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 ΔC/γnTc ~ 1.91 and double s-wave superconducting gaps of 2ΔL(0)/kBTc ~ 6.56 and 2ΔS(0)/kBTc ~ 2.36 accounting for 90% and 10%,respectively. The(Cp-γnT)/T^3 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 Tc in Nb5Ir3O(1-δ)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 T^2-to T^3-dependence at a critical temperature T^*, and a broad peak at a temperature Tp. The reduced Tc under pressure is linearly correlated with lattice parameters c/a ratio and Tp, suggesting the important phonon contributions in Nb5Ir3O as a phonon-medicated superconductor. Possible physical mechanisms are proposed.