Far infrared conductivity of charge density wave materials and the oxygen isotope effect in high-T(sub c) superconductors

摘要

The far infrared reflectance and conductivity of (Ta(sub 1-x)Nb(sub x)Se(sub 4))(sub 2)I and TaS(sub 3) 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 (TaSe(sub 4))(sub 2)I and K(sub 0. 3)MoO(sub 3). The measurements cover frequencies from 3 to 700cm(sup (minus)1) and the temperature range from 15K to 300K. In the niobium-doped alloys (Ta(sub 1-x)Nb(sub x)Se(sub 4))(sub 2)I, the size and frequency of the giant infrared mode remain nearly constant as the impurity concentration x is increased. For TaS(sub 3), the pinned acoustic phason near 0.5cm(sup (minus)1) dominates (var epsilon)((omega)) and an additional small mode lies near 9cm(sup (minus)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(sub 0.3)MoO(sub 3) has been measured at 1.2K 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 YBa(sub 2)Cu(sub 3)O(sub 7) has been determined by substitution of (sup 18)O for (sup 16)O. A series of cross-exchanges was performed on high-quality polycrystalline specimens to eliminate uncertainties due to sample heat treatments and sample inhomogeneities.