Spin excitations carry charge currents: one-dimensional Hubbard model

The problem of spin-charge separation is addressed by considering current carrying states in a one-dimensional system of interacting electrons. Exact results for the Hubbard model and the bosonization method are exploited. We find that away from half filling both spin and charge excitations carry charge currents which are proportional to their momenta for most cases. Being in a qualitative agreement with the noninteracting ld Fermi gas picture, this result contradicts the spin-charge separation concept as it is usually derived e.g. from the bosonization approach or in a strong repulsion picture. For weak coupling the paradox is resolved by taking into account the Fermi velocity dispersion and by reexamining the structure of the current operator within the bosonization approach. For strong coupling we recover an effect of hale drag by a spinon as reflected in their momentum shifts