  We present a comparison of electron-phonon interaction in NbB_2 and MgB_2,
calculated using full-potential, density-functional-based methods in P6/mmm
crystal structure. Our results, described in terms of (i) electronic structure,
(ii) phonon density of states F(\omega), (iii) Eliashberg function \alpha
^2F(\omega), and (iv) the solutions of the isotropic Eliashberg gap equation,
clearly show significant differences in the electron-phonon interaction in
NbB_2 and MgB_2. We find that the average electron-phonon coupling constant
\lambda is equal to 0.59 for MgB_2 and 0.43 for NbB_2, leading to
superconducting transition temperature T_c of around 22 K for MgB_2 and 3 K for
NbB_2.

Superconductivity