Abstract
The electronic structure of single-layer FeSe films on SrTiO3 presents a quandary: experimentally there is no long-range magnetic order, but the observed bands are reasonably well described by density functional calculations assuming the checkerboard antiferromagnetic (CB-AFM) ordering despite this configuration not being the calculated ground state. Here we investigate the paramagnetic nature of this system via first-principles spin-spiral calculations. Fits of the spin-spiral dispersion to spin models place this S = 1 spin system in a region of parameter space where CB-AFM quantum fluctuations lead to a magnetically disordered paramagnetic state. Modeling the paramagnetic state as an incoherent superposition of spin-spiral states arising from thermal and/or quantum fluctuations, the resulting electronic bands around the Fermi level are found to closely resemble those of the ordered CB-AFM configuration, thus providing a consistent explanation of the angle-resolved photoemission observations. These results suggest that CB-AFM fluctuations play a more important role than previously thought.
Highlights
The electronic structure of single-layer FeSe films on SrTiO3 presents a quandary: experimentally there is no long-range magnetic order, but the observed bands are reasonably well described by density functional calculations assuming the checkerboard antiferromagnetic (CB-AFM) ordering despite this configuration not being the calculated ground state
Contrary to most iron-based superconductors, this Fermi surface is characterized by electron pockets centered at the Brillouin zone (BZ) corner (M), with the zone center (Γ) states pushed below the Fermi level, posing a challenge for pairing theories relying on Fermi surface nesting between the Γ and Mcentered pockets[11, 12] through (π,π) collinear stripe antiferromagnetic (CL-AFM) spin fluctuations
Detailed comparisons of the experimental Angle-resolved photoemission spectroscopy (ARPES) data[10] for single-layer FeSe/STO to the density functional theory (DFT) calculations corresponding to non-magnetic and various ordered antiferromagnetic configurations find that the calculated bands of only the checkerboard antiferromagnetic (CB-AFM) configuration are consistent with the experimental data: both the Fermi surface and the band structure throughout the whole BZ are reasonably reproduced, when a small Hubbard U correction is included that pushes the Γ-centered hole-like band completely below the Fermi level
Summary
The electronic structure of single-layer FeSe films on SrTiO3 presents a quandary: experimentally there is no long-range magnetic order, but the observed bands are reasonably well described by density functional calculations assuming the checkerboard antiferromagnetic (CB-AFM) ordering despite this configuration not being the calculated ground state. Modeling the paramagnetic state as an incoherent superposition of spinspiral states arising from thermal and/or quantum fluctuations, the resulting electronic bands around the Fermi level are found to closely resemble those of the ordered CB-AFM configuration, providing a consistent explanation of the angle-resolved photoemission observations. O-vac are found to electron-dope the FeSe layer, and to modify the band structure of the CB-AFM state, including shifting the hole-like band around Γ down so that satisfactory agreement with the ARPES data is achieved without the addition of a phenomenological U17 This seemingly successful agreement between the ARPES data and the DFT calculated bands for the CB-AFM configuration of monolayer films of FeSe/STO, is apparently inconsistent with the fact that the CB-AFM configuration is not the calculated ground state. The Hund’s rule tendency of Fe to form local moments and the existence of magnetic order in related Fe-based superconducting materials, strongly indicate that inclusion of magnetic interactions and (local) moments are essential for a proper description of the ground state properties
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