Magnon-magnon interactions in the Spin-Peierls compound CuGeO3
Abstract
In a magnetic substance the gap in the Raman spectrum, DeltaR, is approximatively twice the value of the neutron scattering gap, DeltaS, if the the magnetic excitations (magnons) are only weakly interacting. But for CuGeO3 the experimentally observed ratio DeltaR/DeltaS is approximatively 1.49-1.78, indicating attractive magnon-magnon interactions in the quasi-1D Spin-Peierls compound CuGe3. We present numerical estimates for DeltaR/DeltaS from exact diagonalization studies for finite chains and find agreement with experiment for intermediate values of the frustration parameter alpha. An analysis of the numerical Raman intensity leads us to postulate a continuum of two-magnon bound states in the Spin-Peierls phase. We discuss in detail the numerical method used, the dependence of the results on the model parameters and a novel matrix-element effect due to the dimerization of the Raman-operator in the Spin-Peierls phase.
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