  We show that strong spin-triplet neutron-proton interaction causes polaronic
protons to occur in neutron matter at subnuclear densities and nonzero
temperature. As the neutron density increases, proton spectra exhibit a smooth
crossover from a bare impurity to a repulsive polaron branch; this branch
coexists with an attractive polaron branch. With the neutron density increased
further, the attractive polarons become stable with respect to deuteron
formation. For two adjacent protons, we find that the polaron effects and the
neutron-mediated attraction are sufficient to induce a bound diproton, which
leads possibly to diproton formation in the surface region of neutron-rich
nuclei in laboratories as well as in neutron stars.
