Millimeter-Wave Spectroscopy of the Organic spin-Peierls System   b'-(ET)2SF5CF2SO3
  The first purely organic BEDT-TTF spin-Peierls system, b'-(ET)2SF5CF2SO3, has been confirmed using a high-frequency electron spin resonance (EPR) cavity perturbation technique. The material exhibits the characteristics of a quasi-one-dimensional (1D) Heisenberg antiferromagnetic spin system above 30 K, but undergoes a second-order transition, at TSP = 33 K, to a singlet ground state, due to a progressive spin-lattice dimerization. The spin-Peierls state is evidenced by a sharp drop in the spin susceptibility below 24 K for the magnetic field parallel to each of the three principle axes (i.e. H7a, H7b, and H7c). The 1D chain axis has been identified as the crystallographic b axis from the g value analysis. The singlet-triplet gap, Ds(0) = 114 (+-21)K, was determined using a modified BCS theory. Also, we describe in some detail the millimeter-wave vector network analyzer (MVNA) for researchers who have interest in precision EPR measurements at higher magnetic fields and frequencies. 
Brian H. Ward, Isaac B. Rutel, James S. Brooks, John A. Schlueter, R.   W. Winter and Gary L. Gard