In all unconventional materials, superconductivity appears in the vicinity of a competing magnetic, electronic and/or lattice instability. The transition temperature of this competing instability progressively decreases under the action of an external parameter (chemical doping, pressure, etc…) and a superconducting dome develops in the vicinity of the critical value of this parameter for which this instability vanishes. During almost three decades, high temperature cupper oxides (cuprates) have mainly been considered as (hole) doped Mott insulators subjected to spin fluctuations giving rise to a – still mysterious – "pseudo-gap" phase from which the superconducting dome emerges. The discovery of a charge ordered phase associated to a major Fermi surface reconstruction in underdoped YBa2Cu3Oy, recently revived the debate on the pairing mechanism in those superconductors, still one of the most challenging issue in modern solid-state physics.
Other systems are also of great interest. BaBiO3 also hosts for instance a charge density wave and superconductivity can then be induced by (hole) doping with Tc ≈ 32 K. However in striking contrast to other systems, no underdoped regime has been observed in this system and Tc abruptly reaches its maximum at the insulator-superconductor transition. Moreover, ab-initio calculations recently suggested that electron doped BaBiO3 might be an unconventional (so-called topological) insulator with mass less surface states. Being both a topological insulator (for electron doping) and a superconductor (for hole doping), this system is then an ideal candidate for the search of Majorana fermions, an intriguing particle being its own antiparticle.
At Néel Institute :
Members of the team