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.