Classical mechanics at T=0: Mobility of negative ions in ³He

We study the dynamics of a heavy particle in a Fermi liquid and find localization transition in the strong-coupling limit in a homogeneous system. The ground state is a wave packet of size <1/k_F. In the localized phase the particle behaves as a quantum-mechanical object at short times (Γt≪1) and distances (k_Fx≪1). Classical behavior is found at T=0 in the long-time and large-distance limit due to the destruction of phase correlations in the system. The particle has a temperature-independent mobility when a uniform field is applied. Our results explain the observed constant mobilty of negative ions in liquid ³He.