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The physics of a two leg spin-ladder with frustrating interactions along the
diagonals is presented in two physical regimes.
The first limit is that of the ladder with the frustrating bonds along the diagonals and those along the legs being of equal strength. In this parameter regime the model, in the presence of a magnetic field, is exactly solvable for the ground state in a substantial region of the parameter space for any value of the spin. In fact, the ladder is shown to be a special case of a large class of models defined on bipartite lattices in any dimension. The ground states can be shown to be very simple product states of total spins along the rungs, which are conserved quantities. These exact solutions and their analysis [1] leads to macroscopic magnetisation jumps connected by plateaus. Interesting first order transitions on a plateau driven by an interplay of the effects of the magnetic field and frustration are reported. The second limit studied is this system in the ferrimagnetic regime, with alternating rungs having spin-1 and spin-1/2 sites. Using DMRG, exact diagonalisation, and spinwave theory we describe phase transitions as the strength of the interaction along the diagonals is increased. The underlying motivation is to study the effects of quantum fluctuations and frustration on a ferrimagnetic system. We find two phase transitions in the model. The first is from a gapped singlet phase to a short ranged phase (which replaces the classical canted phase) at low strength of frustration. The second transition for strong bonds along the diagonals is to a ferrimagnetic phase. Details of numerical data from an ongoing study [2] on large systems (~L=100 rungs) to analyse the transitions will be presented. [1] V. R. Chandra and N. Surendran, Phys. Rev. B, 74, 024421 (2006) [2] V. R. Chandra, N. B. Ivanov, J. Richter (in preparation) |
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