Far infrared linear response and radio frequency nonlinear response of charge density wave conductors and high T(sub c) superconductors

The far-ir (FIR) linear response of the charge-density-wave (CDW) conductor (TaSe4)2I and polycrystalline La(sub 1.85)Sr(sub 0.15)CuO4 and La(sub 1.85) Ca(sub 0.15)CuO4 was measured at 8 to 350/cm and 5 to 300K. At low T in (TaSe4)2I, a mode with giant oscillator strength was found at 38/cm. This giant FIR mode lies between the pinned mode and the Peierls gap. It is suggested that a giant FIR mode distinct from the pinned mode is a common feature of CDW conductors. At low T in the high-T sub c superconductors, a reflectance edge was observed near 2.5k sub B T sub c. The BCS-like temperature-dependence of the reflectance edge is suggestive of an energy gap. However, a simple model shows that a BCS-like temperature dependence is also consistent with an interpretation of the reflectance edge as a low-frequency plasmon. It is not yet possible to deduce the energy gap from the FIR spectra. The RF nonlinear response of the CDW conductor NbSe3 was also measured. In combined RF- and dc-electric fields, mode-locking occurs in NbSe3. Complete mode-locking in conventional samples dramatically suppresses sliding CDW conduction fluctuations. Combined RF and dc electric fields on switching NbSe3 induced a large amplitude ac switching noise for RF frequencies greater than 1 MHz, and a period-doubling route to chaos for RF-frequencies less than 1 MHz. The mode-locking behavior of switching and nonswitching NbSe3 is analyzed. A model of CDW elasticity qualitatively reproduces all of the experimental anomalies for dc-, ac- and combined ac- and dc-electric fields. It is suggested that, during mode-locking, the number of degrees of freedom active in CDW transport is reduced.