Weak-coupling calculation of the gap structure of doped n-leg Hubbard   ladders
  In weak coupling, the spin gap in doped, even, n-leg periodic Hubbard ladders reflects the energy to break a pair into separate quasiparticles. Here we investigate the structure of the gap within a spin-fluctuation exchange approximation. We also calculate the amplitude for removing a singlet pair from two lattice sites separated by a distance (l_x,l_y), which describes the internal structure of a pair. 
N. Bulut and D.J. Scalapino (UCSB)