One-Dimensional Instability in BaVS₃

The 3d¹ system BaVS₃ undergoes a series of remarkable electronic phase transitions. We show that the metal-insulator transition at T_MI=70   K is associated with a structural transition announced by a huge regime of one-dimensional (1D) lattice fluctuations, detected up to 170 K. These 1D fluctuations correspond to a 2k_F=c^*/2 charge-density wave (CDW) instability of the d_z² electron gas. We discuss the formation below T_MI of an unconventional CDW state involving the condensation of the other V⁴⁺ 3d¹ electrons of the quasidegenerate e(t_2g) orbitals. This study stresses the role of the orbital degrees of freedom in the physics of BaVS₃ and reveals the inadequacy of current first principle band calculations to describe its electronic ground state.