Quantum Cafe: J.C. Seamus Davis

Quantum Café is CCQ’s ongoing seminar series: open to all bona fide members of the greater NYC scientific community and held every second week, Quantum Café presents a series of informal, highly interactive talks, typically by external speakers, which present the most interesting recent developments and open questions in our field.

Title: Visualizing Orbital-Selective Mottness and Superconductivity

Abstract: In Cu-based high temperature superconductors (HTS) the undoped phase proximate to the superconductor is a robust Mott insulator. By contrast, the undope phase proximate to Fe-based HTS is never an insulator. But this striking 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 metal, sometimes referred to as an orbital-selective Mott phase or as a Hund’s Metal phase, to appear. In such a Hund’s Metal, the inter-orbital Hund’s coupling aligning the spins suppresses inter-orbital charge fluctuations. The result of this decoupling is that electrons associated with some orbitals are predicted to become strongly incoherent, while coexisting with fully coherent delocalized quasiparticles associated with other orbitals of the same atom. There are two distinguishing experimental signatures of this previously unknown Hund’s Metal phase: (1) quasiparticle spectral weights associated with each different orbital should be highly distinct, effectively resulting in orbital selective quasiparticles and, (2)Cooper pairing should become orbital-selective, meaning that the electrons of one specific orbital character should bind to form the Cooper pairs of the superconductor. We describe a campaign of quasiparticle interference visualization studies of the Fe-based HTS in FeSe, using spectroscopic imaging STM. They reveal directly the orbital selective quasiparticles in the normal state (Kostin et al, PNAS (2018)) and the highly consistent orbital-selective Cooper pairing in the superconducting state.