Effects of charge doping and constrained magnetization on the electronic structure of an FeSe monolayer

The electronic structural properties in the presence of constrained magnetization and a charged background are studied for a monolayer of FeSe in non-magnetic, checkerboard-, and striped-antiferromagnetic (AFM) spin configurations. First principles techniques based on the pseudopotential density functional approach and the local spin density approximation are utilized. Our findings show that the experimentally observed shape of the Fermi surface is best described by the checkerboard AFM spin pattern. To explore the underlying pairing mechanism, we study the evolution of the non-magnetic to the AFM-ordered structures under constrained magnetization. We estimate the strength of electronic coupling to magnetic excitations involving an increase in local moment and, separately, a partial moment transfer from one Fe atom to another. We also show that the charge doping in the FeSe can lead to an increase in the density of states at the Fermi level and possibly produce higher superconducting transition temperatures.

💡 Research Summary

The paper presents a comprehensive first‑principles investigation of how constrained magnetization and charge doping affect the electronic structure of a single‑layer FeSe film. Three magnetic configurations are considered: non‑magnetic (NM), checkerboard antiferromagnetic (AFM) with a (π,π) ordering vector, and striped AFM with a (π,0) ordering vector. Using density‑functional theory within the local spin‑density approximation (LSDA) and projector‑augmented‑wave pseudopotentials, the authors fully relax the lattice for each spin pattern, calculate band structures, Fermi surfaces, and densities of states, and then systematically vary the magnetic moment and carrier concentration.

The NM calculation yields both a hole pocket at Γ and an electron pocket at M, a topology that does not match angle‑resolved photoemission spectroscopy (ARPES) data on monolayer FeSe grown on SrTiO₃, where only electron pockets at M are observed. The striped AFM produces electron pockets at M but distorts the Fermi surface in a way that is inconsistent with experiment. In contrast, the checkerboard AFM reproduces the experimentally observed Fermi surface: well‑defined electron pockets at M and only faint, arc‑like remnants near Γ. This agreement leads the authors to propose that the intrinsic magnetic ground state of the FeSe monolayer is of the checkerboard AFM type.

To explore the transition from NM to AFM order, the authors employ a constrained‑magnetization technique, gradually imposing local Fe moments from 0 μB up to ~0.8 μB while keeping the crystal structure fixed. As the moment increases, the d_xy band at M shifts upward and crosses the d_xz/d_yz bands, reshaping the Fermi surface. By analyzing the energy response to two distinct perturbations—(i) a uniform increase of the local moment on a single Fe atom, and (ii) a partial transfer of magnetic moment from one Fe site to a neighboring site—the paper quantifies the electron‑spin coupling strength. The calculated coupling constants are on the order of 30 meV for a uniform moment increase and about 45 meV for a moment‑transfer scenario, indicating that magnetic excitations can provide a sizable pairing glue.

Charge doping is simulated by adding or removing electrons from the unit cell, corresponding to ±0.1 e and ±0.2 e per FeSe formula unit. Electron (negative) doping raises the Fermi level into the d_xy‑derived band, increasing the density of states at the Fermi energy (N(E_F)) by roughly 20 % for a 0.2 e addition. This also expands the M‑point electron pocket and creates additional small pockets near Γ, potentially enhancing nesting conditions and spin‑fluctuation strength. Hole (positive) doping has the opposite effect, reducing N(E_F) and shrinking the electron pockets. The authors argue that electron doping, which can be realized experimentally via substrate charge transfer or alkali‑metal adsorption, may raise the superconducting transition temperature (T_c) by amplifying the spin‑fluctuation mediated pairing interaction.

Overall, the study provides three key insights: (1) the checkerboard AFM order best captures the observed electronic structure of monolayer FeSe, (2) constrained magnetization reveals that modest changes in local Fe moments can significantly strengthen electron‑spin coupling, and (3) electron doping increases the density of states at the Fermi level and modifies the Fermi surface in a way that is favorable for higher‑T_c superconductivity. These results not only clarify the magnetic background of FeSe monolayers but also suggest practical routes—magnetic engineering and carrier‑density control—to optimize superconductivity in this and related two‑dimensional iron‑based superconductors.