Optical investigation of the quasi two-dimensional monophosphate tungsten bronzes K_xP_4W_8O_(32) (x = 0 - 1.57)

摘要

The potassium doped monophosphate tungsten bronzes K_xP_4W_8O_(32) are two-dimensional metals which show a metal-to-metal transition at a critical temperature which depends on the doping level. The metal-to-metal transition is accompanied by the formation of a commensurate charge density wave with wave vector (π/b, 0) which is independent of the doping level. Undoped P_4W_8O_(32), on the other hand, has two metal-to-metal transitions which are connected to the formation of incommensurate charge density waves. We measured the infrared reflectivity of the series K_xP_4W_8O_(32) (x = 0 - 1.57) in the spectral range from 100 to 10 000 cm~(-1) for room temperature and well below the critical temperature. Polarization-dependent infrared spectra find a two-dimensional behavior in the normal and the charge density wave state and show signatures of hybridization between one- and two-dimensional conduction bands. In undoped P_4W_8O_(32) the essentials of the charge density wave state can be understood from the nesting vectors of the calculated Fermi surface and two gaps are observed in the infrared spectra. The gap sizes are a factor of about 2.5 bigger than the predictions from mean-field theory in the weak-coupling limit which suggests medium- or strong electron-phonon coupling. For potassium doped K_xP_4W_8O_(32) one gap is observed in the charge density wave state. The energetics of the charge density formation may be dominated by the energy required for the lattice modulation.