Quantum Phases Of Frustrated Two-Leg Spin-1/2 Ladders With Skewed Rungs

The quantum phases of two-leg spin-1/2 ladders with skewed rungs are obtained using exact diagonalization of systems with up to 26 spins and by density-matrix renormalization-group calculations to 500 spins. The ladders have isotropic antiferromagnetic (AF) exchange J(2) > 0 between first neighbors in the legs, variable isotropic AF exchange J(1) between some first neighbors in different legs, and an unpaired spin per odd-membered ring when J(1) >> J(2). Ladders with skewed rungs and variable J(1) have frustrated AF interactions leading to multiple quantum phases: AF at small J(1), either ferromagnetic or AF at large J(1), as well as bond-order-wave phases or reentrant AF (singlet) phases at intermediate J(1).