NMR studies of metal-insulator transition in the spinel-type Cu(Ir1−xVx)2S4

In order to investigate the physical properties of a metal-insulator transition (MIT) on Ir rich side in Cu(Ir1-xVx)2S4 from a microscopic point of view, 63Cu NMR measurements for Cu(Ir1-xVx)2S4 (x = 0.00, 0.03 and 0.05) have been carried out from 4.2 to 300 K. The temperature dependences of the line width, Knight shift and spin-lattice relaxation time (T1) have been measured for these samples. A sudden decreases of the 1/T1T and the negative contribution to the Knight shift from the core polarization of Cu conduction electrons for x = 0.00 and 0.03 samples show clearly the existence of the MIT at TMI = 226 and 175 K, respectively, being attributed to the opening of a band gap below TMI. However, the temperature dependences of the 1/T1T and the Knight shift for x = 0.05 sample change continuously as the band gap is remarkably suppressed below TMI. The density of states at Fermi level in the insulating phase is reduced to about 20 % of that in the metallic phase.