Far Infrared ESR Study of Spin-Peierls Compound MEM(TCNQ)2

Abstract
Electron spin resonance experiments have been performed on spin-Peierls compound MUM(TCNQ) 2 at far infrared (H 0 =255 kOe) and X -band( H 0 =3.3 kOe) frequencies, with a special interest in the predicted high field incommensurate phase. The low field ESR revealed a rapid increase of the resonance width below the spin-Peierls transition temperature T sp =17.8 K, while no change was observed in the high field experiments down to 4.2 K within the present experimental resolution Δ H res ≃50 Oe (Δ H res / H 0 ≃2×10 -4 ). It is most likely that the predicted incommensurate phase does not exist, and that the high temperature uniform phase survives at high field region down to the lowest temperature studied.