Hot electron driven structural expansion and magnetic collapse in bilayer FeSe

Quantum phenomena emerging from the interaction of light and matter in low-dimensional systems hold great potential for future quantum technologies. Here using finite-temperature density functional calculations incorporating nonlocal 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 nonmagnetic, as observed experimentally, double-layer FeSe exhibits robust antiferromagnetic order at low temperatures with a net site magnetization of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>∼</a:mo><a:mn>2.75</a:mn><a:mspace width="0.28em"/><a:msub><a:mi>μ</a:mi><a:mi>B</a:mi></a:msub></a:mrow></a:math>/Fe. Increasing the density of photoexcited electrons systematically enhances the internal electronic entropy, leading to a complete collapse of the 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.