Supersolidity in the ultracold: when atoms behave as solid and superfluid at the same time

Physicists in the US believe they have created a new phase of matter by cooling helium-4 to ultracold temperatures. Eun-Seong Kim and Moses Chan of Pennsylvania State University say their "supersolid" behaves like a superfluid–a liquid that flows without resistance–yet it has all the characteristics of a crystalline solid. If confirmed, their results will mean that Bose–Einstein condensation has now been achieved in the gas, liquid and solid phases (Nature 427 225).

Alessio Recati, S. Stringari

Jozef T. Devreese, Jacques Tempere

Tim Langen

Ultracold atomic gases with the right balance of interactions enter a phase that demonstrates a superposition of seemingly opposing properties.

Philip Zupancic, Julian Léonard, Andrea Morales +2 more

Matthew A. Norcia, Claudia Politi, Lauritz Klaus +6 more

Abstract Supersolid states simultaneously feature properties typically associated with a solid and with a superfluid. Like a solid, they possess crystalline order, manifesting as a periodic modulation of the particle density; but unlike a typical solid, they also have superfluid properties, resulting from coherent particle delocalization across the system. Such states were initially envisioned in the context of bulk solid helium, as a possible answer to the question of whether a solid could have superfluid properties1–5. Although supersolidity has not been observed in solid helium (despite much effort)6, ultracold atomic gases provide an alternative approach, recently enabling the observation and study of supersolids with dipolar atoms7–16. However, unlike the proposed phenomena in helium, these gaseous systems have so far only shown supersolidity along a single direction. Here we demonstrate the extension of supersolid properties into two dimensions by preparing a supersolid quantum gas of dysprosium atoms on both sides of a structural phase transition similar to those occurring in ionic chains17–20, quantum wires21, 22 and theoretically in chains of individual dipolar particles23, 24. This opens the possibility of studying rich excitation properties25–28, including vortex formation29–31, and ground-state phases with varied geometrical structure7, 32 in a highly flexible and controllable system.