Xiaoyu Guo

Sunlight hits a crystal, and inside, a pair is born — a swift electron and a hole. Ideally, they immediately race off to the contacts and produce current. But in the material the authors studied, things go differently. The moment a charge stumbles upon a void, a missing atom in the lattice, a cloud of vibrations tightens around it.

Toss a handful of phosphorus into silicon, and at a certain impurity concentration the material suddenly starts conducting electricity. This is a metal-insulator phase transition, and it happens abruptly. But what exactly changes in the electron wave functions when an insulator becomes a metal?