Anisotropic ab-plane optical response of the charge-density-wave superconductor P₄W₁₄O₅₀

We report the polarized optical reflectance of P₄W₁₄O₅₀, the only charge-density-wave (CDW) superconductor in the monophosphate tungsten bronze family, over a wide frequency range. The 300-K infrared spectra are dominated by two-dimensional behavior: high reflectance with weak phonons in the conducting ab plane and low reflectance with strong vibrational modes along the interlayer direction anisotropic intra-t_2g d→d excitation (∼ 10000 cm^-1) reveals the "hidden" one-dimensional band structures in P₄W₁₄O₅₀. The variable temperature ab-plane spectra clearly show a suppression of the optical conductivity along the b axis below 140 K, giving rise to charge localization and anisotropic CDW gap formation at ∼1400 cm^-1. Although the oscillator strength is redistributed among the free-carrier response, the CDW gap absorption, and the d→d transition in the density wave states, the spectral weight is largely conserved below the plasma frequency. The ab-plane phonons become well resolved in the CDW states due to reduced screening. Based upon these observations, P₄W₁₄O₅₀ is another example of a superconductor with an unusual normal state.