A crystal of mono ammonium phosphate

A crystal of monoammonium phosphate revealed signatures of the strange phase of matter called a time crystal. Credit: Michael Marsland/Yale University

Physics

An unexpected twist in ‘crystallized’ time

Compound’s bizarre pulsing behaviour challenges convention.

A weird phase of matter called a time crystal was once thought to be theoretically impossible. But in 2017, two teams reported seeing the first hints of these objects, which pulse in patterns that repeat over time, much as the atoms in a crystal repeat in space. Now scientists have seen this behaviour in the most unexpected system so far.

Sean Barrett and his colleagues at Yale University in New Haven, Connecticut, aimed pulses of radio waves at phosphorous nuclei in an ordered crystal of monoammonium phosphate (pictured). This caused the nuclei’s spins to oscillate, in a stable way, with a frequency of half that of the pulses — the key hallmarks of a time crystal.

Previously described time crystals were thought to maintain their repeating pattern, which might otherwise break down, in part by drawing on elements of disorder in the system. However the latest set-up includes no obvious source of disorder, which challenges assumptions about how time crystals arise.