Optical properties of the charge-density-wave polychalcogenide compounds R<Subscript> 2</Subscript>Te<Subscript> 5</Subscript> (R=Nd, Sm and Gd)

We investigate the rare-earth polychalcogenide R<Subscript>2</Subscript>Te<Subscript>5</Subscript> (R=Nd, Sm and Gd) charge-density-wave (CDW) compounds by optical reflectivity measurements. We obtain the optical conductivity through Kramers-Kronig transformation of the reflectivity spectra. From the real part of the optical conductivity we then extract the excitation energy of the CDW gap and estimate the fraction of the Fermi surface which is gapped by the formation of the CDW condensate. In analogy to previous findings on the related RTe<Subscript>n</Subscript> (n=2 and 3) families, we establish the progressive closing of the CDW gap and the moderate enhancement of the metallic component upon chemically compressing the lattice. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2008