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A new theory of the spin-Peierls transition with special relevance to the experiments on TTFCuBDT

Cross, M. C. and Fisher, Daniel S. (1979) A new theory of the spin-Peierls transition with special relevance to the experiments on TTFCuBDT. Physical Review B, 19 (1). pp. 402-419. ISSN 0163-1829.

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We develop a new theory of the spin-Peierls transition in spin-½ Heisenberg chains, treating the phonons in a mean-field random-phase approximation (RPA) as in previous work, but calculating the relevant response functions of the spin system using the procedure of Luther and Peschel. We show that the RPA on the phonons (and therefore our whole calculation) should be good for the experimentally important system tetrathiafulvalenium bis-cis- (1,2-perfluoromethylethylene-1,2-dithiolato)-copper (TTFCuBDT). It is also exact for a model system in which planes of atoms perpendicular to the chains are constrained by lattice forces to move together: we have derived some exact results for the spin-Peierls transition in this model system. We find a new linear dependence of the transition temperature Tc on the spin-phonon coupling, and an enhancement of Tc and also of the rate of phonon softening above Tc. Predictions of some other signatures of the transition, such as the specific-heat jump ratio, are however not much changed from previous work. Exact results are found for the leading dependence of the ground-state energy E and gap Δ in the excitation spectrum on the lattice distortion δ: E∝δ^(4/3), Δ∝δ^(2/3).

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Additional Information:©1979 The American Physical Society Received 8 June 1978 The authors thank W. F. Brinkman for bringing this topic to their attention; D. E. Moncrton for discussions of the experimental results, both published and unpublished; and T. M. Rice, P. A. Lee, and B. I. Halperin for many invaluable suggestions. One of us (DSF) wishes to thank Bell Laboratories for its hospitality during the time most of this research was carried out and the NSF for support via a Post-Graduate Fellowship.
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Deposited On:09 Oct 2006
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