An international research team including DESY scientists has observed tiny quantum vortices in cold droplets of liquid helium. The team reports in the journal Science that the exotic vortices arrange themselves as densely packed lattices inside the nanodroplets. It is the first time that the quantum vortices, which have already been observed in larger samples of what is known as superfluid helium, have been detected in nanodroplets. "The experiment has exceeded our best expectations," says Andrey Vilesov of the University of Southern California, one of the experiment's three leads.
The noble gas helium becomes liquid at minus 269 degrees Celsius. Below minus 271 degrees a quantum effect occurs, through which the liquid helium loses all internal friction and becomes superfluid. In this exotic state it can even crawl up walls. To explore the dynamics of superfluid helium, the scientists X-rayed tiny helium nanodroplets with what is currently the world's strongest X-ray laser, the Linac Coherent Light Source LCLS at the SLAC National Accelerator Laboratory in California.
The production of the cold droplets with an average of only 0.2 to 2 thousandths of a millimetre was no easy task. The scientists sprayed the liquid helium through a fine nozzle into a vacuum chamber. A portion of the helium evaporated on the way, and the remaining portion of the drop continued cooling due to evaporative cooling. "After a distance of few millimeters, the drops reached the superfluid state and were struck by the intense X-ray laser flash further downstream," explains DESY scientist Daniel Rolles of the Center for Free-Electron Laser Science CFEL, a joint facility run by DESY, the University of Hamburg and the Max-Planck Society.
An especially detailed recording of the X-ray scattering patterns of the nanodroplets was made possible using the CAMP experimental vacuum chamber, developed by a Max Planck Group in Hamburg at CFEL. "CAMP has two large d
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Deutsches Elektronen-Synchrotron DESY