Excitation spectrum of a He-3 impurity in superfluid He-4

We present a variational Monte Carlo calculation of the excitation spectrum of one He-3 impurity in superfluid He-4 over the full momentum range. It includes backflow effects to high order. This is achieved with a shadow wave function in which the phase factors of the excitation are introduced in the subsidiary variables. Energy, effective mass and strength of the single excitation peak of the impurity are computed. Above q similar or equal to 0.5 Angstrom(-1) the effective mass increases significantly with q in agreement with experiment. No roton like minimum is found and the impurity excitation brand crosses the phonon-roton spectrum at q similar or equal to 1.6 Angstrom(-1).