Nuclear spin-lattice relaxation of 93Nb in a superconducting NbN synthesized by SHS

A study is reported of the temperature dependence of the nuclear spin-lattice relaxation rate 1/T-1 of Nb-93 in a superconducting NbN with cubic B I structure and with T-C of about 16 K. Powdered samples of NbN mainly prepared by self-propagating high-temperature synthesis (SHS) have been used. The recovery of the nuclear magnetization from saturation did not show a simple exponential one and is interpreted by a multi-exponential recovery appropriate in this case of spin 9/2. Prominent features of the experimental results are as follows. (1) The relaxation rate 1/T-1 of Nb-93 follows the Korringa relation above T-C. (2) The rate has a bump just below T-C, which is considered to be a so-called coherence peak. (3) The data at low temperature can be interpreted by an activation-type temperature dependence, which allows estimating the value of 2Delta/k(B)T(C) to be 3.5. (4) The NMR shift of Nb-93 is decreased below T-C. These experimental results seem to indicate that the superconducting mechanism in NbN is due to the conventional BCS mechanism. (C) 2004 Elsevier B.V. All rights reserved.