In 1958 Philip Anderson published a paper suggesting that certain materials can suffer a sudden phase transition, from conductor to insulator, under a slight change in the amount of disorder in the material. At the time, every physicist would have understood that disorder hinders electron mobility and thus decreases the material’s conductivity. But until then, no one had predicted its complete cancellation past a certain amount of disorder. Even 20 years after his seminal article, when Anderson received the Nobel Prize in Physics in 1977, the effect was still far from fully understood. (For a historical account, see the article by Ad Lagendijk, Bart van Tiggelen, and Diederik Wiersma on page 24.) Today, questions still remain—among them, what is the exact critical value of the disorder at the transition? and how do interactions change the picture? Fortunately, researchers are now in a position to answer some of those questions....

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