Nuclear Magnetic Resonance Study on the Charge Density Wave (CDW) in Copper Sulfide Spinel CuV_2S_4

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CuV_2S_4 shows the charge density wave (CDW) transition which is a typical low dimensional property although CuV_2S_4 is a three dimensional substance. At high temperatures, all the ^<63>Cu, ^<65>Cu and ^<51>V spin-lattice relaxation rates obey the Korringa law, and CuV_2S_4 is considered to be a simple metal at high temperatures. ^<51>V relaxation rate shows the Arrhenius type temperature dependence below 90 K, which indicates an energy gap due to CDW.Below 75 K, ^<51>V relaxation rate obeys the Korringa law again, which indicates its energy gap is anisotropic. The ratio of ^<63>Cu and ^<65>Cu relaxation rates is close to the square of the ratio of their gyromagnetic ratios at high temperatures, which tells us its relaxation mechanism is magnetic and is consistent with obeying the Korringa law. But with approaching CDW transition temperature, it reverses and becomes the square of the ratio of their quadrupole moments. The ^<63>Cu and ^<65>Cu relaxation rate becomes to diverge at CDW transition temperature, which indicates that the electric field gradients at Cu site becomes larger. This fact agrees with the fact that ^<63>Cu and ^<65>Cu NMR spectra disappear due to CDW below 83 K.^<51>V Knight shift ^<51>K is negative at high temperatures because the core polarization is effective due to V 3d electrons. The magnitude of ^<51>K becomes smaller with approaching CDW transition temperature and changes to a positive value after CDW transition, which indicates that the paired two electrons are distributed in each CDW wave length. The hyperfine coupling constant of Cu is positive, which seems to indicate the existence of the super transferred hyperfine coupling in CuV_2S_4 as observed in copper oxide high T_c superconductors.