Bosonic spectral density of optimally and overdoped LSCO superconductors from optical spectroscopy

Optical spectroscopy on single crystals of optimally doped La$_{2- x}$Sr$_{x}$CuO$_{4}$ (x=0.17) show two bosonic peaks at ~50 meV and ~18 meV at low temperatures (30K) as reported by Hwang et al[1]. The bosonic spectrum is acquired through the Eliashberg formalism by inverting the measured optical spectra, and shows a remarkable similarity to the spin excitation spectrum achieved through inelastic neutron scattering results. The optical study is extended into the overdoped region (x=0.22) for which detailed neutron scattering results suggest a suppression of the strong 50 meV response and a characteristic shift from 18 meV to 10 meV of the low energy response. [1] J. Hwang, E. Schachinger, J.P. Carbotte, F. Gao, D.B. Tanner, T. Timusk, Phys. Rev. Letters 100, 137005 (2008)