Physicists have observed for the first time a quantum phase transition in which a superfluid turns into a supersolid and then reverts, advancing the study of exotic states of matter. A superfluid flows without friction, a hallmark of quantum behavior at near-absolute zero temperatures, while a supersolid combines that frictionless flow with the rigid structure of a crystal. That combination has long been a theoretical prediction of quantum condensed-matter physics but has proven difficult to realize in a natural and reversible way, tells Live Science.
The new experiment, published in Nature, used ultracold excitons, quasiparticles formed by electron-hole pairs, confined in two layers of graphene under a strong magnetic field. Those conditions allowed the excitons to behave as a superfluid. As researchers adjusted temperature and particle density, the exciton system transitioned into an insulating state, which they interpreted as a supersolid, because its properties resembled a crystalline arrangement with suppressed motion. Increasing the temperature again caused the system to return to the superfluid phase. The reversibility of the transition sets this work apart from earlier attempts, where supersolid behavior was induced artificially with external traps but did not appear as a natural ground state.
This result has implications for quantum phase transition research and condensed-matter physics more broadly. Observing a supersolid form without engineered constraints suggests that certain quantum systems can settle into states that simultaneously support long-range order and frictionless flow, challenging assumptions about energy landscapes at ultralow temperatures. The experiment also opens new avenues for exploring quantum materials and their potential uses, for example, in precision sensors or in fundamental tests of quantum mechanics.
Researchers plan to explore whether similar reversible transitions occur in other materials and whether the unique characteristics of supersolids can be harnessed in practical technologies. By demonstrating a controlled, bidirectional quantum phase change, the work adds a critical piece to understanding the rich behavior of matter under quantum rules.