Electronic properties of the dimerized organic conductor $\kappa$-(BETS)$_2$Mn[N(CN)$_2$]$_3$

TL;DR

This study investigates the electronic and vibrational properties of the organic conductor $k$-(BETS)$_2$Mn[N(CN)$_2$]$_3$ across its metal-insulator transition at 21 K, revealing anisotropic optical conductivity, vibrational mode splitting, and magnetic ordering effects.

Contribution

It provides comprehensive infrared, ESR, and transport data showing the anisotropic optical response and vibrational mode splitting associated with the metal-insulator transition in this compound.

Findings

Pronounced in-plane optical anisotropy observed.

Splitting of charge-sensitive vibrational modes below $T_{MI}$.

ESR properties show significant $g$-factor shifts and increased spin susceptibility at low temperatures.

Abstract

The two-dimensional molecular conductor $\kappa$-(BETS)$_2$Mn[N(CN)$_2$]$_3$ undergoes a sharp metal-to-insulator phase transition at $T_{\rm MI}\approx$ 21 K, which has been under scrutiny for many years. We have 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 compounds. The metal-insulator transitions affects 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 $T_{\rm MI}$ 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 ESR properties are affected by the Mn$^{2+}$ 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.