Gate-Tunable Renormalization of Spin-Correlated Flat-Band States and Bandgap in a 2D Magnetic Insulator

Emergent quantum phenomena in two-dimensional van der Waal (vdW) magnets are largely governed by the interplay between exchange and Coulomb interactions. The ability to precisely tune the Coulomb interaction enables the control of spin-correlated flat-band states, band gap, and unconventional magnetism in such strongly correlated materials. Here, we demonstrate a gate-tunable renormalization of spin-correlated flat-band states and bandgap in magnetic chromium tribromide (CrBr₃) monolayers grown on graphene. Our gate-dependent scanning tunneling spectroscopy (STS) studies reveal that the interflat-band spacing and bandgap of CrBr₃ can be continuously tuned by 120 and 240 meV, respectively, via electrostatic injection of carriers into the hybrid CrBr₃/graphene system. This can be attributed to the self-screening of CrBr₃ arising from the gate-induced carriers injected into CrBr₃, which dominates over the weakened remote screening of the graphene substrate due to the decreased carrier density in graphene. Precise tuning of the spin-correlated flat-band states and bandgap in 2D magnets via electrostatic modulation of Coulomb interactions not only provides effective strategies for optimizing the spin transport channels but also may exert a crucial influence on the exchange energy and spin-wave gap, which could raise the critical temperature for magnetic order.