Low-energy properties of ferrimagnetic two-leg ladders:: A Lanczos study -: art. no. 054432

We apply the Lanczos method to a two-leg ladder with mixed spins of magnitudes (S-1 S-2) = (1,1/2) located at alternating positions along the ladder. The effect of dimerization gamma is also considered according to two different patterns. A spin-wave theory (SWT) is applied to this model predicting one gapless branch with ferromagnetic properties and another gapful branch with antiferromagnetic nature as low-energy excitations of the model. We compute the ground-state energies, ferromagnetic and antiferromagnetic excitation gaps, mag netizations, and correlation functions as a function of J' and gamma which results into a fine estimate of the phase diagram. The Lanczos results are compared with the SWT analysis and a qualitative agreement is found but with numerical discrepancies. We also study numerically the spin-Peierls instability and find that it is conditional for any value of J' is an element of (0,2) and both dimerization patterns.