Electron spin resonance investigations of organic spin chains and two-dimensional organic conductors ; Elektronspinresonanz-Untersuchungen an organischen Spinketten und zweidimensionalen organischen Leitern

In this work, detailed electron spin resonance and SQUID investigations on single crystals of the organic spin chain compounds (TMTTF)2X (X = SbF6, AsF6, BF4 and SCN) were performed to explore the spin and the charge ordered states in these materials. Furthermore, the transport and ESR measurements of the quasi-two dimensional organic superconductor Theta-(BEDT-TTF)2I3, its tempered phase (Theta)T-(BEDT-TTF)2I3, and the new synthesized organic conductor (BEDT-TTF)2(B12H12)(CH2Cl2) are presented and discussed. The temperature dependence of the spin susceptibility at constant volume (χs)v for T>100K of the investigated (TMTTF)2X (X = SbF6, AsF6, BF4 and SCN) salts can be modeled using the EAT (Eggert, Affleck and Takahashi) model for a S = 1/2 antiferromagnetic spin chain. The obtained values of the antiferromagnetic exchange constant |J| are 400 K, 410 K, 430 K and 460 K for (TMTTF)2X with X=SbF6, AsF6, BF4 and SCN, respectively. The linewidth decreases more slowly for (TMTTF)2SbF6 and (TMTTF)2AsF6 and increases by decreasing the temperature for (TMTTF)2SCN below the charge ordering transition temperature TCO. Also the anisotropy of the linewidth changes below TCO. These observations are attributed to the loss of the inversion symmetry between the TMTTF molecules along the stacking direction below TCO. Doubling in the periodicity in the orientation dependence of the linewidth along the ab'-plane is observed in the (TMTTF)2SbF6 and (TMTTF)2AsF6 salts in the charge order region. The linewidth have a minimum along the a-axis and the b'-axis and maximum at 45 degree and 135 degree between the two axes. The doubling of the periodicity in the orientation pattern can be due to the coexistence of two inequivalent magnetic TMTTF sites in the CO region. The spin Peierls transition in (TMTTF)2AsF6 (TSP = 13K) and the anion ordering transition in (TMTTF)2BF4 (TAO=41 K) were studied. Below the transition temperature of these spin-singlet ground states, the spin susceptibility decreases rapidly by lowering the temperature ...