Focus: Electric Current in Big Chunks

Phys. Rev. Focus 7, 20
A single-atom bridge proves that the irreducible unit of current between two superconductors can be many times the charge of the electron.
Figure caption
R Cron and M. Goffman/CEA-Saclay
Flimsy bridge. A 3-µm-long bridge with a single atom at its narrowest point allows precise verification of a surprising theory: The irreducible unit of current between two superconductors can be many... times the charge of the electron. This bridge was manipulated after the experiments and contains millions of atoms at its midpoint. Show more

You can’t hear the individual raindrops hitting the roof during a storm, but condensed matter physicists are not so limited. By carefully “listening” to a steady electric current, they can measure the size of the individual units of charge that make up the current. In the 30 April PRL a team reports such an analysis and finds the equivalent of golf-ball-sized hail. In experiments where two superconducting electrodes were connected by only a single atom, they found units of charge anywhere from one to ten times the charge of the electron–all of which agreed with the strange predictions of superconductivity theory. The team was able to verify the theory in the most direct way possible because they could explicitly account for every electron path through their pint-sized circuit.

High school level electronics principles like Ohm’s Law don’t always hold up when your resistor is a single atom, so Cristian Urbina and his colleagues at the French Atomic Energy Commission in Saclay are trying to learn the new rules. In some ways, things are simpler: If two microscopic electrodes are bridged by an aluminum atom, there are only three ways for electrons to get across, because the atom has only three available p-orbitals. “We are getting to a scale in which chemistry and solid state circuit physics are really related,” says Urbina.

The team can precisely measure the probability for an electron crossing a one-atom bridge via any of the three “channels,” which gives a set of three numbers they call the “PIN code” that characterizes a particular atomic contact. “If you know this code, you can predict all the properties of the system,” Urbina explains. The PIN code allows a direct connection between measurable properties like conductance and theoretical constructs involving electron wave functions. So atomic contacts allow direct comparison with quantum mechanical theories of electron transport.

Urbina and his colleagues wanted to test a surprising prediction of superconductivity theory. Normal current is carried by single electrons; within superconductors the charge units are electron pairs called Cooper pairs. But for current traveling between two superconductors connected by a “weak link”–a piece of nonsuperconducting material–theory predicts an even more strange result: The size of the charge units increases with decreasing applied voltage and also depends on the details of the link. For an extremely small link, the units of charge are integer multiples of the electron’s charge.

To verify these predictions, Urbina and his colleagues measured the fluctuations in the current–the so-called current shot noise–across the atomic contact at temperatures where aluminum becomes superconducting. The method is akin to listening for the variations in the pitter-patter of raindrops to gauge the drops’ size. Just as theory predicts, they saw the units of charge increase in steps of approximately one electron charge as they reduced the applied voltage. By plugging the contact’s PIN code into the theory, the team came up with a theoretical curve that matched the data’s rounded steps exactly, without any adjustable parameters.

“It’s absolutely fantastic. It’s beautiful,” says Jan van Ruitenbeek of the University of Leiden in the Netherlands. He is impressed with the team’s experimental prowess and with the precise agreement between their data and the theory: “It fits just like a glove.” van Ruitenbeek says the work demonstrates the recent progress in nanoscale electronics. “It firmly establishes that we know how to think of conductance at the atomic scale.”

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