Far infrared conductivity of charge density wave materials and the oxygen isotope effect in high-Tc superconductors

The far infrared reflectance and conductivity of Ta(1-x)Nb(x)(Se4)2I and TaS3 have been measured to determine the origin of a huge infrared resonance that dominates the charge density wave (CDW) dynamics along with the pinned acoustic phason mode in the related materials, (TaSe4)2I and K(0.3)MoO3. The measurements cover frequencies from 3 to 700 cm exp -1 and the temperature range from 15 to 300 K. In the niobium-doped alloys, Ta(1-x)Nb(x)(Se4)2I, the size and frequency of the giant infrared mode remain nearly constant as the impurity concentration x is increased. For TaS3, the pinned acoustic phason near 0.5 cm exp -1 dominates epsilon(omega) and an additional small mode lies near 9 cm(exp -1). The latter mode is much smaller than the infrared mode in other CDW materials. These results rule out several models of a 'generic infrared mode' in CDW excitations. They are compared in detail to the predictions of a recent theory attributing the infrared mode to a bound collective mode localized at impurity sites within the crystal. The transmittance of K(0.3)MoO3 has been measured at 1.2 K with a strong dc electric field applied across the crystal. Under these conditions, the charge density wave depins abruptly and carries large currents with near-zero differential resistance. For some samples, the low-frequency transmittance is enhanced slightly when the CDW depins. The magnitude of the oxygen isotope effect in the high-(Tc) superconductor YBa2Cu3O7 has been determined by substitution of O-18 for O-16. A series of cross-exchanges was performed on high-quality polycrystalline specimens to eliminate uncertainties due to sample heat treatments and sample inhomogeneities.