Ground State Phases of the Doped 4-Leg t-J Ladder
  Using density matrix renormalization group techniques, we have studied the ground state of the 4-leg t-J ladder doped near half-filling. Depending upon J/t and the hole doping x, three types of ground state phases are found: (1) a phase containing d_{x^2-y^2} pairs; (2) a striped CDW domain-wall phase, and (3) a phase separated regime. A CDW domain-wall consists of fluctuating hole pairs and this phase has significant d_{x^2-y^2} pair field correlations. 
Steven R. White (University of California, Irvine) and D.J. Scalapino   (University of California,Santa Barbara)