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The Rapid Communications section is intended for the accelerated publication of important new results g. ince manuscripts submitted to this section are given priority treatment both in the editorial once and in production, authors sltould explain in their submittal letter why the work justiftes this special handling A. Rapid Communication should be no longer titan 3' printed pages and must be accompanied by an abstract Pa.ge proofs are sent to authors, but, because of the accelerated schedule, publication is not delayed for receipt of corrections unless requested by the author or noted by tile editor

We present results for the reflectivity, R, of two bulk crystals of Ba2YCu3C>7-$ which, below the superconducting transition at T c , show behavior consistent with perfect reflectivity up to an energy of (3-4)ICBT C .We interpret the reproducibility and high R values (especially /?~1 below T c ) as indicative of intrinsic behavior in the highly conducting crystal planes probed by the light. Results above T c show that the charge carriers have an enhanced effective mass and low scattering rate at low frequency and temperature, indicating a strong interaction with some excitation.PACS numbers: 78.30.Er, 74.70.VyWe have studied the absolute optical reflectivity, R, as a function of frequency, co, and temperature, T, of a series of bulk, crystalline samples of Ba2YCu307-<5 . These crystals 1 have negligible misalignment of the c axis but are microtwinned in the (a,b) plane. Six samples that we studied with T c -90 K showed relatively low and sample-dependent R (in the range 0.85 to 0.95 near 7-£ with reduced O content and 7Vs of 50 and 70 K which show higher R than the above values. Results on these samples are presented here with emphasis on the normal-state behavior of the sample with highest R. Both samples show qualitatively similar behavior: The charge carriers in the normal state interact strongly with some excitation enhancing their effective mass by a factor -10 at low co and T. As co and T are increased, the scattering rate increases drastically and the effective mass is reduced. For T<£T C , we find results consistent with R = 1 for co below the energy gap, 2A. The central importance of this reproducibility and these high R values (especially R -1 below T c ) is an indication of intrinsic behavior in the region of the crystals near the surface probed by the light.R was measured with focusing mirror optics and a rapid-scanning Michelson interferometer from 500 to 20000 cm -1 , and with light-pipe optics and a steppingmirror instrument from -30 to 900 cm -1 . For all our experiments, light (with a spot diameter < 1.75 mm) was incident at an angle -10° so that the electric field was predominantly in the basal plane. Our absolute uncertainty of 1% in R arises from the difficulties of verifying the precise optical alignment of the Au reference and sample at low T, from the sample to reference variation of the detector linearity (negligible at low co), and from questions about the absolute R of our polished, solid Au reference. Relative uncertainties of about j % apply to all the data, and arise partly from noise and partly from regular cavity oscillations at small co which have been eliminated from the data by averaging. Figure 1 shows R of the sample with 7^=50 K as a function of co from -30 to 15 000 cm" 1 , with the entire range in the inset and the low-frequency range in the main figure. We have fixed the magnitude by setting ^...

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