Large Exchange Coupling Between Localized Spins and Topological Bands in MnBi₂ Te₄.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Phenomena

Background:

• Topological materials with magnetism exhibit quantized transport phenomena.
• Emergence of these phases depends on strong spin-band interaction and exchange gap formation.
• Experimental quantification of this interaction in intrinsic magnetic topological materials is lacking.

Purpose of the Study:

• Quantify the interaction between localized spins and topological bands in MnBi2Te4.
• Investigate the dynamics of this interaction on ultrafast timescales.
• Validate materials-by-design strategies for manipulating topological phases.

Main Methods:

• Optical excitation of Bi-Te p states in MnBi2Te4.
• Ultrafast electron scattering and magneto-optic measurements.
• Resonant X-ray scattering and atomistic simulations.

Main Results:

• Demagnetization of topological bands via electron-phonon scattering at picosecond timescales.
• Concurrent disordering of Mn 3d spins, despite energetic decoupling.
• Exchange coupling is at least 100 times stronger than superexchange, implying an exchange gap of at least 25 meV.

Conclusions:

• Quantified the spin-band interaction in MnBi2Te4, a key step for magnetic topological materials.
• Demonstrated concurrent spin dynamics between topological bands and localized spins.
• Validated the materials-by-design approach for ultrafast control of topological phases.