Raman cross section of spin ladders

Abstract

We demonstrate that a two-triplet resonance strongly renormalizes the Raman spectrum of two-leg spin-ladders and moreover suggest this to be the origin of the asymmetry of the magnetic Raman continuum observed in CaV2O5. PACS. 78.30.-j Infrared and Raman spectra – 75.10.Jm Quantized spin models – 75.50.Ee Antiferromagnetics Magnetic Raman scattering is a powerful tool to investigate the total spin-zero excitations near zero momentum in low-dimensional quantum-spin systems [1]. In a recent Raman scattering study by Konstantinović and collaborators [2] a strongly asymmetric magnetic continuum, see fig. 1(b), has been observed in the spin-ladder compound CaV2O5. It has been realized by the authors of this study that the continuum defies an interpretation in terms of non-interacting two-triplet excitations as given in ref. [3]. The latter would imply two van-Hove-type intensity maxima, one at the lower and one at the upper edge of the two-triplet continuum. Noteworthy, the magnetic Raman intensity for the two-leg spin-ladder has been evaluated also by exact diagonalization (ED) [4]. Within the limitations of finite system analysis the ED results are consistent with the observed intensity if the intra-rung coupling on the ladder is assumed to be strong in CaV2O5, moreover, the ED is incompatible with the non-interacting spectra of ref. [3]. While this clearly emphasizes the relevance of interaction effects, it is unfortunate that no simple physical picture can be extracted from the ED data to allow for a direct interpretation of the measured Raman spectrum. In this brief note we clarify that the physical origin of the asymmetric Raman continuum of two-leg spin-ladders is a two-triplet bound state of total spin zero which merges with the two-triplet continuum at small wave vector to form a resonance. Our analysis is focussed on the limit of strong intra-rung coupling which is one likely scenario also for the magnetic properties of CaV2O5 [2,5]. In this limit we can profit from an exact evaluation of the two-triplet propagator which has been carried out including all twotriplet interactions in a different study of phonon-assisted two-triplet optical absorption (PTA) of spin-ladders [6]. The Hamiltonian of the two-leg spin-ladder reads

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Cite this paper

@inproceedings{Jurecka2001RamanCS, title={Raman cross section of spin ladders}, author={Christoph Jurecka and Verena Gr{\"{u}tzun and Andreas Friedrich and Wolfram Brenig}, year={2001} }