Visualizing Orbital Selective Mottness/Hundness and Cooper Pairing in FeSe

In Cu-based high temperature superconducting (HTS) materials, the undoped phase proximate to the superconductor is a robust Mott insulator. By contrast, the undoped phase proximate to Fe-based HTS is never an insulator. But this distinction may be deceptive because, while only a single Cu d-orbital is active in the former compounds, up to five Fe d-orbitals are active in the latter. Theory has long predicted that such orbital multiplicity allows an unusual new correlated state, sometimes referred to as an Orbital-selective Mott phase or as a Hund's Metal phase, to appear.

In a Hund’s Metal, the Hund coupling suppresses inter-orbital fluctuations so that electrons associated with some orbitals become strongly incoherent while coexisting with coherent delocalized electronic quasiparticles associated with other orbitals.

There are two distinguishing experimental signatures of this novel phase: (1) quasiparticle spectral weights associated with each different orbital should be highly distinct, effectively resulting in orbital selective quasiparticles and, (2) under these circumstances, Cooper pairing should become orbital-selective, meaning that the electrons of predominantly one specific orbital character bind to form the Cooper pairs.

We describe a campaign of quasiparticle interference visualization studies of the Fe-based HTS in FeSe, that reveal directly the orbital selective quasiparticles in the normal state and, from Bogoliubov quasiparticle interferences, the highly consistent orbital-selective Cooper pairs in the superconducting state.