Supersolid

A supersolid is a phase of matter that combines crystalline order with superfluid behavior. It retains a periodic structure while supporting flow without viscosity.

Superfluidity: A phase of matter that flows without viscosity and exhibits macroscopic quantum coherence. Its defining flow behavior appears alongside the supersolid’s crystalline order.
Eugene Gross: A theoretical physicist whose work helped establish the concept of supersolidity. His early analysis treated supersolidity as a possible quantum phase of solid helium.
Ultracold quantum gas microscopy: Imaging methods that resolve the positions or density of individual atoms in ultracold gases. Spatially resolved measurements can reveal density modulation in supersolid states.
Supersolid helium debate: The dispute over whether solid helium-4 exhibits genuine supersolid behavior. The debate exposed how ordinary elastic effects can imitate a supersolid signal.
Bose–Einstein condensate: A state in which bosons occupy a shared macroscopic quantum state, typically at very low temperatures. Condensation provides a route to coherent flow in many supersolid experiments.
Andreev–Lifshitz theory: A theory proposing that vacancies in a quantum crystal could become mobile and condense. It supplied an influential microscopic route to supersolidity in solid helium.
Atom interferometry: A measurement technique that uses matter-wave interference to detect phase shifts. It can probe coherence and phase dynamics in ultracold supersolid systems.
Supersolid versus superfluid: A comparison between a phase with both crystalline order and superfluidity and one with superfluidity alone. The distinction is whether periodic density order survives alongside frictionless flow.
Off-diagonal long-range order: Long-distance coherence in a quantum many-body system’s one-particle density matrix. It characterizes the coherence associated with superfluid behavior.
Solid helium-4: A crystalline phase of helium-4 stabilized by pressure at low temperatures. It became the central material in early searches for supersolid behavior.

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