▎ 摘　　要

In a two-dimensional electron gas, the electron-electron interaction generally becomes stronger at lower carrier densities and renormalizes the Fermi-liquid parameters, such as the effective mass of carriers. We combine experiment and theory to study the effective masses of electrons and holes m(e)(*) and m(h)(*) in bilayer graphene in the low carrier density regime on the order of 1 x 10(11) cm(-2). Measurements use temperature-dependent low-field Shubnikov-de Haas oscillations observed in high-mobility hexagonal boron nitride supported samples. We find that while m(e)(*) follows a tight-binding description in the whole density range, m(h)(*) starts to drop rapidly below the tight-binding description at a carrier density of n = 6 x 10(11) cm(-2) and exhibits a strong suppression of 30% when n reaches 2 x 10(11) cm(-2). Contributions from the electron-electron interaction alone, evaluated using several different approximations, cannot explain the experimental trend. Instead, the effect of the potential fluctuation and the resulting electron-hole puddles play a crucial role. Calculations including both the electron-electron interaction and disorder effects explain the experimental data qualitatively and quantitatively. This Rapid Communication reveals an unusual disorder effect unique to two-dimensional semimetallic systems.