Electronic phase diagram of Fe1+yTe1-xSex revealed by magnetotransport measurements

Among the Fe-based superconductors, Fe1+yTe1-xSex is unique in that its crystal structure is the simplest and the electron correlation level is the strongest, and thus it is important to investigate the doping (x)-temperature (T) phase diagram of this system. However, inevitably incorporated excess Fe currently prevents the establishment of the true phase diagram. We overcome the aforementioned significant problem via developing a new annealing method termed as "Te-annealing" wherein single crystals are annealed under Te vapor. Specifically, we conducted various magnetotransport measurements on Te-annealed superconducting Fe1+yTe1-xSex. We observed that crossover from the incoherent to the coherent electronic state and opening of the pseudogap occurs at high temperatures (approximate to 150 K for x = 0.2). This is accompanied by a more substantial pseudogap and the emergence of a phase with a multi-band nature at lower temperatures (below approximate to 50 K for x = 0.2) before superconductivity sets in. Based on the results, the third type electronic phase diagram in Fe-based high-T-c superconductors is revealed.