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Electronic and magnetic properties of orthorhombic iron selenide (FeSe)

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