Anisotropicab-plane optical response of the charge-density-wave superconductorP4W14O50

Abstract

We report the polarized optical reflectance of ${\mathrm{P}}_{4}{\mathrm{W}}_{14}{\mathrm{O}}_{50},$ 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 $\mathrm{ab}$ plane and low reflectance with strong vibrational modes along the interlayer direction. An anisotropic intra-${t}_{2g}\stackrel{\ensuremath{\rightarrow}}{d}d$ excitation $(\ensuremath{\sim}10000 {\mathrm{cm}}^{\ensuremath{-}1})$ reveals the ``hidden'' one-dimensional band structures in ${\mathrm{P}}_{4}{\mathrm{W}}_{14}{\mathrm{O}}_{50}.$ The variable temperature $\mathrm{ab}$-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 $\ensuremath{\sim}1400 {\mathrm{cm}}^{\ensuremath{-}1}.$ Although the oscillator strength is redistributed among the free-carrier response, the CDW gap absorption, and the $\stackrel{\ensuremath{\rightarrow}}{d}d$ transition in the density wave states, the spectral weight is largely conserved below the plasma frequency. The $\mathrm{ab}$-plane phonons become well resolved in the CDW states due to reduced screening. Based upon these observations, ${\mathrm{P}}_{4}{\mathrm{W}}_{14}{\mathrm{O}}_{50}$ is another example of a superconductor with an unusual normal state.