We present the c-axis optical conductivity σ1c(ω,T) of underdoped (x = 0.12) and optimally doped (x = 0.15) LSCO from 4 meV to 1.8 eV obtained by a combination of reflectivity and transmission spectra. In addition to the opening of the superconducting gap, we observe an {\it increase} of conductivity above the gap up to 270 meV with a maximal effect at about 120 meV. This may indicate a new collective mode at a surprisingly large energy scale. The Ferrell-Glover-Tinkham sum rule is violated for both doping levels. Although the relative value of the violation is much larger for the underdoped sample, the absolute increase of the low-frequency spectral weight, including that of the condensate, is higher in the optimally doped regime. Our results resemble in many respects the observations in YBCO.
Cite
@article{arxiv.cond-mat/0302394,
title = {The c-axis optical sum rule and possible new collective mode in LSCO},
author = {A. B. Kuzmenko and N. Tombros and H. J. A. Molegraaf and M. Grueninger and D. van der Marel and S. Uchida},
journal= {arXiv preprint arXiv:cond-mat/0302394},
year = {2009}
}