012Eoprime
not rated yet Aug 02, 2012Torbjorn_Larsson_OM
5 / 5 (2) Aug 02, 2012@ Eoprime:
It's funny, usually these things are on arxiv. But Mebrahtu publishes very sparsely there.
@ HannesAlfven:
Pollack is not to be believed, since he is working towards finding "transitional science" instead of results wherever they come from. It is very unlikely that he is correct in his more speculative ideas.
Indeed, few of his ideas seems useful for research, since apparently systems are known to work differently than he speculates. [Note that speculative ideas as such can pass peer review. When books are describing such instead of the state of the art in a field, beware though!] Despite his insistence, he is analogous to a homeopath in that way.
MRI has nothing to do with quantum phases or water as such, except in as much that it works on hydrogen nuclear spin.
Torbjorn_Larsson_OM
5 / 5 (2) Aug 02, 2012As always, when you hear hoof beats, think horses not zebras.
Cells are known to be crowded environments with sparse molecular machines, and the many effects from that environment, including surface effects remains to be investigated. It is just recently that tools have been developed that can look at that crowded volumes or sparse reactions.
In general, it is the liquid that implies that quantum effects are not important for cell function at larger scales. Quantum system decoheres faster than biological action, see physicist Tegmark's paper on that. It has become the accepted theory of the generic state.
In specific molecular machines, quantum effects abound of course. It is above all responsible for chemistry, likely some of protein folding (tunneling increasing possible folds and rates), and recently it has been found in antenna functions in photoactive compounds.