  The pair spin-orbit interaction (PSOI) is the spin-orbit component of the
electron-electron interaction that originates from the Coulomb fields of the
electrons. This relativistic component, which has been commonly assumed small
in the low-energy approximation, appears large and very significant in
materials with the strong SOI. The PSOI, being determined by the spins and
momenta of electrons, has highly unusual properties among which of most
interest is the mutual attraction of the electrons in certain spin
configurations. We review the nature of the PSOI in solids and its
manifestations in low-dimensional systems that have been studied to date. The
specific results depend on the configuration of the Coulomb fields in a
particular structure. The main actual structures are considered:
one-dimensional quantum wires and two-dimensional layers, both suspended and
placed in various dielectric media, as well as in the presence of a metallic
gate. We discuss the possible types of the two-electron bound states, the
conditions of their formation, their spectra together with the spin and orbital
structure. In a many-particle system, the PSOI breaks the spin-charge
separation as a result of which spin and charge degrees of freedom are mixed in
the collective excitations. At sufficiently strong PSOI, one of the collective
modes softens. This signals of the instability, which eventually leads to the
reconstruction of the homogeneous state of the system.
