FAR-INFRARED ELLIPSOMETRY USING SYNCHROTRON-RADIATION - THE OUT-OF-PLANE RESPONSE OF LA2-XSRXCUO4

We coupled a home-made rotating analyzer ellipsometer to the Fourier-s pectrometer of the U4IR beamline at the National Synchrotron Light Sou rce in Brookhaven. The set-up is designed to operate in the spectral r ange from 30 to 700 cm(-1). The higher brightness of the synchrotron c ompared to conventional light sources allows us to measure on small su rfaces (approximate to 1 x 1 mm(2)). Furthermore. the opening angle of the light focused on the sample can be kept at a minimum, which is a requirement for ellipsometric measurements with high accuracy. We appl y this new technique to determine the out-of-plane response of the hig h-T-c system La2-xSrxCuO4 with dopant concentrations ranging from the underdoped to the overdoped regime (x = 0.1; 0.12; 0.15; 0.18 and 0.2) , The out-of-plane response of these compounds contains contributions from the c-polarized infrared active phonons as well as from free char ge carriers. No anomalies attributed to superconductivity are seen for any of the observed phonons. The low frequency plasmon appearing in t he superconducting state is discussed in terms of a Josephson-plasmon in a layered superconductor. (C) 1998 Elsevier Science S.A.