Alternating-Spin Ladders in a Magnetic Field: New Magnetization Plateaux

Abstract

We study numerically the formation of magnetization plateaux with the Lanczos method in 2-leg ladders with mixed spins of magnitudes $(S_1,S_2)=(1,1/2)$ located at alternating positions along the ladder and with dimerization $γ$. For interchain coupling $J&#39;>0$ and $γ=0$, we find normalized plateaux at $m=1/3$ starting at zero field and $m=1$ (saturation), while when $γ\ne 0$ is columnar, another extra plateau at $m=2/3$ shows up. For $J&#39;<0$, when $γ<γ_c(J&#39;)$ we find no plateau while for $γ>γ_c(J&#39;)$ we find four plateaux at $m=0,1/3,2/3,1$. We also apply several approximate analytical methods (Spin Wave Theory, Low-Energy Effective Hamiltonians and Bosonization) to understand these findings and to conjeture the behaviour of ferrimagnetic ladders with a bigger number of legs.