  Emergent quantum phenomena in two-dimensional van der Waal (vdW) magnets are
largely governed by the interplay between the exchange and Coulomb
interactions. The ability to tune the Coulomb interaction in such strongly
correlated materials enables the precise control of spin-correlated flat-band
states, bandgap (Eg) and unconventional magnetism, all of which are crucial for
next-generation spintronics and magnonics applications. Here, we demonstrate a
giant 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 inter-flat-band spacing and bandgap of CrBr3 can be continuously tuned by
120 meV and 240 meV respectively via electrostatic injection of carriers into
the hybrid CrBr3/graphene system, equivalent to the modulation of the Cr
on-site Coulomb repulsion energy by 500 meV. This can be attributed to the
self-screening of CrBr3 arising from the gate-induced carriers injected into
CrBr3, which dominates over the opposite trend from the remote screening of the
graphene substrate. Precise tuning of the spin-correlated flat-band states and
bandgap in 2D magnets via electrostatic modulation of Coulomb interactions not
only provides new 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.
