Electron-electron interaction induced effective mass suppression in bilayer graphene

The effective mass of carriers m* captures fundamental properties of a material. In a two-dimensional electron system, the electron-electron (e-e) interaction manifests in the renormalization of m*. Extending previous studies[1] to lower carrier densities, where the interaction effect is expected to be stronger, we present precision measurements of the electron and hole effective mass m$_e$* and m$_h$* in high-quality ($\mu\sim$30,000cm$^2$/Vs) hexagonal boron nitride supported bilayer graphene using temperature-dependent Shubnikov-de Hass oscillations. Our measurements probe carrier densities down to 2$\times$10$^{11}$/cm$^2$. Comparison to tight-binding bands and previous data shows excellent agreement at carrier densities above 5$\times$10$^{11}$/cm$^2$, where m$_e$* and m$_h$* can be well described by a renormalized Fermi velocity of v$_F$= 1.11$\times$10$^6$m/s. At lower carrier densities, m$_h$*continuously decreases from the tight-binding band value, reaching m$_h$*=0.0234m$_e$ at n= 2$\times$10$^{11}$/cm$^2$. This corresponds to a suppression of 30\% and an increased v$_F$=1.37$\times$10$^6$m/s. The deviation is much smaller for electrons. We compare our results with theory and discuss its implications. [1] K. Zou, X. Hong, and J. Zhu, Phys. Rev. B 84, 085408 (2011).