Optical properties of the quasi-two-dimensional dichalcogenides 2H-TaSe2 and 2H-NbSe2

We present a comprehensive analysis of the optical constants of the two- dimensional dichalcogenide materials 2H- TaSe2 and 2H- NbSe2, in an attempt to address the physics of two- dimensional correlated systems. The title compounds were studied over several decades in frequency, from the far- infrared to the ultraviolet. Measurements with linearly polarized light have allowed us to obtain both the in- plane and out- of- plane components of the conductivity tensor. Although the electromagnetic response of dichalcogenides is strongly anisotropic, both the in- plane and out- of- plane components of the conductivity tensor share many common features, including the presence of a well-defined metallic component, as well as a " mid- infrared band". We discuss the implications of these results in the context of the spectroscopic results of other classes of low- dimensional conductors such as the high- temperature superconducting cuprates. In particular, the analysis of the redistribution of the spectral weight as a function of temperature, as well as the behavior of the quasiparticles relaxation rate, points to significant distinctions between the charge dynamics of dichalcogenides and other classes of low dimensional conductors.