In conventional superconductivity, electrons combine into Cooper pairs, and those pairs collectively enter into a single quantum state in which current can flow with zero electrical resistivity; there is no current dissipation and no Joule heating of the material. A multinational collaboration led by Valerii Vinokur of Argonne National Laboratory in the US and Tatyana Baturina of the Institute of Semiconductor Physics in Russia recently reported on an analogous but opposite situation in which electrical current is vanishingly small, effectively zero. The group studied a thin film of superconducting titanium nitride. Below critical values of temperature and applied voltage, the system went through an abrupt transition from an insulator with normal, linear resistivity to one with apparently infinite resistivity. What's more, the transition could be crossed by tuning a magnetic field for a given threshold voltage, as shown in the figure. As with a superconductor, the superinsulator has zero Joule loss—but now because there is no current rather than no resistance. The experimental system was successfully modeled and analyzed as an array of superconducting islands or droplets connected by Josephson weak links. The researchers conjecture that such a network is also essential to the superconductor-to-insulator transition in thin films. (V. M. Vinokur et al., Nature 452, 613, 2008 [MEDLINE].) — Phillip F. Schewe