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Making Supersolids with Ultracold Atoms

Artist's rendition of a counterflow superfluid state. In this type of superfluid, the motion of one type of atom is perfectly matched with an atom of the other type moving in the opposite direction.

Artist's rendition of a counterflow superfluid state. In this type of superfluid, the motion of one type of atom is perfectly matched with an atom of the other type moving in the opposite direction.

Credit: Ludwig Mathey/JQI

The Flash, a comic-book superhero, can walk through solid walls by vibrating fast enough. Physics hasn’t quite gotten there yet, but JQI researchers predict that in a weird new state of matter called a “supersolid” two types of atoms could flow through each other frictionlessly while each maintaining a regular crystalline arrangement.

Earlier this year the JQI team proposed a recipe for turning ultracold “boson” atoms into just such a supersolid, an exotic state of matter that behaves simultaneously as a solid and a friction-free superfluid. While scientists have found evidence for supersolids in complex liquid helium mixtures, a supersolid formed from such weakly interacting gas atoms would be simpler to understand, potentially providing clues for making a host of new “quantum materials” whose bizarre properties could expand physicists’ notions of what is possible with matter.

First theorized in 1970, a supersolid displays the essential characteristics of a solid, with atoms arranged in regularly repeating patterns like that of a crystal lattice, and of a superfluid, with the particles flowing frictionlessly and without losing any energy. Able to exist only at low temperatures, a supersolid behaves very differently from objects in the everyday world. “If you add more clothing to a spinning washing machine, you increase the mass of its rim, and the machine needs to exert a greater force to make the wheel reverse direction,” explains lead author Ludwig Mathey. “But in a supersolid washing machine, some of the clothes would mysteriously hover in space, staying stationary as the washer spins and making it easier for the wheel to reverse direction. Moreover, these hovering, frictionless clothes would form a predictable pattern—such as frictionless socks alternating with frictionless shirts—just as atoms arrange themselves in a repeating pattern in a crystal.”

In 2004, Moses Chan and Eun-Seong Kim of Pennsylvania State University published a groundbreaking experiment on helium at low temperatures and gathered evidence for a supersolid phase. However, the interpretation of their observations has considerable uncertainties due to the complex nature of the particular system used in their experiments.

Physicists Ludwig Mathey, Ippei Danshita and Charles Clark have identified a technique for making a simpler-to-understand supersolid using two species of ultracold atoms confined in an optical lattice, a “web of light” that traps atoms in regular positions. In a paper recently published in Physical Review A (see reference publications), the JQI team identifies conditions under which a cloud of ultracold atoms of two species (such as rubidium and sodium, or two slightly different forms of rubidium) can spontaneously condense into a state in which there is crystalline structure in the relative positions of atoms, e.g. a chain in which the two different types of atoms alternate regularly, but in which the entire cloud exhibits the frictionless, superfluid properties of a Bose-Einstein condensate (BEC). This remains hard to visualize in familiar terms—the accompanying image shows an artist’s conception of it—but the team identified clear experimental signatures (essentially photographs of the cloud), that could verify the simultaneous existence of these two seemingly incompatible properties.

In work presented at the March 2009 APS meeting, JQI researcher Anzi Hu will present an extension of this theory that makes some new predictions. The expanded theory predicts a phenomenon called “counterflow superfluidity,” in which two species of atoms can flow frictionlessly in opposite directions while maintaining their crystalline patterns. It’s like opposite streams of traffic flowing through each other frictionlessly. Examining the flow of atoms in both directions, the researchers find the net flow to be zero. But it can be regarded a superfluid, with nonzero net flow, if one species of atom is considered as an “anti-atom” or a hole (just like in electronic devices). The underlying technologies of optical lattices and Bose-Einstein condensation were pioneered at NIST and have sparked a renaissance in atomic physics with applications to NIST’s fundamental measurement missions, such as time and frequency standards and improved sensors of magnetic and gravitational forces. The supersolid is an example of a further direction of research in ultracold atomic physics: the design of quantum materials with fundamental properties not previously found in familiar matter.

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