Optical Properties of the Charge Density Wave Materials Tantalum-Trisulfide and Zirconium-Tritelluride

It has been demonstrated that TaS(,3) is an excellent example of a material that undergoes a metal-semiconductor transition at low temperature due to the formation of a charge density wave and the opening of an energy gap at the Fermi surface, resulting from a Peierls lattice distortion. This distortion is a consequence of the quasi-one dimensional character of TaS(,3) which exhibits highly anisotropic conductivity. The steep resistance versus temperature behavior below the transition temperature allows for the sample to be used as a bolometer such that a measurement of the power absorbed versus incident photon energy (or frequency) can be made to determine the edge of the Peierls energy gap. The results of this experiment have shown that orthorhombic TaS(,3) is a simpler Peierls semiconductor than previously suggested. The gap energy at 96(DEGREES)K is slightly greater than twice the activation energy (400cm(' -1)), implying that the Fermi level is near the center of the gap and not pinned to the gap edge by impurities. Although the gap exceeds the mean field estimate by a factor (TURN)2, this is less than would be expected for a strictly one dimensional system. Despite the linear chain structure of ZrTe(,3), its conductivity is quasi-two dimensional. At a transition temperature of 63(DEGREES)K it undergoes a phase transition which most strongly affects the conductivity component perpendicular to the conducting chains, which is not typical behavior for other transition metal trichalcogenides. We have measured the room temperature polarized reflectivity in order to help understand the two dimensional semimetallic properties of ZrTe(,3). Our measurements show low Drude edges (<1eV) for both polarizations, but high values for the core dielectric constants imply unscreened plasma energies consistent with the large carrier densities ( p = n = 10('21)/cm('3)) inferred from Hall measurements. The optical scattering time is anisotropic, but also consistent with the dc conductivity, implying reasonable values of the effective masses. An oscillator at 0.435eV is identified with the direct p-d gap, suggesting that the overlap is small (<0.1eV).