NMR studies of CDW transition in the spinel-type Cu(Ir1?xVx)2S4

The spinel CuV2S4 exhibits two-step anomalies at about 92 K (Tt1) and 56 K (Tt2), reflecting the CDW state. In order to investigate the CDW state on V rich side in Cu(Ir1?xVx)2S4 microscopically, NMR measurements of 51V for Cu(Ir1?xVx)2S4 (x = 1.00, 0.98 and 0.95) have been carried out between 4.2 and 300 K. The line width of 51V for x = 1.00 is very narrow above Tt1 and become wider with decreasing x. For x = 1.00 and 0.98, the line width is suddenly broadened at Tt1 due to an electrical quadruple interaction between 51V nucleus and CDW charge modulation, but for x = 0.95, the line width is continuously increased around Tt1. The Knight shifts of 51V for x = 1.00, 0.98 and 0.95 show almost the same negative shift due to the influence of d-electrons above Tt1. At Tt1, the values of the Knight shift for x = 1.00 and 0.98 suddenly change to positive values, but for x = 0.95 continuously shifts to a positive one. The 1/T1T's of 51V for x = 1.00, 0.98 and 0.95 obey the Korringa law above Tt1. Below Tt1, the 1/T1T values of 51V for x = 1.00, 0.98 and 0.95 take constant values, which shows the Korringa law resulting from Pauli paramagnetic state.