Possible orbital ordering in a spin-singlet ground state: V-51 NMR and NQR study of BaVS3

V-51 NMR and NQR measurements of the S = 1/2 system, BaVS3, which show a metal-insulator transition at T-MI congruent to 70 K, revealed a nonmagnetic ground state, presence of a huge and extraordinary asymmetric electrical field gradient at V sites below T-X congruent to 30 K, and its motional fadeout at T-X. The temperature dependences of the V-51 Knight shift and relaxation time indicate the presence of a spin gap (Delta congruent to 250 K) below T-MI. These characteristic features may be interpreted as orbital ordering in a spin-singlet state. The possible existence of low-lying collective excitations of ordered orbitals is also argued. [S0031-9007(97)04475-X].