BEGIN:VCALENDAR VERSION:2.0 PRODID:Data::ICal 0.22 BEGIN:VEVENT DESCRIPTION:David Tanner\, University of Florida\n\n
It was in April 1986 when Bednorz and Mueller of the IBM Zü\;rich laboratories sent a pape r about &ldquo\;possible high-Tc superconductivity&rdquo\; to Zeitschr ift fü\;r Physik B. \; The resulting bombshell changed conden sed-matter physics forever. \; Experimenters and theorists developed m ethods to measure and calculate in ways that were much improved over prior years. However\, despite 30 years of intense study\, the description of t hese materials remains incomplete. I&rsquo\;ll discuss the \; discover y of the high Tc cuprates from the perspective of a participant. \; I& rsquo\;ll then turn to what infrared spectroscopy can tell us about their properties. Measurements for a number of cuprate families of optical refle ctance over a wide spectral range (far-infrared to ultraviolet) have been analyzed using Kramers-Kronig analysis to obtain the optical conductivity\ , s(w)\, and (by integration of the real part of the con ductivity) the spectral weight of low- and mid-energy excitations. For the Kramers-Kronig analysis to give reliable results\, accurate high-frequenc y extrapolations\, based on x-ray atomic scattering functions\, were used. When the optical conductivities of the normal and superconducting states are compared\, a transfer of spectral weight from finite frequencies to th e zero-frequency delta-function conductivity of the superconductor is seen . The strength of this delta function gives the superfluid density\, r s. There are two ways to measure rs\, usin g either the low energy spectral weight or by examination of the imaginary part\, s2(w)\; both estimates show that 98% of the ab-plane superfluid density comes from low energy scales\, below abou t 0.15 eV. Moreover\, there is a notable difference between clean metallic superconductors and the cuprates. \; In the former\, the superfluid d ensity is essentially equal to the conduction electron density. The cuprat es\, in contrast\, have only about 20% of the ab-plane low-energy spectral weight in the superfluid. The rest remains in finite-frequency\, midinfrared absorption. In underdoped materials the superfluid fraction i s even smaller. The consequences of this observation for the electronic st ructure will be addressed.
\n DTSTART:20160429T193000Z LOCATION:Physics Building\, Room 204 SUMMARY:30 years of high Tc: Superfluid and normal-fluid densities in the cuprate superconductors END:VEVENT END:VCALENDAR