Local electronic structure in Mg B 2 from B 12 β -NMR

We performed $\ensuremath{\beta}$-NMR spectroscopic studies on $^{12}\mathrm{B}$ which was implanted into $\mathrm{Mg}{\mathrm{B}}_{2}$ to get insight into the local electronic structure of this high-temperature superconducting material. By measuring electric field gradients we probed the electronic charge distribution anisotropy around boron in this solid and thus obtained experimental information about the bonding by $p$ electrons. Besides the absolute value, the sign of the electric field gradient was also experimentally determined. By comparison with quantum chemical calculations, some of which were performed in the present work, and with results from conventional NMR spectroscopy on $^{11}\mathrm{B}$ we were able to identify regular boron lattice sites as well as interstitial sites.