Dynamical domain walls and spin-Peierls order in doped antiferromagnets: Evidence from exact diagonalization of small clusters

The hole-doped two-dimensional(2D) t-J model is studied by exact diagonalization on a 5x4 cluster which, unlike the standard tilted-square clusters, can in principle accomodate an antiphase domain wall. For hole concentration 10% and J/tgreater than or equal to0.5 the ground-state energy/site is lower than the conventional tilted-square root20 x root20 cluster. In the ground state two holes form a loosely bound pair pinned to an antiphase domain wall. The dynamical density correlation function shows sharp quasiparticlelike peaks, reminiscent of the "holons" in 1D chains, which suggest the existence of solitonlike propagating domain walls. The dynamical correlation function of the bond-singlet operator has a low-energy peak structure characteristic of columnar spin-Peierls order, the dynamical spin-correlation function shows an intense and isolated "resonance peak" near (pi,pi).