Reduction in the electron density of states in superconducting MgB₂ disordered by neutron irradiation: ¹¹B and ²⁵Mg NMR estimates

The NMR line shift and nuclear spin-lattice relaxation rate T₁^-1 of ¹¹B and ²⁵Mg were measured in superconducting MgB₂(T_c^ons=38K) structurally disordered by nuclear reactor neutrons up to the fluence of thermal neutrons Φ=1×10¹⁹cm^-2. The temperature of the superconducting transition was shifted down to T_c,irrad^ons=7K under irradiation. The change due to irradiation in the partial electron density of states (DOS) at the Fermi energy of boron and magnesium were traced by taking into account that the T₁^-1 of ¹¹B and ²⁵Mg is determined by hyperfine magnetic interactions with carriers. It was revealed that electronic states near E_F of Mg are influenced negligibly by irradiation whereas the partial DOS of the B 2p_x,y states reduce greatly in irradiated MgB₂. Acording to the Mc Millan formula, NMR data show that critical temperature decreases in irradiated MgB₂ mainly due to a reduction in the partial DOS of p states of boron.