Electron/phonon coupling in group-IV transition-metal and rare-earth nitrides

Transport electron/phonon coupling parameters and Eliashberg spectral functions alpha F-2(tr)((h) over bar omega) are determined for group-IV transition-metal (TM) nitrides TiN, ZrN, and HfN, and the rare-earth (RE) nitride CeN using an inversion procedure based upon temperature-dependent (4 andlt; T andlt; 300 K) resistivity measurements of high-crystalline-quality stoichiometric epitaxial films grown on MgO(001) by magnetically-unbalanced reactive magnetron sputtering. Transport electron/phonon coupling parameters lambda(tr) vary from 1.11 for ZrN to 0.82 for HfN, 0.73 for TiN, and 0.44 for CeN. The small variation in lambda(tr) among the TM nitrides and the weak coupling in CeN are consistent with measured superconducting transition temperatures 10.4 (ZrN), 9.18 (HfN), 5.35 (TiN), and andlt; 4 K for CeN. The Eliashberg spectral function describes the strength and energy spectrum of electron/phonon coupling in conventional superconductors. Spectral peaks in alpha F-2(andlt;(h)over barandgt;omega), corresponding to regions in energy-space for which electrons couple to acoustic (h) over bar omega(ac) and optical (h) over bar omega(op) phonon modes, are centered at (h) over bar omega(ac) = 33 and (h) over bar omega(op) = 57 meV for TiN, 25 and 60 meV for ZrN, 18 and 64 meV for HfN, and 21 and 39 meV for CeN. The acoustic modes soften with increasing cation mass; optical mode energies remain approximately constant for the TM nitrides, but are significantly lower for the RE nitride due to a lower interatomic force constant. Optical/acoustic peak-intensity ratios are 1.15 +/- 0.1 for all four nitrides, indicating similar electron/phonon coupling strengths alpha(tr)((h) over bar omega) for both modes. Funding Agencies|Swedish Research Council (VR)||Swedish Government Strategic Research Area Grant in Materials Science (SFO Mat-LiU) on Advanced Functional Materials