Tunable band inversion in trilayer graphene

Displacement field control of elecronic bands in low-dimensional systems is a promising route toward engineering emergent quantum phases. We report first experimental observation of displacement-field-induced band inversion and modulation of the Berry phase of low-energy quasiparticles in high-mobility Bernal-stacked trilayer graphene (TLG). Quantum oscillation measurements reveal a striking sequence of transitions: at low displacement field D , the gullies are characterized by a Berry phase of 2⁢π and large effective mass, indicating massive fermions. As D increases, the Berry phase abruptly shifts to π and the effective mass reaches a minimum, signaling the onset of massless Dirac behavior. At higher D , the Berry phase returns to 2⁢π , and the effective mass increases again, consistent with a band inversion. These findings demonstrate a rare, reversible topological phase transition—massive → massless → massive—driven entirely by an external displacement field. Our results establish TLG as a tunable platform for nanoscale control of band topology. They establish a means to tune between massive and Dirac-like dispersions dynamically providing a foundation for exploring field-switchable topological phenomena in layered two-dimensional systems.