Electronic properties of the dimerized organic conductor κ-(BETS) 2 Mn[N(CN) 2 ] 3

The two-dimensional molecular conductor kappa-(BETS)2Mn[N(CN)2]3 undergoes a sharp metal-to-insulator phase transition at TMI approximate to 21 K, which has been under scrutiny for many years. We performed comprehensive infrared investigations along the three crystallographic directions as a function of temperature down to 10 K, complemented by electron spin resonance and dc-transport studies. The in-plane anisotropy of the optical conductivity is more pronounced than in any other kappa-type BEDT-TTF or related compound. The metal-insulator transitions affect the molecular vibrations due to the coupling to the electronic system; in addition we observe a clear splitting of the charge-sensitive vibrational modes below TMI that evidences the presence of two distinct BETS dimers in this compound. The Mn[N(CN)2]-3 layers are determined by the chain structure of the anions resulting in a rather anisotropic behavior and remarkable temperature dependence of the vibronic features. At low temperatures the electron spin resonance properties are affected by the Mn2+ ions via pi-d coupling and antiferromagnetic ordering within the pi spins: The g factor shifts enormously with a pronounced in-plane anisotropy that flips as the temperature decreases; the lines broaden significantly, and the spin susceptibility increases upon cooling, with a kink at the phase transition.