title: c-Axis optical sum rule and a possible new collective mode in La2-xSrxCuO4.
abstract: We present the c-axis optical conductivity sigma(1c)(omega,T) of underdoped (x=0.12) and optimally doped (x=0.15) La2-xSrxCuO4 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 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 under-doped 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 YBa(2)Cu(3)O(7-delta).
authors.affiliation: Material Science Center, University of Groningen, Nijenborgh 4, 9747AG, Groningen, The Netherlands.
authors.forename: A B
authors.initials: AB
authors.lastname: Kuzmenko
authors.forename: N
authors.initials: N
authors.lastname: Tombros
authors.forename: H J A
authors.initials: HJ
authors.lastname: Molegraaf
authors.forename: M
authors.initials: M
authors.lastname: Grüninger
authors.forename: D
authors.initials: D
authors.lastname: Van Der Marel
authors.forename: S
authors.initials: S
authors.lastname: Uchida