Details are given of a technique for measuring the reflectance at near-normal incidence of small, irregular, submillimeter-sized samples from the far IR (40 cm(-1)) to the visible (40000 cm(-1)) between 10 and 300 K by using a modified Michelson interferometer or grating spectrometer. The sample and a reference mirror are mounted on nonreflecting cones. At the focus the size of the beam is larger than either the sample or the reference, so that the entire area of the sample is utilized. The positions are interchanged by a 90° rotation by using preset mechanical stops. The scattering caused by geometrical effects is corrected for by the in situ evaporation of gold or aluminum onto the sample. The effect of diffraction is estimated from Mie theory by assuming the sample and reference to be spheres. For frequencies above ≈ 40 cm(-1) and sample diameters of ≈ 1 mm with a detector field of view of 30°, the calculations show that the ratio of the backscattered intensities gives a good approximation of the specular reflectance.

By combining reflectance spectroscopy and spectroscopic ellipsometry, the complex dielectric function of SrTiO, in the frequency range 40-5000 cm-' at 20, 100, 200, and 300 K has been determined. Using a factorized description, analytical expressions for the optical quantities were derived, giving excellent agreement with the experimental data. These can be used for two-layer fits of films on SrTiOs, e.g., of high-T, superconductors. The fit parameters complement very well those found at higher temperatures. 0 1995 American Institute of Physics.

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We show that the strong absorption structure observed in the a-^-plane optical conductivity of the high-Tc superconductors is correlated with c-axis longitudinal optical (LOc) phonons. This suggests a resolution to the long-standing controversy over the origin of these features. The interaction with LOc phonons is forbidden when the incident wave vector q is normal to the c axis which leads to the surprising result, confirmed with experiments, that the optical properties along a or b are different when measured on the fl-^-plane face and a face containing the c axis. PACS numbers: 74.30.Gn, 63.20.Kr, 74.70.Vy, 78.20.CiThe unconventional response of the high-Tc superconductors to electromagnetic radiation has been well established [1]. A prominent manifestation of the non-Drude behavior is the "knee" that develops in the a-6-plane reflectance of YBa2Cu307-5 near 400 cm "' at low temperatures leading to a '*notch"-like absorption feature in the optical conductivity [2,3]. This structure has been the focus of much discussion. It has been variously attributed to the superconducting energy gap [4] and to pho-It has become clear that YBaiCusOj-s is not the only cuprate superconductor to exhibit such structure. Bi2Sr2CaCu208 [6], Tl2Ba2Ca"-iCu"02"+4 [7,8], Pb2-Sr2(Y/Ca)Cu308 [9], and Nd2-xCe;cCu04-5 [10] all show strong low-frequency absorption features in the non-Drude component of the a-6-plane optical conductivity. In each case the structure appears as a minimum in (7\((o). There are, however, significant material to material diff'erences in the number, magnitude, width, and position of the minima. For example, whereas YBa2-CU3O7-5 is dominated by one broad feature near 420 cm~^ [ Fig. 2(b)], Tl2Ba2CaCu208 shows two deep minima at 350 and 600 cm~^ [ Fig. 3(b)], while Pb2Sr2(Y/Ca)Cu308 exhibits two closely spaced sharper minima at 525 and 585 cm "^ and a third broader feature centered near 435 cm ~' [Fig. 2(a)].The relative independence of the position of this structure to both temperature [3] and level of doping [2] suggests that it is unrelated to the superconducting energy gap and that phonons are involved. It has been shown that a sharp level, interacting with an electronic continuum, results in a notchlike minimum in the spectrum of the continuum (an antiresonance) if the external field does not interact with the sharp transition [11,12]. This phenomenon, discussed by Fano for atoms [11] and Rice for phonons and an electronic continuum [12], is common in quasi-one-dimensional organic conductors, where charge [13] or spin density [14] waves break symmetry and couple the low-lying charge-transfer continuum to totally symmetric phonon modes not normally ir active [12]. In high-T^c superconductors, the interaction of the phonons is with bound (midinfrared) and not Drude-like

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