Superconducting state in Ba_1−xSr_xNi₂As₂ near the quantum critical point

In the phase diagram of the nickel-based superconductor Ba_1−xSr_xNi₂As₂, T_c has been found to be enhanced sixfold near the quantum critical point (QCP) x = 0.71 compared with the parent compound. However, the mechanism is still under debate. Here, we report a detailed investigation of the superconducting properties near the QCP (x ≈ 0.7) by utilizing scanning tunneling microscopy and spectroscopy. The temperature-dependent superconducting gap and magnetic vortex state were obtained and analyzed in the framework of the Bardeen–Cooper–Schrieffer model. The ideal isotropic s-wave superconducting gap excludes the long-speculated nematic fluctuations while preferring strong electron–phonon coupling as the mechanism for T_c enhancement near the QCP. The lower than expected gap ratio of Δ/(k_BT_c) is rooted in the fact that Ba_1−xSr_xNi₂As₂ falls into the dirty limit with a serious pair breaking effect similar to the parent compound.