12.28.2009

Evidence of macroscopic quantum tunneling detected in nanowires

http://news.illinois.edu/news/09/0527tunneling.html
Mitrabhanu Sahu, Alexey Bezryadin, and Paul Goldbart - news.illinois.edu
A team of researchers at the University of Illinois has demonstrated that, counter to classical Newtonian mechanics, an entire collection of superconducting electrons in an ultrathin superconducting wire is able to “tunnel” as a pack from a state with a higher electrical current to one with a notably lower current, providing more evidence of the phenomenon of macroscopic quantum tunneling.
Physics professors Alexey Bezryadin and Paul Goldbart led the team, with graduate student Mitrabhanu Sahu performing the bulk of the measurements. Their research was published on the Web site of the journal Nature Physics on May 17.
Quantum tunneling is the capability of a particle to inhabit regions of space that would normally be off-limits according to classical mechanics. This research observes a process called a quantum phase slip, whereby packs of roughly 100,000 electrons tunnel together from higher electrical current states to lower ones. The energy locked in the motion of the electrons as they phase slip is dissipated as heat, causing the nanowires to switch from a superconducting state to a more highly resistive one.
It’s through this switching of states that allows the tunneling of the phase slip to be observed, the researchers say.
Goldbart, who is also a researcher at the university’s Frederick Seitz Materials Research Laboratory, describes a quantum phase slip as a phenomenon that allows the spatially extended structure of superconductivity “to undergo a kind of quantum mechanical rip or tear, one where the entire extended behavior of the superconductivity tunnels its way through a classically forbidden set of configurations.”
“Semiconductors, insulators and metals all hinge upon the ability of particles to make it through classically forbidden regions, despite apparently having negative kinetic energy there, as quantum physics allows,” Goldbart said.
In Newton’s world, according to Goldbart, particles would be reflected from such regions.
Although quantum mechanics governs the realm of atoms and molecules and smaller, quantum phenomena sometimes “leak up” to macroscopic scales, he said.
“Observing switching events in superconducting nanowires at high-bias currents provides strong evidence for quantum phase slips. Our experiments provide further evidence that the laws of quantum mechanics continue to govern large systems, composed of many thousands of electrons, acting as a single entity.”

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