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Speed limit on a superfluid helium nano-highway

Posted October 10, 2013
Scientists from the University of Amsterdam (UvA), the Ecole Polytechnique Fédérale de Lausanne (EPFL) and the Universitat de Barcelona have been able to determine the so-called Landau velocity for helium nanodroplets down to a radius of 2.5 nanometer, containing only a thousand helium atoms. Their results, confirming the existence of superfluidity at the nanoscale, are published this week by Physical Review Letters.
These computer simulations show how a silver atom is launched from a helium nanodroplet. The moment the silver atom is ‘spat out’ is a spectacle in its own right, as it is accompanied by various other processes such as excitation of ‘riplons’ (surface waves). Credit: Fundamental Research on Matter (FOM)

The fact that helium becomes superfluid at extremely low temperatures is a well-known quantum mechanical phenomenon. Objects do not experience any friction when they move through superfluid helium – at least, as long as they move slower than the critical Landau velocity. So far, scientists have only been able to verify this for bulk helium. The new research has now shown that even in tiny nanodroplets, helium still exhibits superfluidity.

Pioneering experiment

The pioneering experiment is the result of a collaboration between dr. Marcel Drabbels (EPFL, former postdoc at FOM Institute AMOLF) and FOM workgroup leader and UvA researcher prof.dr. Wybren Jan Buma. The experiment starts with the production of extremely cold helium nanodroplets having a temperature of 0.4 K and a size that can be varied from a few million to less than thousand helium atoms. Subsequently, the researchers place one single metal atom or one single molecule in such a nanodroplet.

Due to the weak attractive interactions it experiences with helium, the ‘impurity’ will initially be located at the center of the nanodroplet. Next, the researchers activate the impurity with a nanosecond laser pulse. The electronically excited particle now experiences a repulsive interaction with the helium. As a result the atom or molecule is launched out of the droplet. There its speed is determined.

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