Optical spectroscopy evidence of a second-order Mott phase transition

By means of Raman scattering and infrared reflectivity spectroscopies under high pressure, we investigate the metallic phase induced by pressure at Pcr = 17.5 GPa in the Mott insulator (NaMn3)Mn4O12 (NMO). Upon approaching Pcr, two signatures of the presence of mobile carriers become increasingly pronounced: a Fano-like lineshape of the Raman modes and a Drude-like peak in the optical conductivity spectra. Remarkably, all Raman and infrared modes display a continuous evolution with pressure up to 22 GPa, indicating that no structural distortion occurs at the transition owing to the very compact quadruple perovskite structure of NMO. We therefore propose a unique scenario of a second-order Mott phase transition where only the electronic degrees of freedom are active, which opens the possibility of probing the effective Hamiltonian of the transition disentangled from the lattice degrees of freedom.