Jie Wang

A crystal is like a city where houses stand in neat rows. But if you slightly stretch the ground beneath the city, the houses may tilt, and the tilt can ripple as a wave from block to block. In SrTiO3 and SrMnO3 crystals, scientists have found exactly such a hidden mode: a soft wave of tilting oxygen octahedra that had gone unnoticed before.

Two paper-thin sheets of a semiconductor are stacked and slightly rotated — by about three to four degrees. The atomic lattices don't match, creating a large-scale pattern — a moiré, like the ripples from two washboards shifted by a small angle. In this pattern, electrons move quite differently than in a regular crystal.

A tuning fork sounds incredibly pure, but touch it and the pitch changes almost instantly. Engineers faced a similar challenge: creating a laser that simultaneously possesses two seemingly incompatible properties — exceptional spectral purity (a very narrow emission line, like a tuning fork producing a single note without wobble) and the ability to rapidly tune that frequency.