NMR Studies on Spin-Gap Behavior of Monoclinic CuNb ₂O ₆

The NMR Knight shift and the spin-lattice relaxation rate 1/T-1 have been measured at Nb sites in two polymorphs of quasi one-dimensional zig-zag spin chain compounds CuNb2O6. The temperature (T) dependence of NMR-1/T-1 of the monoclinic CuNb2O6 indicates the existence of spin excitation gap (spin gap) whose magnitude (similar to 20K) is consistent with the result of other kinds of measurements. The difference of the T-dependences between 1/T1T and K is similar to that well-known for high-T-c oxides. A simultaneous growing of the antiferromagnetic and the singlet correlation has been observed with decreasing T at around the temperature of the susceptibility maximum (greater than or equal to 20 K). Results of the NMR measurements for the orthorhombic CuNb2O6, which undergoes an antiferromagnetic transition at about 7.3 K; are also presented.