Supersolid at Room Temperature Achieved in Liquid Crystal Microcavities

Light and matter instead of ultracold atoms

The idea of a supersolid dates back to the 1960s: a rigid crystal lattice coexisting with superfluidity, a state with zero viscosity. It was first observed in the lab in 2017, in ultracold atomic Bose-Einstein condensates.

The European team sidestepped the need for cryogenics by replacing slow atoms with quasiparticles of light and matter. Using strong coupling between photons trapped in an optical microcavity and excitons, they created hybrid exciton-polaritons. Their lifetime is only a few picoseconds, yet that is enough to form a Bose-Einstein condensate that behaves like a quantum superfluid.

The transition was recorded as regularly spaced bright interference fringes on the sample surface, ordered simultaneously in two independent channels: light intensity and polarisation. It was accompanied by direct evidence of spontaneous symmetry breaking. Prof. Guillaume Malpuech of Institut Pascal (CNRS) noted that each laser pulse produced fringes at different, random positions, matching theoretical predictions that the system picks one of many equivalent ground states.

A race against chemistry

The microcavities, filled with a specially composed liquid crystal, were designed and made at WAT, and the IBM Research Zurich team deposited ultrathin layers of an organic semiconductor on them. The semiconductor reacted slowly with the liquid crystal, so a sample remained stable for only a few hours. All optical measurements had to fit in that window, and every repeat required synthesising a new sample.

Despite this, the results show that voltage-tunable liquid crystal microcavities are a flexible platform for exploring new quantum phases of light and matter.