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

CuV_2S_4具有电荷密度波(CDW)相变，这是一种典型的低维性质，尽管CuV_2S_4是三维物质。在高温下，所有的^&lt；63&gt；Cu，^&lt；65&gt；Cu和^&lt；51&gt；V自旋晶格驰豫速率都服从Korringa定律，CuV_2S_4在高温下被认为是一种简单的金属。弛豫速率在90K以下表现出Arrhenius型温度依赖关系，这是由于CdW引起的能隙；当温度低于75K时，51&gt；V弛豫速率再次服从Korringa定律，表明其能隙是各向异性的。高温下^&lt；63&gt；Cu和^&lt；65&gt；Cu的驰豫速率之比接近其旋磁比的平方，说明其驰豫机制是磁性的，符合Korringa定律。但当接近CDW转变温度时，它发生反转，变为四极矩之比的平方。在CDW转变温度下，^&lt；63&gt；Cu和^&lt；65&gt；Cu的驰豫速率变得发散，表明铜位的电场梯度变大。这一事实与^&lt；63&gt；铜和^&lt；65&gt；铜的核磁共振谱因CDW低于83K而消失的事实一致。^&lt；51&gt；V Knight Shift^&lt；51&gt；K在高温下是负的，因为核心极化是由于V3D电子的有效。随着CDW转变温度的升高，^&lt；51&gt；K的值变小，并在CDW转变后变为正值，表明成对的两个电子分布在每个CDW波长上。铜的超精细耦合常数为正，这似乎表明在CuV_2S_4中存在铜氧化物高T_c超导体中的超转移超精细耦合。

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.