Antiferromagnetic fluctuations in the metallic state of organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Br observed by Raman scattering

Charge properties of the $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]$X$ ($X$=Cl, Br) materials are well studied and are well explained within the approach of the Hubbard model, but magnetic properties leave some questions. In particular, the presence of anti-ferromagnetic (AF) fluctuations in the metallic state close to Mott Insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Br observed by NMR and other methods, but do not agree with the observed Fermi-liquid behavior. We study magnetic excitations in these materials by Raman scattering. In B$_{1g}$ polarization at temperatures below 100 K we observe a band at 500 cm$^{-1}$, assigned to two-magnon excitations basing on results for AF-ordered Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Cl. The band shows very similar intensity and position for the metallic $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Br. We discuss this result in terms of two possible interpretations, AF fluctuations vs metal-AF insulator mixed phase close to the line of the first order phase transition between Mott Insulator and metallic states.