Imaging Orbital-selective Quasiparticles in the Hund's Metal State of FeSe

TL;DR

This study uses quasiparticle interference to reveal orbital-selective strong correlations in FeSe, supporting the Hund's Metal state hypothesis as key to understanding iron-based high-temperature superconductivity.

Contribution

It provides experimental evidence of orbital-dependent quasiparticle weights in FeSe, confirming the Hund's Metal model in the parent state of iron-based superconductors.

Findings

Signatures of strong orbital selectivity in quasiparticle weights.

Quasiparticle interference amplitudes show Z_xy < Z_xz << Z_yz.

Orbital-selective correlations dominate FeSe's parent state.

Abstract

Strong electronic correlations, emerging from the parent Mott insulator phase, are key to copper-based high temperature superconductivity (HTS). By contrast, the parent phase of iron-based HTS is never a correlated insulator. But this distinction may be deceptive because Fe has five active d-orbitals while Cu has only one. In theory, such orbital multiplicity can generate a Hund's Metal state, in which alignment of the Fe spins suppresses inter-orbital fluctuations producing orbitally selective strong correlations. The spectral weights $Z_m$ of quasiparticles associated with different Fe orbitals m should then be radically different. Here we use quasiparticle scattering interference resolved by orbital content to explore these predictions in FeSe. Signatures of strong, orbitally selective differences of quasiparticle $Z_m$ appear on all detectable bands over a wide energy range. Further, the quasiparticle interference amplitudes reveal that $Z_{xy}<Z_{xz}<<Z_{yz}$, consistent with earlier orbital-selective Cooper pairing studies. Thus, orbital-selective strong correlations dominate the parent state of iron-based HTS in FeSe.