Condensed Matter Seminars

Low carrier densities and short coherence lengths in the recently discovered semi-metallic Fe-based superconductors (FBS) can result in exotic behaviors at strong magnetic fields due to the interplay of multiband superconductivity, unconventional pairing symmetry and the Zeeman and orbital pairbreaking. I this talk I will give an overview of these interesting new materials, particularly their anomalous temperature dependencies of the upper critical field Hc2(T) which often extrapolate to very high Hc2 > 100 T at low temperatures. The materials features of FBS and the multiband s pairing symmetry with the sign change of the order parameter on different sheets of the Fermi surface can significantly increase Hc2(T) and facilitate the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) transition to the state with a spatially modulated order parameter. Small shifts of the chemical potential upon doping of FBS can produce new small pockets of the Fermi surface, triggering the FFLO instability due to the Lifshitz transition.