Laser thermal spectroscopy of highly granular NbN

Far-infrared thermal spectroscopy of low-Tcand high-Tcsuperconductor films

“…In the high quality sample (A) it corresponds well to BCS calculations, whereas in the polycrystalline sample (B) the observed maximum is somewhat broader and its level a little lower than theoretical. This behavior is in accordance with other experiments which were performed on granular films 19 . In the YBCO samples, the low temperature variation of transmission is non-measurably small and the peak below T c predicted by the BCS theory is missing.…”

“…Near the critical temperature the Mattis-Bardeen theory predicts a sharp peak in transmission at frequencies comparable with the optical gap. Some indistinct maximum just below T c was already observed in classical superconductors by Perkowitz, who measured the temperature dependence of far infrared transmission of NbN films, one homogenous and one highly granular 19 . The peak in transmission predicted by the theory was observed at a reduced level in the homogeneous sample, but it was absent in the granular material.…”

“…The exponential temperature dependence of surface resistance or penetration depth was verified for NbN 16 and other materials 15,17,18 . The FIR transmission of NbN films was fitted well using the Mattis-Bardeen theory 19 . The high-T c superconductors were investigated also in the far-infrared region but the results are still ambiguous.…”

Optical conductivity to probe boson structure*

“…( 3) was obtained earlier by Mitrović and Fiorucci [23] based on a generalization of Allen's second order perturbation theory approach. They use their equation to analyse optical data in A15 compounds [24,25] where the band structure electronic density of states shows structure on the scale of a few times the phonon energy. The finite temperature formula (3) was applied to the oxides by Hwang et al [32] to treat the pseudogap present in the normal state of underdoped cuprates.…”

Terahertz thermal spectroscopy of a NbN superconductor

“…It enables precise measurement of transmission at fixed laser frequencies as a function of temperature in the vicinity of the superconducting transition, where a peculiar behavior was observed, and has proved to be a very useful experimental method. 3,4 Earlier we performed experiments in the terahertz frequency range, 4,5 but a satisfactory theoretical explanation has not been found. Our previous experiments suffered from the fact that the NbN sample we used displayed a rather broad superconducting transition.…”