Mode coupling effect in infrared spectra of Tl-based cuprates

We study the mode coupling effect in infrared spectra of Tl-based cuprates. For samples near optimal doping, the reflectance roughly has a linear-frequency dependence in the normal state, but displays a pronounced knee structure followed by a dip-like feature at higher frequency below Tc. We illustrate that the structure could not be caused solely by the strong coupling effect of electrons with a bosonic mode. A gap in the density of states has to be taken into account. The spectral evolution with doping for Tl-2201 crystals in the overdoped side was also investigated. Similar to Bi-2212 system, the mode coupling feature weakens dramatically with overdoping and tends to vanish in the heavily overdoped sample. We elaborate that the observed trend does not favor a phonon origin of the mode, but could be reconciled with coupling to magnetic excitations.