Separation of Orbital Contributions to the Optical Conductivity of ${\mathrm{BaVS}}_3$

The correlation-driven metal-insulator transition (MIT) of ${\mathrm{BaVS}}_3$ 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 $m_{\mathrm{eff}}\approx 7m_e$ and scattering rate $\Gamma \ge 160\mathrm{meV}$ of the quasiparticles in the $A_{1g}$ band. There is a pseudogap above $T_{\mathrm{MI}}=69\mathrm{K}$, and in the insulating phase the gap follows the BCS-like temperature dependence of the structural order parameter with ${\Delta }_{\mathrm{ch}}\approx 42\mathrm{meV}$ in the ground state. The MIT is described in terms of a weakly coupled two-band model.

Figure 1

Temperature dependence of the resistivity ${\rho }_{\parallel }$ (upper blue line) and the inverse magnetic susceptibility ${\chi }^{-1}$ (lower red line). Open circles corresponds to the zero-frequency extrapolation of the optical spectra according to ${\rho }_{\parallel }\equiv 1/{\sigma }_{\parallel }(\omega \to 0)$.

Figure 2

Upper panel: Reflectivity spectra of ${\mathrm{BaVS}}_3$ at temperatures below 300 K for $E\parallel c$ and $E\perp c$. Lower panel: Optical conductivity at $T=10$ and 100 K with polarization parallel (red lines) and perpendicular (black lines) to the $c$ axis. Labels show the assignment of the different contributions.

Figure 3

Low-energy optical conductivity for $E\parallel c$ (top panel) and $E\perp c$ (bottom panel). Solid circles represent the dc conductivity at the same temperatures obtained from resistivity measurements. For $T<T_{\mathrm{MI}}$ the extrapolation to estimate the gap energy is shown by dash-dotted gray lines. Note that the scales for ${\sigma }_{\parallel }$ and ${\sigma }_{\perp }$ differ by a factor of $\sim 5$.

Figure 4

Top panel: Temperature dependence of the charge gap in the insulating phase of ${\mathrm{BaVS}}_3$ as determined from the optical conductivity (solid symbols), and the structural order parameter (open symbols, on an arbitrary vertical scale) reproduced from Refs. 1, 2. The dashed line corresponds to the BCS gap profile. Bottom panel: Low-energy conductivity of the $A_{1g}$-type $\mathrm{V}(3d)$ electrons in the metallic phase of ${\mathrm{BaVS}}_3$ at $T=115\mathrm{K}$ evaluated according to ${\sigma }_{A_{1g}}={\sigma }_{\parallel }-{\sigma }_{\perp }$ (see text for details). The dashed line indicates the Drude fit.