Collective and single-particle excitations in two-and three-dimensional ³He

We utilize structural information from microscopic variational and diffusion Monte Carlo calculations for two- and three-dimensional ³ He to calculate the dynamic structure function and the effective mass of ³ He. Static effective interactions are constructed from the density- and spin structure functions using sumrules. It is shown that multiparticle excitations are important for understanding the dynamic structure function. We formulate a theory that contains such excitations and maintains the m₀ and m₁ sumrules. Our implementation of the theory describes, at a semi-quantitative level, the salient experimental features of S(k, ω): A lowering of the collective mode energy, a reduction of the strength of the particle-hole band, and multiparticle excitations above the particle-hole continuum. The contribution of both spin-and density-fluctuations to the effective mass are of comparable size, spin-fluctuations are somewhat more important in 2D than in 3D. Our results show, in agreement with experiments, a flattening of the single-particle self-energy with increasing density, which eventually leads to a divergent effective mass.