Condensed Matter Seminars

Recent measurements on the tetragonal phase of the iron-based superconductor FeSe,S support the existence of a remarkable phase where the superconducting state supports a finite residual  density of states arising from patchlike nodal surfaces[1,2].  This ``ultranodal"> state can arise in situations where conventional intraband spin singlet pairing is highly anisotropic and coexists with time-reversal symmetry breaking  interband spin triplet interactions [3].  Here I present a  microscopic scenario including ferromagnetic interactions that can account for nonunitary pairing and C4 symmetry breaking in the superconducting state that is also observed in recent experiments.

1) Sato, Y. et al. Abrupt change of the superconducting gap structure at the nematic critical point in FeSe1-xSx. Proc. Natl Acad. Sci. 115, 1227??1231 (2018).

2) Hanaguri, T. et al. Two distinct superconducting pairing states divided by the nematic end point in FeSe1-xSx. Sci. Adv. 4, eaar6419 (2018).

3) ``Topologically protected ultranodal state in iron-based superonductors", S. Setty, S.

Bhattacharyya, Y. Cao, A. Kreisel and P.J. Hirschfeld,  Nat. Comm. 11, 523 (2020).