Evolution of the Spin Gap Upon Doping a 2-Leg Ladder
  The evolution of the spin gap of a 2-leg ladder upon doping depends upon the nature of the lowest triplet excitations in a ladder with two holes. Here we study this evolution using various numerical techniques for a t-t'-J ladder as the next-near-neighbor hopping t' is varied. We find that depending on the value of t', the spin gap can evolve continuously or discontinuously and the lowest triplet state can correspond to a magnon, a bound magnon-hole-pair, or two separate quasi-particles. Previous experimental results on the superconducting two-leg ladder Sr12Ca2Cu24O41 are discussed. 
D. Poilblanc & O. Chiappa (CNRS Toulouse), J. Riera (Univ. Rosario),   S.R. White (UC Irvine), D.J. Scalapino (UCSB)