Evidence for spin-fluctuation-mediated superconductivity in electron-doped cuprates

In conventional superconductors, one of the key parameters fixing the robustness of the superconductivity is the electron–phonon coupling strength λ. This in turn is closely related to λ_tr, the parameter that defines the transport scattering rate associated with the linear-in-temperature resistivity that is characteristic of a normal metal. This link between the coefficient of the T-linear resistivity α and the superconducting transition temperature T_c is enshrined in the old adage; ‘‘good metals make bad superconductors”. In certain unconventional superconductors, including the high-T_c cuprates, a similar correlation exists, albeit with a T-linear resistivity that extends to anomalously low temperatures indicative of a unconventional or ‘strange’ metal. Despite this complication, the search for an associated λ has been prolonged, intense but ultimately unsuccessful. Here, I will present a high-field magnetotransport study of combinatorial thin films of the electron-doped cuprate La_2-xCe_xCuO₄ that reveal two key findings. Firstly, we have succeeded to identify the relevant λ as well as its origin. Specifically, we find evidence that T_c in electron-doped cuprates is determined by the strength of coupling to commensurate antiferromagnetic fluctuations. Secondly, on the hole-doped side, magnetotransport suggests an altogether different origin for the T-linear resistivity (i.e. one that is not related to scattering off a bosonic bath) and, in turn, a possibly new paradigm for high-T_c superconductivity.