Wigner islands of electrons over helium

Wigner islands of electrons over helium

E. Rousseau, D. Ponarin, L. Hristakos, O. Avenel, E. Varoquaux, Yu. Mukharsky
SPEC Groupe de Nanoélectronique – Hydrodynamique superfluide

The study of few-electron devices, from Single Electron Transistors (SET) to qubit devices as the quantronium, to metallic nanowires and carbon nanotubes is a distinctive hallmark of SPEC. The description in Physical Review B of a novel such system, a Wigner island of electrons on superfluid helium, which can condense into a Wigner molecule, by Emmanuel Rousseau, Dmitri Ponarin, Likourgos Hristakos, Olivier Avenel, Eric Varoquaux, and Yuri Mukharsky (Phys. Rev. B 79, 045406 (2009)) has recently been singled out by the Editors of Physical Review B and promoted to the Editors' choice status.

This work has also been put in the limelight by a viewpoint written by François Peeters, a leading expert in the theory of few-electron systems (Physics 2, 4 (2009, http://physics.aps.org/articles/v2/4 ) and has appeared in the January 26, 2009 issue of Virtual Journal of Nanoscale Science & Technology, which covers a focused area of frontier research (http://www.vjnano.org).

The system is made of a few electrons trapped over a film of liquid helium in a nanofabricated potential well. Electrons floating over helium are very mobile on the film surface on which they move a few tens of nanometres away from the underlying substrate impurities and crystalline defects. Thus, they form a very clean system.

The behaviour of these electrons is controlled with a set of electrodes located on the substrate under the helium film. Their motion is detected through the change of the electric charge of the trap with a Single Electron Transistor, an extremely sensitive microscopic electrometer.

The device operation is depicted in the animated figure, which shows a micro-photograph of the nanofabricated structure. Electrons are stored in the reservoir to the right. Upon adjustment of the electrode potentials, as depicted at the bottom part of the figure, electrons either rush leftward into the circular trap, or ooze out one by one to the right back to the reservoir. The change in the total electric charge in the trap is shown on top; when an electron leaves the trap, the charge jumps upward by a quantum amount. A system with a given, small, number of electrons can thus be formed at will.

Using this newly developed technique, it becomes possible to study the ordering of a trapped electron cluster into concentric rings, the Wigner molecules. Also, it is hoped that, when only one electron is left in the trap, the device can be turned into a highly coherent qubit, the basic element of a quantum computer.