  The spinon and holon excitations of two-leg Heisenberg or lightly doped t-J
ladders are shown to be bound in pairs by string confinement forces given
approximately by the antiferromagnetic exchange energy across the rungs. These
forces originate from the fictitious flux tubes associated with the half-fermi
statistics of the excitations. It is conjectured that similar confinement
forces, determined by the antiferromagnetic exchange energy across the layers,
are responsible for the spin gap and the pairing of charge carriers in CuO
superconductors.
