FAR-INFRARED C-AXIS RESPONSE OF LA(1.87)SR(0.13)CNO(4) DETERMINED BY ELLIPSOMETRY

We have measured the c-axis optical response of a large, high-quality single crystal of La1.87Sr0.13CuO4 between 30 and 700 cm(-1) in the 10 -300 K temperature range with a far-infrared Fourier ellipsometer. Sin ce ellipsometry provides both amplitude and phase information, we dire ctly obtain the complex dielectric function, thereby avoiding the erro rs introduced by the Kramers-Kronig transformation or the reference de termination. A purely algebraic treatment of our data within the two-f luid Gorter-Casimir model reveals no indication of a reduction of the normal carrier scattering rate Gamma below T-c. We obtain Gamma approx imate to 130+/-70 cm(-1) and a plasma frequency for the superconductin g carriers of omega(ps)approximate to 260+/-40 cm(-1). Thus, assuming a weak-coupling BCS gap (3.5k(B)T(c) approximate to 77 cm(-1), with T- c=31.5 K) we conclude that the c-axis optical response of La1.87Sr0.13 CuO4 Can neither be modeled within the clean nor with the dirty limit.