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Quantum tornadoes in a nanodroplet

25 September 2014
New x-ray-diffraction experiments reveal the hidden order inside a spinning bead of superfluid.

Spin a vessel of water, and the fluid will move in uniform, solid-body rotation. Replace the water with a superfluid—a liquid chilled to a viscosity-free quantum state—and the flow instead spawns an array of tiny tornadoes, each with quantized angular momentum. The so-called quantum vortices have been widely studied in rotating cryostats, but those experiments have limitations. Defects along a cryostat’s surface inevitably disturb the flow in ways that aren’t neatly captured by theory. Plus, bulky cryostats can be spun only so fast, a few rotations per second at most, and many interesting behaviors are thought to emerge at higher speeds. Now an international collaboration led by Christoph Bostedt (SLAC), Oliver Gessner (Lawrence Berkeley National Laboratory), and Andrey Vilesov (University of Southern California) has conducted a miniature, container-free version of the rotating-vessel experiment using liquid-helium nanodroplets. Sprayed into a vacuum, the droplets can acquire angular velocities of several million rotations per second. To reveal the quantum vortices, the researchers doped the droplets with xenon atoms, which cluster along the vortex cores and provide x-ray-scattering contrast. The ordered vortex arrays therefore show up in x-ray-diffraction patterns as Bragg peaks, as seen in the image here. Curiously, the peaks vanish at rotation frequencies above about 10 million hertz, possibly indicating a transition to a disordered or nonstationary phase. Those fastest-spinning droplets also show an unexpected morphology: They bulge at their equators and flatten at their poles, reminiscent of a cheese wheel. Under the same conditions, a classical droplet would form a two-lobed structure resembling a peanut. (L. F. Gomez et al., Science 345, 906, 2014.)

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