Effects of Fe substitution on TC suppression of MgB2: A B11 NMR study

We have performed a systematic study of Fe-doped MgB2 alloys using B11 nuclear magnetic resonance spectroscopy. The Knight shift, line shape, as well as spin-lattice relaxation time (T1) on each individual composition have been identified. The central transition and quadrupole linewidths increase with Fe concentration, attributed to the effect of disorder on these measurements. The temperature independence of the central transition linewidth further confirms no magnetic moment associated with these dopants. In addition, the value of T1 T remains unchanged with Fe content, indicative of little or no effect on the B-2p Fermi level density of states (DOS) by substituting Fe in the Mg sublattice of MgB2. These observations clearly revealed that the drop of TC cannot be accounted for either by the pair breaking or by the electronic DOS reduction. It thus revealed the importance of the phonon contributions to the change of TC, presumably due to the decreased phonon frequency and/or the reduced electron-phonon coupling strength through Fe doping.