Infrared Studies of the Superconducting Energy Gap and Normal State Dynamics of Y 1 Ba 2 Cu 3 O 7 and Ba 0.6 K 0.4 BiO 3
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ir traditional rôle as a probe of the superconducting energy gap and for their ability to probe the dynamics of the normal state by measuring the conductivity vs. frequency in the crucial infrared range (~1 meV to 2 eV). Occupying the frequency range between about 5 and 15,000 cm-1, infared encompasses the région in which the characteristic energy scales of correlated Systems tend to lie (e.g., superconducting and charge density wave energy gaps, Kondo énergies, etc.). As such, infrared measurements provide a vital probe of the dynamics of thèse Systems. The use of infrared techniques to measure the superconducting energy gap began with the work of Glover and Tinkham4-5 in 1957. Their infrared work on Pb films provided the first spectroscopic measurement of a superconducting gap, and slightly preceded the advent of the BCS theory. The use of infrared measurements to probe the Eliashberg coupling spectrum, a2F(a>), was demonstrated in 1969 by Joyce and Richards.67 More recentiy, Sievers and co-workers8"10 hâve pioneered the use of infrared measurements to study the dynamics of highly correlated Systems such as CePd3 and UPt3. Thèse measurements provide historical examples of the utility of infra-
red, and serve as a guideline to the potential ways infrared measurements can be used to study layered cuprate compounds. Length considérations require this review to be highly sélective and thus a great deal of significant work is not covered. We focus primarily on issues dealing with the electronic response, specifically the nature of the normal state dynamics, and the occurrence of a superconducting energy gap in the electronic excitation spectra. Infrared Measurement Techniques Measurements of conducting samples can be performed using either transmission measurements on thin spécimens or reflectivity measurements on bulk samples. In the layered cuprate superconductors, the electromagnetic pénétration depth for the electric field parallel to the planes is about 1,500 Â. To perforai transmission measurements with results représentative of intrinsic bulk properties requires a homogeneous film, no thicker than the pénétration depth, grown on a substrate that is transparent over the frequency range of interest (e.g., 100 to 2,000 cm1). At the présent rime thèse conditions are rather difficult to satisfy, and consequently, transmission measurements are still in their early stages. This review will concentrate on reflectivity measurements, which are performed on bulk samples (crystals) or thick films (-4,000 Â) on SrTi03 substrates. Typically, reflectivity is measured at normal incidence as a function of frequency and température. One can study the température and frequency dependence of the reflectivity itself, or use a Kramers-Kronig transformation11 to obtain the conductivity and dielectric response of the material as a function of frequency (at each température). The infrared thus yields fundamental information in the frequency domain, which generally corresponds with information obtained in the température domain by other
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