Role of bosonic modes in the mechanism of high temperature Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$ superconductors using ultrafast optical techniques

Abstract

Using ultrafast optical techniques, we probe the hole-doping dependence of the electron-boson coupling constant $\lambda$ in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$. In the overdoped region, we observe a correlation between ($\lambda$) and the superconducting transition temperature $T_c$. Upon performing the McMillan analysis, however, we find that $\lambda$ is too small to explain the high $T_c$'s, and that the Coulomb pseudopotential ${\mu }^{\ast }$ is negative. Our analysis therefore reveals two components in the mechanism of high-$T_c$ superconductivity -- a dominant pre-existing pairing interaction, together with a weaker electron-phonon interaction that fine-tunes $T_c$.

4 pages, 4 eps figures