Ultra-fast pump-probe determination of electron-phonon coupling in cuprate superconductors

Fresh femtosecond spectroscopy experiments show the electron-phonon interaction strength $\lambda$ to be 0.7 and 1.4 for YBCO and LSCO respectively and not around 0.2 as previously reported [1]. The revised estimates arise primarily from improved time-resolution, and also partly from improved modeling. Comparison with classical superconductors and pnictides shows non-monotonic correlation of $\lambda$ with $T_c$. Systematic new measurements of the condensate vaporization energy ($U_v$) in cuprates [2] and pnictides reveals a power-law dependence on $T_c$ with exponent 2. However, $U_c$ is 16-18 times greater than the BCS condensation energy $U_c$, implying that a significant heat capacity of the ``bosonic glue.'' In contrast, charge-density wave systems with electronically driven ordering transitions have $U_v \simeq U_c$. The data suggest BCS and Eliashberg-based models to be inappropriate for describing the physics of high-temperature superconductors, and point towards polaron models which consider strong or intermediate $\lambda$.\\[4pt] [1] C.Gadermeier et al., arXiv:0902.1636\\[0pt] [2] P.Kusar et al., Phys. Rev. Lett. 101, 227001 (2008)