Fluctuation effects in low-dimensional spin-Peierls systems: Theory and experiment.

Abstract

A distinctive feature of a large variety of phase transitions in quasi-one-dimensional compounds is their broad regime of 1D fluctuations precursor to the true critical point. Peierls or charge-density-wave systems stand as classical examples for which 1D lattice softening found by either x-ray or neutron scattering experiments has a marked influence on electronic spin degrees of freedom well above the transition [1]. Surprisingly, it is only very recently that similar effects were observed in insulating quasi-1D spin-Peierls (SP) systems. The organic series [2] sBCPTTFd2X (BCPTTF stands as benzocyclopentyltetrathiafulvalene, X ­ AsF6, PF6) and the cuprate compound [3] CuGeO3, for example, can be considered among the first few spin-Peierls systems for which 1D fluctuations effects are clearly visible in x-ray diffraction and magnetic susceptibility measurements. Correspondingly, despite an apparent similarity existing between the Peierls and spin-Peierls instabilities, there is so far no theoretical description of coupled lattice and spin fluctuations in the spin-Peierls case [4]. In this Letter, we propose a microscopic treatment for such fluctuation effects combining the renormalization group (RG) and functional-integral methods. In the Jordan-Wigner (JW) fermion representation of spins, we first show that