Phase diagram of the frustrated asymmetric ferromagnetic spin ladder

We present a systematic investigation on the ground state of an asymmetric two-leg spin ladder (where exchange couplings of the legs are unequal) with ferromagnetic (FM) nearest-neighbor interaction and diagonal anti-ferromagnetic frustration using the density matrix renormalization group method. When the ladder is strongly asymmetric with moderate frustration, a magnetic canted state is observed between an FM state and a singlet dimerized state. The phase boundaries are dependent on the asymmetric strength. On the other hand, when the asymmetric strength is intermediate, a so-called spin-stripe state (spins align parallel on the same legs, but antiparallel on rungs) is discovered, and the system experiences a first-order phase transition from the FM state to the spin-stripe state upon increasing frustration. Numerical evidence is presented for interpretation of the phase diagram in terms of frustration and the asymmetric strength.