Femtosecond Manipulation of Two-Dimensional Ruderman-Kittel-Kasuya-Yasuda Interaction

The interaction between magnetism and itinerant electrons leads to the broken time-reversal symmetry for the key electronic states in topological insulators, enabling dissipationless and spin-polarized quantum transport. This interaction fundamentally determines the operational temperature scale for topological electronics. Here we use a unique combination of time-resolved photoemission spectroscopy and time-resolved magneto-optical Kerr effect measurements to elucidate the simultaneous evolutions of the electronic and magnetic subsystems in an intrinsic magnetic topological insulator MnBi₂Te₄. Our experiments selectively reveal the details of the 2D Ruderman-Kittel-Kasuya-Yasuda (RKKY) interaction on the material surface; our theoretical model quantitatively reproduces the demagnetization time scale as well as the order-of-magnitude for the exchange gap quenching. This distinct 2D RKKY mechanism offers a direct explanation for the sizable gap in the quasi-2D electronic state and for the nonzero residual magnetization in even-layer MnBi₂Te₄. Furthermore, it promises a unique ultrafast path to effectively manipulate magnetism and topological orders through the p-d interaction and paves the road toward future topotronic devices.