Profiles of magnons and spinons in a spin ladder

We have studied quantum phase transitions between magnetization plateaux and gapless incommensurate states in a frustrated spin ladder placed in a magnetic field \cite{Mila98}. The transitions are triggered by a condensation of magnons (spinons) at the edge of an integer (half-integer) magnetization plateau. In this talk we will describe magnetization patterns formed in a finite ladder just off a magnetization plateau, i.e. at low concentrations of magnons or spinons. In these regimes the magnons and spinons behave as noninteracting fermions carrying spins $S^z = \pm 1$ and $\pm 1/2$, respectively. The spinons also act as domain walls, thus causing a dramatically different magnetization pattern from that of the magnons. The analytical results obtained agree well with numerical data. \begin{thebibliography}{1} \bibitem{Mila98} F. Mila, Eur. Phys. J. B {\bf 6}, 201 (1998). \end{thebibliography}