Hot Electron-Driven Structural Expansion and Magnetic Collapse in Bilayer FeSe

Abstract

Quantum phenomena emerging from the interaction of light and matter in low-dimensional systems hold great potential for future quantum technologies. Here, using first-principles calculations incorporating non-local van der Waals interactions and Hubbard corrections, we report simultaneous structural expansion and magnetic collapse in bilayer FeSe induced by photoexcited hot electrons. Our calculations reveal that, while bulk FeSe is paramagnetic, as observed experimentally, double-layer FeSe exhibits robust {\it staggered} antiferromagnetic order at low temperatures with a net site magnetization of $\sim 2.75~μ_B$/Fe. However, increasing the density of photoexcited electrons systematically enhances the internal electronic entropy, leading to a complete collapse of antiferromagnetic order accompanied by an abrupt expansion of the interlayer separation. Our findings suggest the structural and magnetic properties of FeSe thin films can be finely tuned via ultrafast laser excitation, offering a pathway to control quantum phases in iron-based compounds through electronic temperature.

Figures (6)

• Figure 1: Double layer FeSe shows metallic antiferromagnetic properties at low temperature. Photo excited electrons causes an antiferromagnetic to paramagnetic phase transition and simultaneously the interlayer separation increases due to hot electron thermal pressure. Iron and selenium atoms are shown by red and yellow spheres, respectively.
• Figure 2: Free energy contour plot of tetragonal FeSe crystal as a function of lattice parameters obtained by GGA+$U$ with U=1, vdW, and GGA. The white triangle shows the minimum of the fitted polynomial function. Similar plots using GGA+$U$ with $U=$4 and 6 are reported in the Supplementary Materials Suppl.
• Figure 3: (Up left panel) Optimized interlayer separation of double-layer FeSe obtained with vdW, GGA, and GGA$+U=1$ methods. Three systems of not-spin-polarised, antiferromagnetism with linear spin configuration, and antiferromagnetism with stagger spin configuration are considered. The appearance of magnetic ordering increases the distance between two layers. (Upright panel) The energy difference between the studied phases as a function of interlayer separation. The phase diagram is obtained using GGA+$U$ with $U=1$. The antiferromagnetic phase with stagger spin configuration has the lowest energy. The spin-up and spin-down iron atoms are highlighted by red and cyan colours, respectively. (Bottom left panel) The $d$-orbital of spin-up-Fe atom projected density of states (pDOS) for liner and stagger antiferromagnetic configurations. The inset shows the pDOS near the Fermi energy. (Bottom right panel) The percentage of Löwdin atomic charge of Fe atom located on a layer of double-layer FeSe. GGA+$U$ with $U$=1 are used in both calculations.
• Figure 4: The stagger and linear spin configurations of Fe atoms in each layer of double-layer FeSe. The spin-spin first (1$^\text{st}$) and second (2$^\text{nd}$) nearest-neighbour (NN) exchange interactions are represented by $\text{J}_1$, and $\text{J}_2$, respectively. E$_\text{S}$ and E$_\text{L}$ are ground state Ising energy of stagger and linear spin configurations, respectively.
• Figure 5: The Helmholtz free energy $F$ as a function of interlayer separation $d$ of anti-ferromagnetic double-layer FeSe with stagger spin configuration. The system is fully optimized at electronic temperatures $T_{el} = 0.34, 0.68, 0.816, 1.02, 1.36 ~eV$. The minimized $d$ is obtained by fitting a polynomial on the data points which are calculated using GGA+$U$ with $U=1$. The last plot on the right side shows the absolute magnetization $|M|$ and the electronic entropy $TS$ per iron atom as a function of the electronic temperature T$_{el}$.