Enhanced electron-phonon coupling near the lattice instability of superconducting NbC1−xNx from density-functional calculations

Simon Blackburn; Michel Côté; Steven G. Louie; Marvin L. Cohen

doi:10.1103/PhysRevB.84.104506

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Enhanced electron-phonon coupling near the lattice instability of superconducting NbC1−xNx from density-functional calculations

Simon Blackburn, Michel Côté, Steven G. Louie, Marvin L. Cohen

DOI 10.1103/PhysRevB.84.104506 · Physical Review B

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Abstract

Using density-functional theory within the local-density approximation, we study the electron-phonon coupling in NbC1−xNx and NbN crystals in the rocksalt structure. The Fermi surface of these systems exhibits important nesting. The associated Kohn anomaly greatly increases the electron-phonon coupling and induces a structural instability when the electronic density of states reaches a critical value. Our results reproduce the observed rise in Tc from 11.2 to 17.3 K as the nitrogen doping is increased in NbC1−xNx. To further understand the contribution of the structural instability to the rise of the superconducting temperature, we develop a model for the Eliashberg spectral function in which the effect of the unstable phonons is set apart. We show that this model together with the McMillan formula can reproduce the increase of Tc near the structural phase transition.

Source-reported materials — not catalogue approval

Formula	Reported Tc (K)	Pressure (GPa)	Type
NbC Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	11.2	Pressure not reported	unknown
NbC0.9N0.1 Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	14.3	Pressure not reported	unknown
NbC0.8N0.2 Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	17.4	Pressure not reported	unknown
NbC0.7N0.3 Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	20.1	Pressure not reported	unknown
NbC0.6N0.4 Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	23.1	Pressure not reported	unknown
NbN Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	7.1	Pressure not reported	unknown
NbN0.9 Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	8.9	Pressure not reported	unknown
NbN0.8 Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	9.8	Pressure not reported	unknown
NbN0.7 Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	14.6	Pressure not reported	unknown
NbN0.6 Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	17.3	Pressure not reported	unknown
NbN0.5 Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	20.4	Pressure not reported	unknown
NbC Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	11.2	Pressure not reported	unknown
NbN Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	17.3	Pressure not reported	unknown