This graphic shows an electron using a resonant level (center well) to pass through an energy barrier (orange wall). Credit: Gleb Finkelstein; Duke.
Scientists have designed a simple system to study how electrons travel through energy barriers instead of over them.
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.
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.
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.
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 F, 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.
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.”
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.
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.
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.
with Baranger, Finkelstein and Mebrahtu, Ivan Borzenets, Dong Liu,
Huaixiu Zheng and Yuriy Bomze of Duke and Alex Smirnov of North Carolina
State University contributed to the work.
Citation: Quantum Phase Transition in a Resonant Level Coupled to Interacting Leads