Effective Mass of Two-Dimensional [Formula presented]

We use structural information from diffusion Monte Carlo calculations for two-dimensional [Formula presented] to calculate the effective mass. Static effective interactions are constructed from the density and spin-structure functions using sum rules. We find that both spin and density fluctuations contribute about equally to the effective mass. Our results show, in agreement with recent experiments, a flattening of the single-particle self-energy with increasing density, which eventually leads to a divergent effective mass.