| Posted: Jul 22, 2016 |
Friction-like effect observed in quantum system
(Nanowerk News) Applying a voltage to an array of electrons hovering above a sea of liquid helium (Fig. 1) causes the electron lattice to undergo ‘stick-slip’ motion resembling that between two sliding rough surfaces, an all-RIKEN team has discovered (Physical Review Letters, "Stick-slip motion of the Wigner solid on liquid helium").
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| Figure 1: Electrons (green spheres) form a crystalline lattice when placed above liquid helium. By applying a voltage, researchers studied the motion of this lattice. (Image: APS/Joan Tycko)
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Diamonds and table salt are common examples of crystals; they are three-dimensional arrays of periodically spaced atoms or ions. But electrons can also form crystals—when placed above a liquid helium surface, they arrange themselves into a flat crystal lattice. This crystalline arrangement arises as the electrons seek to maximize their distances from each other due to their mutual repulsion. In addition, a dimple forms on the helium surface immediately below each electron. This electron–helium system is ideal for exploring physical phenomena because it is free from defects that blight more complex systems.
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When a rough solid surface is dragged across another surface it displays a kind of stop–start motion due to friction. This is because the pulling force has to build up until it exceeds the frictional force that resists motion. After the surface moves a short distance, the two surfaces become interlocked again until the pulling force increases sufficiently.
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A similar stick–slip motion has now been observed in an electron lattice above a liquid helium surface by Kimitoshi Kono of the RIKEN Center for Emergent Matter Science and his three RIKEN co-workers. By applying a voltage across a 100-micrometer-long channel with electrons floating above helium, they were able to drive the electron lattice across the channel (Fig. 2).
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| Figure 2: Schematic depiction of array of electrons (brown dots) moving above liquid helium (gray area) due to the application of electric field. (Image: Kimitoshi Kono, RIKEN Center for Emergent Matter Science)
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To their surprise, the researchers found that as they ramped up the voltage, the electron motion remained constant for a while, but accelerated dramatically when the electric field exceeded a critical value, before dropping back to its original level. “We did not expect to see this repeating process,” says Kono. “We were very excited when we first saw it.”
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The motion of the electron crystal is restricted by the emission of ‘ripplons’—field quanta of capillary waves that form on the helium surface. These ripplons resemble the ripples produced by a moving boat. The electrons transfer momentum to the ripplons, and in so doing their own motion is slowed. At the critical electric field, the electron lattice overcomes a ‘sound barrier’ and all the ripplons are left behind, drastically reducing the lattice mobility.
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The team is very excited about their discovery. “The present observation suggests the possibility of studying hydrodynamics on a nanoscale,” says Kono. “I believe that we have opened up a huge area of nanofluidics.”
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