TEMPERATURE-DEPENDENCE OF SPIN RELAXATION IN QUASI-ONE-DIMENSIONAL ORGANIC METALS

A study of the temperature dependence of the spin-relaxation rate is presented for various organic conductors in which the spin flip occurs via spin-orbit coupling during an electron-phonon scattering process. The temperature dependence is in no way universal; some compounds such as TTF-TCNQ, TSeF-TCNQ, and HMTTF-TCNQ exhibit an increasing linewidth with decreasing temperature, while others such as TMTTF-TCNQ and TMTSeF-DMTCNQ (the last one over most of the temperature range) exhibit a decreasing linewidth with decreasing temperature. The existence of these very different temperature dependences is interpreted in terms of the existence of at least two competitive processes: (i) Peierls fluctuations on one hand tending to increase the rate of spin-flip scattering with the increase in the amplitude of fluctuations. This effect will tend to increase the linewidth with decreasing temperature, and (ii) the decrease of the number of available states into which the electron can be scattered (as evidenced by the decreasing magnetic susceptibility with decreasing temperature) which tends to reduce the rate of spin flip with decreasing temperature. The actual temperature dependence of the spin relaxation rate depends on which of these two competing mechanisms prevails. The supportive experimental evidence in favor of the role of Peierls fluctuations in spin relaxation is the following: (a) The temperature at which the linewidth reaches its maximum value, for all the compounds for which the linewidth increases with decreasing temperature, is the temperature at which the phase transition occurs. (b) The effects due to doping either the donor or acceptor stack on the temperature dependence of the linewidth are consistent with the effects of doping on the growth of the correlation lengths. © 1979 The American Physical Society.