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

doi:10.1088/1674-1056/ab4047

Abstract

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/γ_nT_c~1.91 and double s-wave superconducting gaps of 2△_L(0)/k_BT_c~6.56 and 2△_S(0)/k_BT_c~2.36 accounting for 90% and 10%, respectively. The (C_p-γ_nT)/T³ 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) 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.

Keywords: new superconductor high-pressure effect strongly coupled superconductor

Received: 16 June 2019
Revised: 15 August 2019
Accepted manuscript online:

PACS:	74.70.-b	(Superconducting materials other than cuprates)
	74.62.Fj	(Effects of pressure)

Corresponding Authors: Bosen Wang
E-mail: bswang@iphy.ac.cn

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Bosen Wang(王铂森), Yaoqing Zhang(张尧卿), Shuxiang Xu(徐淑香), Kento Ishigaki, Kazuyuki Matsubayashi, Jin-Guang Cheng(程金光), Hideo Hosono, Yoshiya Uwatoko Robust two-gap strong coupling superconductivity associated with low-lying phonon modes in pressurized Nb₅Ir₃O superconductors 2019 Chin. Phys. B 28 107401

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

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