supersolid

A supersolid is a phase of matter that combines the rigid, periodic structure of a crystal with the frictionless flow characteristic of a superfluid. In a supersolid, atoms maintain a crystalline lattice while a portion of the material participates in coherent, dissipationless motion typically associated with superfluidity.

The concept originated in theoretical discussions in the 1960s by Andreev, Lifshitz, and Chester, who proposed that zero-point vacancies in a quantum crystal could condense into a Bose-Einstein condensate without destroying crystalline order.

Experimental searches in solid helium-4 produced initial reports of non-classical rotational inertia, suggestive of supersolidity, but later work attributed the effects to defects, grain boundaries, or surface phenomena. A bulk supersolid phase in solid helium remains unconfirmed, and the broader interpretation of early results is still debated.

More robust realizations have emerged in ultracold atomic gases, where density modulations with preserved phase coherence have been observed in dipolar Bose-Einstein condensates and related systems. These setups show a genuine coexistence of crystalline order and superfluidity in a controlled setting, providing clearer demonstrations of the supersolid concept.

Today, supersolidity is an active area of research. In helium, the existence of a bulk supersolid is still debated; in ultracold gases, the phenomenon offers a tunable platform to study the interplay between order and superfluidity, roton physics, and collective excitations.