Separation of orbital contributions to the optical conductivity of BaVS${}_3$

The correlation-driven metal-insulator transition (MIT) of BaVS${}_3$ was studied by polarized infrared spectroscopy. In the metallic state two types of electrons coexist at the Fermi energy: The quasi 1D metallic transport of $A_{1g}$ electrons is superimposed on the isotropic hopping conduction of localized $E_g$ electrons. The “bad-metal” character and the weak anisotropy are the consequences of the large effective mass $m_{eff}\approx 7m_e$ and scattering rate $\mathrm{\Gamma }\ge 160$ meV of the quasi-particles in the $A_{1g}$ band. There is a pseudo-gap above $T_{MI}=69$ K, and in the insulating phase the gap follows the BCS-like temperature dependence of the structural order parameter with ${\mathrm{\Delta }}_{ch}\approx 42$ meV in the ground state. The MIT is described in terms of a weakly coupled two-band model.