Robust two-gap strong coupling superconductivity associated with low-lying phonon modes in pressurized Nb₅Ir₃O superconductors

We report robust superconducting state and gap symmetry of Nb₅Ir₃O via electrical transport and specific heat measurements. The analysis of specific heat manifests that Nb₅Ir₃O is a strongly coupled superconductor with and double s-wave superconducting gaps of and accounting for 90% and 10%, respectively. The vs. T plot shows a broad peak at ∼23 K, indicating phonon softening and the appearance of low-lying phonon mode associated with the interstitial oxygen. This behavior explains the monotonic increase of T_c in Nb₅Ir₃O(1-δ) in by strengthening the electron-phonon coupling and enlarging the density of states at Fermi level. The Hall coefficient is temperature independent below 200 K, and changes its sign from positive to negative above 250 K, suggesting that carrier is across the hole- to electron-dominant regions and the multi-band electronic structures. On warming, the resistivity shows a gradual crossover from T² - to T³-dependence at a critical temperature T∗, and a broad peak at a temperature T _p. The reduced T _c under pressure is linearly correlated with lattice parameters c/a ratio and T _p, suggesting the important phonon contributions in Nb₅Ir₃O as a phonon-medicated superconductor. Possible physical mechanisms are proposed.