Electrical resistance is one of the few physical quantities that is spanning all the way from zero (in superconductors) to infinity (in insulators at T = 0 K). Doping insulators to obtain new superconductors may hence seem to be a surprising approach. However, in (conventional) superconductors, superconductivity is the result of the coupling between electrons and lattice vibrations (phonons), and this coupling is particularly strong in so-called covalent insulators (such as diamond or silicon). This new route has been first highlighted by the discovery of MgB2, in which the boron covalent bounds are "self-doped" by the magnesium atoms leading to a critical temperature ∼ 40 K, or in so-called "cage structures" such as C60 Bucky balls for which Tc reaches ∼ 30 K in K3C60. A few years latter, the discovery of superconductivity in boron doped diamond confirmed that covalent insulator might be a fruitful starting point towards new superconductors.
Doped insulators are also a powerful platform to investigate the physics of the superconductor to insulator transition (SIT). Indeed, although superconductivity is not affected by a small amount of disorder, numerous experimental works (in systems such as InO and NbTi) have shown that superconductors can be gradually driven to an insulating phase when the disorder gets large. The microscopic origin of this transition is however still an open question. Indeed, two fundamentally contradictory effects come into play : on the one hand, the formation of a macroscopic fully delocalized superconducting condensate and, on the other hand, the localisation of the charge carriers in presence of disorder and/or strong correlations.
At Néel Institute :
Members of the team