Phase Diagram and Soliton Picture of an Ideal Spin-Peierls Compound D-F$_{5}$PNN

We investigate magnetic properties of an $S$=1/2 quasi-one dimensional organic antiferromagnet, D-F$_{5}$PNN using magnetization measurements taken at temperatures as low as 0.5 K. Three distinct phases were observed consisting of uniform, dimerized (D), and incommensurate (I) spin structures in the magnetic field versus temperature plane, where a significant hysteresis appears between D-I transitions in the field scan measurements. A combination of magnon ($S$=1) and soliton ($S$=1/2) excitations have successfully reproduced the observed magnetic susceptibility. In addition, such excitations provide a reasonable interpretation of the temperature dependent electron spin resonance (ESR) spectra. By comparison with the theoretical study, we conclude that D-F$_{5}$PNN is an ideal compound for investigating the spin-Peierls transition.