(23)NA-NMR study in NAV(6)O(11)

A mixed valent vanadium compound NaV6O11 shows a magnetic phase transition at T-t = 245 K. For the microscopic investigation of the transition at T-t, Na-23-NMR measurements from 70 K to 300 K were carried out on NaV6O11. The Knight shift, K, and the nuclear spin-lattice relaxation rate, 1/T-1, were measured. While 1/T-1 above T-t has an almost constant value, 1/T-1 below T-t shows a gap-type temperature dependence. This is the first direct evide nce that the d-electrons at V(1) ions, which form a kagome lattice in NaV6O 11, exhibit a spin-singlet ground state below T-t. The formation of the spi n-singlet ground state of V(1) seems to be closely related to the frustrati on effect.