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/T1 of 93Nb in a superconducting NbN with cubic B1 structure and with TC of about 16K. 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/T1 of 93Nb follows the Korringa relation above TC. (2) The rate has a bump just below TC, 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 2Δ/kBTC to be 3.5. (4) The NMR shift of 93Nb is decreased below TC. These experimental results seem to indicate that the superconducting mechanism in NbN is due to the conventional BCS mechanism.