Thursday 4.12.2008, 15h15-16h15, HS P.


Asymmetric Spin Ladders

D. N. Aristov, University of Karlsruhe

We consider a family of the asymmetric spin-1/2 ladders, where two coupled chains (legs) have non-equal magnetic exchange along the legs. One limiting case in this family is a usual symmetric ladder, extensively studied in the literature. Another limiting case is the single-pole ladder, which is a system of dangling spins attached to the main leg. Large scale Quantum Monte Carlo calculations were used for the investigation of various characteristics of such ladders. Ferromagnetic rung coupling leads to a formation of effective spin-1 chain, possessing topological string order parameter and the Haldane gap in the spectrum. In the case of single pole ladder the dependence of this gap on the rung coupling is very unusual and we discuss it to some detail. We propose an interpretation of the numerical findings in terms of Suhl-Nakamura interaction induced in the subsystem of dangling spins. The long-range character of such interaction leads to formation of large blocks of spins, each acting as an effective spin-1 on a decimated lattice.