Collective and Single-Particle Excitations in Two- and Three-Dimensional ³He

Abstract

We utilize structural information from microscopic variational and diffusion Monte Carlo calculations for two- and three-dimensional ³He to calculate the dynamic structure function and the effective mass of ³He. Static effective interactions are constructed from the density- and spin structure functions using sumrules. It is shown that mugtiparticle excitations are important for understanding the dynamic structure function. We formulate a theory that contains such excitations and maintains the mo and ml 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 mugtiparticle excitations above the particle-hole continuum. The contribution of both spinand density fluctuations to the effective mass are of comparable size, spinfluctuations are somewhat more important in 2D than in 3D. Our resugts show, in agreement with experiments, a flattening of the single particle selfenergy with increasing density, which eventually leads to a divergent effective mass.