Partial ferromagnetism in the hubbard chain with next-nearest-neighbor hopping

We study the one-dimensional Hubbard model with next-nearest-neighbor hopping by means of the numerical diagonalization for finite-size clusters. It is found that the ground state is evidently partially spin-polarized in a wide region of intermediate values of on-site Coulomb repulsion, which provides the first example as a minimal model without orbital degeneracy. Analysis of correlation functions makes it clear that the partially spin-polarized states form a spin polaron. Polaron's size grows with the increasing interaction.