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Polka dot pattern upends superfluid hypothesis

Exotic behaviors emerge in atoms when cooled to near absolute zero, a temperature so cold that atoms cease their jittery movement. By bringing the isotope helium-3 to the brink of that threshold and confining it to a tiny space, Cornell researchers discovered that a surprising polka dot pattern spontaneously appeared in the superfluid.

“We found clear evidence of a pattern emerging, essentially out of the blue. Systems are not supposed to do that,” said Jeevak Parpia, M.S. ’77, Ph.D. ’79, professor of physics who specializes in low-temperature physics.

The work was described in the paper “Evidence for a Spatially Modulated Superfluid Phase of 3He Under Confinement,” published in February in Physical Review Letters. Parpia collaborated with researchers at Royal Holloway, University of London, (led by physics professor John Saunders and researcher Lev Levitin) where the experiments were conducted using special confinement chambers constructed at Cornell.

Superfluids are exotic quantum systems that behave in a unified manner, without resistance or viscosity. When cooled to a few degrees above absolute zero, liquid helium-3 (an isotope composed of two protons and a single neutron) surrenders its random motion in favor of coordinated movement. The material can be described as a single wave function with specific properties to it.

The superfluid state of liquid helium has been a hot topic of research since its discovery in the early 1970s at Cornell. The work earned Cornell physicists David M. Lee, Robert C. Richardson and Douglas Osheroff, M.S. ’71, Ph.D. ’73, the 1996 Nobel Prize and ignited decades of research to better understand quantum physics.

Two phases of superfluid helium-3 (A phase, B phase) are known to exist. In the early 2000s at Northwestern University, theoretical physicists James Sauls and Anton Vorontsov proposed that the B phase would arrange in a striped pattern of plus and minus orientations in equal ratio when confined to a nearly two-dimensional space.

Testing the hypothesis was stymied due to the difficulty of engineering a confinement device barely a micron – one millionth of a meter – in height. The Cornell researchers designed and built special magnetic resonance chambers at the Cornell NanoScale Science and Technology Facility (CNF), with dimensions of 1 centimeter in length and width, and 1.1 microns in height, creating a nearly flat cavity to conduct their experiments.

By confining the superfluid, physicists found that the superfluid state diverged into positive and negative domains, with the positive domain appearing at four times the concentration of the negative. The findings ruled out the striped pattern, suggesting instead an either regular or disordered array of island domains – in other words, polka dots.

The appearance of a spontaneous pattern is evidence of a broken symmetry, an unusual occurrence. “Usually such patterns impose a significant cost in energy, but the polka dot appears to lower its energy cost enough to compensate,” Parpia said. He said the findings could one day help inform principles used for quantum computing.

“To create a chamber with uniform height extending that large of a distance is remarkable. It’s hard to come up with schemes that maintain that profile,” said Parpia.

The chamber was designed by Robert Bennett while working as a postdoctoral research associate and created by Nikolay Zhelev, M.S. ’13, Ph.D. ’16.

“Superfluids are remarkable materials, and we continue to be surprised by their richness and unexpected behavior when we confine them in smaller and smaller systems,” Parpia said. “Discoveries of these exotic forms of matter in a system that we can control with great precision give us knowledge that can be applied to more practical electronic systems.”

CNF is supported by Cornell, the National Science Foundation (NSF), Empire State Development's Division of Science, Technology and Innovation, industry partners and other users. The research was supported by grants from the Engineering and Physical Sciences Research Council in the U.K., the NSF (in the U.S.) and the European Microkelvin Platform (in the U.K.).

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