Anisotropic Optical Spectra ofPrBa2Cu4O8: Possible Tomonaga-Luttinger Liquid Response of the Quasi-One-Dimensional Metallic CuO Double Chains

The optical spectra of ${\mathrm{PrBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ show large in-plane anisotropy. For the $a$ polarization ( $\mathbf{E}\perp \mathrm{chain}$), the spectrum is characterized by a gap of 1.4 eV, indicating the charge-transfer insulating nature of the ${\mathrm{CuO}}_2$ planes. For the metallic chain direction ( $\mathbf{E}\parallel b$), the spectrum deviates from a simple Drude response; reflectivity $R_b\left(\omega \right)$ shows a sharp edge at $\sim 1\mathrm{eV}$ but it also shows a dip at $\sim 15\mathrm{meV}$, which splits the conductivity spectrum into two parts—a zero-energy mode with small weight and a pronounced $40\mathrm{meV}$ mode. These features are discussed in terms of a Tomonaga-Luttinger liquid in a doped 1D Mott insulator and compared with 1D Bechgaard salts.