Electronic and magnetic properties of orthorhombic iron selenide (FeSe)

TL;DR

This paper investigates the unique electronic and magnetic properties of orthorhombic FeSe, revealing polar multipoles, absence of conventional magnetic dipoles, and the presence of hidden magnetic order involving monopoles and quadrupoles, which differ from other iron-based superconductors.

Contribution

It introduces a new magnetic order in FeSe involving hidden magnetic multipoles, expanding understanding of its electronic structure and magnetic phenomena.

Findings

Polar multipoles contribute to diffraction patterns.

No conventional magnetic dipoles are observed during the phase transition.

Hidden magnetic order involving monopoles and quadrupoles is identified.

Abstract

Iron orbitals in orthorhombic iron selenide (FeSe) can produce charge-like multipoles that are polar (parity-odd). Orbitals in question include Fe(3d), Fe(4p) and p-type ligands that participate in transport properties and bonding. The polar multipoles may contribute weak, space-group forbidden Bragg spots to diffraction patterns collected with x-rays tuned in energy to a Fe atomic resonance (Templeton & Templeton scattering). Ordering of conventional, axial magnetic dipoles does not accompany the tetragonal-orthorhombic structural phase transition in FeSe, unlike other known iron-based superconductors. We initiate a new line of inquiry for this puzzling property of orthorhombic FeSe, using a hidden magnetic-order that belongs to the m'm'm' magnetic crystal-class. It is epitomized by the absence of ferromagnetism and axial magnetic dipoles, and the appearance of magnetic monopoles and magneto-electric quadrupoles. A similar magnetic order occurs in cuprate superconductors, YBCO & Hg1201, where it was unveiled with the Kerr effect and in Bragg diffraction patterns revealed by polarized neutrons. PACS number 75.25.-j