Acoustical-Mode-Driven Electron-Phonon Coupling in Transition-Metal Diborides

TL;DR

This study reveals that in transition-metal diborides NbB2 and TaB2, electron-phonon coupling is mainly driven by the longitudinal acoustical mode, differing from MgB2 where optical modes dominate, with implications for their superconducting properties.

Contribution

The paper provides ab initio analysis showing the dominance of acoustical phonons in electron-phonon coupling in NbB2 and TaB2, contrasting with MgB2's optical mode dominance.

Findings

NbB2 and TaB2 have weaker electron-phonon coupling than MgB2.

E_{2g} linewidth is much larger in MgB2.

E_{2g} phonon frequency is higher in NbB2 and TaB2.

Abstract

We show that the electron-phonon coupling in the transition-metal diborides NbB2 and TaB2 is dominated by the longitudinal acoustical (LA) mode, in contrast to the optical E_{2g} mode dominated coupling in MgB2. Our ab initio results, described in terms of phonon dispersion, linewidth, and partial electron-phonon coupling along Gamma to A, also show that (i) NbB2 and TaB2 have a relatively weak electron-phonon coupling, (ii) the E_{2g} linewidth is an order of magnitude larger in MgB2 than in NbB2 or TaB2, (iii) the E_{2g} frequency in NbB2 and TaB2 is considerably higher than in MgB2, and (iv) the LA frequency at A for TaB2 is almost half of that of MgB2 or NbB2.