Structural fluctuations and spin-peierls transitions revisited

Abstract

Pretransitional structural fluctuations have been measured in the organic compounds (BCPTTF)2X (X = PF6, AsF6) and MEM(TCNQ)2, which undergo spin-Peierls phase transitions at TSP=36 K, 32.5 K and 18 K respectively. In the (BCPTTF)2X materials, the structural fluctuations have been measured up to TF=100 K and 120 K respectively, temperatures at which the in-chain correlation length almost amounts to the inter-spin distance. In the AsF6 compound, accurate measurements show that above 60 K these fluctuations are quasi one-dimensional. Consistently, magnetic susceptibility measurements show deviaations from the Bonner-Fisher law below TF, that we interpret as caused by a local pairing of the spins due to the lattice critical fluctuations. In contrast MEM(TCNQ)2, where the magnetic susceptibility is known to perfectly follow the Bonner-Fisher law down to TSP, exhibits tridimensional critical effects only 20 K above TSP. These results show that the (BCPTTF)2X compounds exhibit a true spin-Peierls transition triggered by one-dimensional fluctuations while an unconventional spin-Peierls transition occurs in MEM(TCNQ)2.