On the connection between high superconducting transition temperature, Tc, and lattice instability in transition metal compounds: NbN

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Abstract
Using the result of a band structure calculation the electron- phonon mass enhancement factor lambda is estimated for NbN. It is shown that the large lambda and therefore the high Tc is due to the combination of two factors: (i) the small mass of the light nitrogen atoms and (ii) the strong scattering of electrons at the Fermi energy by the p resonance in the nitrogen scattering phase shifts. It is argued that in NbN as well as in other similar compounds (NbC, PdAgH), the large oscillations of a strong scattering light component give rise to a high Tc and signal the beginning of a lattice instability. For these high Tc compounds, the existence of what might be called the 'second isotope effect' is predicted.