K. Kamaras, S.L. Herr, C.D. Porter, N. Tache, D.B. Tanner, "In a clean high-Tc superconductor you do not see the gap," Physical Review Letters, 64:84-87, 1990.
Katalin Kamaras (University of Florida, Gainesville; now at the Institute for Solid State Physics, Budapest, Hungary): "This paper reported one of the first few measurements of perfect (100 percent) infrared reflectivity in a high-temperature superconductor (see also Physical Review Letters, 61:1313-16, 1988; Europhysics Letters, 8:679-84, 1989), thanks to the excellent quality of the samples prepared by coauthor T. Venkatesan's group at Bellcore [Red Bank, N.J.] and Rutgers University [Piscataway, N.J.]. In it, we demonstrated that YBa2Cu3O7 is a `clean-limit' superconductor, until now merely a textbook example.
"With the title, we wanted to stress that the `clean' nature of the material (meaning that most of the normal-state free-carrier absorption occurs below the superconducting gap frequency), together with a strong mid-infrared absorption, renders the gap unobservable ...
