Electronic states of boron in superconducting MgB2 studied by 11B NMR

Nuclear magnetic resonance (NMR) spectra and nuclear spin-lattice relaxation rate T-1(-1) of B-11 have been measured in superconducting polycrystalline MgB2 with T-c(ons) = 39.5 K. It is shown that (T1T)(-1) and the Knight shift K-s are independent of temperature and nearly isotropic above T-c. Both of these quantities are decreased gradually in going to the superconducting state. According to NMR data the density of states near the Fermi level is flat at the scale of about 500 K. Some conclusions on the orbital content of the density of states at the Fermi level were drawn and compared with the results of the band structure calculations.