Kunal Mukherjee

A crystal of germanium with tin is like a mosaic where each tile is an atom. If you lay the tiles randomly, the mosaic has one color, but if you arrange them in a pattern, it has another. Physicists measured how the atomic pattern changes a semiconductor's most important property: its band gap, that is, the energy needed to get an electron moving and creating current.

Hot gas settles on a substrate, building up a transparent film layer by layer. But instead of a perfect mirror, the surface sometimes gets covered with tiny bumps, like frost patterns on glass in severe cold. Usually, people try to remove them because they get in the way of making smooth components. Physicists from Oxford and their colleagues did the opposite: they looked inside the bumps.

A stack of playing cards lies still. You can slide the top card with your finger: that's crude intervention. Or you can blow so that the whole stack momentarily changes shape, but the cards don't scatter. That's roughly how light, in a new experiment, rearranges a crystal of tin selenide (SnSe). Usually, to change the structure of a solid, you heat it or pass a current through it.