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

The two-dimensional molecular conductor $\ensuremath{\kappa}$-(${\mathrm{BETS})}₂\mathrm{Mn}{[\mathrm{N}{(\mathrm{CN})}₂]}₃$ undergoes a sharp metal-to-insulator phase transition at ${T}_{\mathrm{MI}}\ensuremath{\approx}21$ 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 $\ensuremath{\kappa}$-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 ${T}_{\mathrm{MI}}$ that evidences the presence of two distinct BETS dimers in this compound. The $\mathrm{Mn}[\mathrm{N}{(\mathrm{CN})}₂{]}₃^{\ensuremath{-}}$ 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 ${\mathrm{Mn}}²⁺$ ions via $\ensuremath{\pi}\text{\ensuremath{-}}d$ coupling and antiferromagnetic ordering within the $\ensuremath{\pi}$ 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.