Theoretical physicists have found evidence for a novel quantum state called a "metallic superfluid" in numerical calculations. If observed in experiments the superfluid would be a new member of the family of "super" states that already includes superconductors, superfluid liquids and gases, and supersolids. The new state could be seen in hydrogen or its isotopes under high pressure (Phys. Rev. Lett. 95 135301).
At ultra-low temperatures the de Broglie wavelength of particles becomes comparable to the distance between them and this results in new states of matter with exotic and counterintuitive properties. In superconductors, for instance, electrons move without electrical resistance, while superfluids can flow without any internal friction.
Last year, Egor Babaev and Asle Sudbo of the Norwegian University of Science and Technology in Trondheim and Neil Ashcroft of Cornell University predicted that liquid metallic hydrogen – essentially a liquid of protons and electrons that is formed under extreme pressure – might form two superstates. One of these states was a superconducting superfluid that had no viscosity or electrical resistance, while the other was a metallic superfluid that displayed electrical resistance but not viscosity (Nature 431 666).
Now Babeav, Sudbo, Eivind Smorgrav and Jo Smiseth have found evidence for the metallic superfluid state in numerical calculations performed at the Norwegian High-Performance Computing Center in Trondheim (see figure). The new state consists of electronic and protonic vortices — which Sudbo refers to as “quantum tornadoes”. When the electrons and protons flow in the same direction, they do not experience any electrical resistance. However, when they move in opposite directions, they encounter resistance.
“At present it is not possible to reach the pressures of about 4 million atmospheres that are needed to see the metallic superfluid state,” says Sudbo, “but recent breakthroughs in the synthesis of ultrahard artificial diamonds at the Carnegie Institution in Washington means that pressures of ten million atmospheres are envisaged within the next few years, so hopefully the metallic superfluid will be seen in experiments within the next five to ten years.”
The new superfluid state is thought to contain Cooper pairs of both protons and electrons at low temperatures, which means that it would be radically different from other quantum fluids. However, the theoretical work done so far has been based on fundamental symmetry principles and a more detailed theory needs to be developed.