Why do some solids conduct electricity like a metal, and others act like insulators? Quantum mechanics has provided some relatively simple (and quite successful) models for electron conductivity, but the underlying physics is often complex, because electrons interact with each other through Coulomb forces and because real materials are not perfectly ordered. On page 1520 of this issue, Schneider et al. (1) address the microscopic distinction between a conductor and an insulator by examining the conducting properties of repulsively interacting ~(40)K atoms, which, like electrons, are fermions-they have half-integer spin and obey the Pauli Exclusion Principle, which allows only one fermion to occupy a quantum state. By placing ultracold ~(40)K atoms in an artificial crystal held in place through optical fields, they can manipulate the energy scales of the system so that it varies all the way from a metallic state to different kinds of insulating phases.
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