Emergence of a real-space symmetry axis in the magnetoresistance of the one-dimensional conductor Li _0.9 Mo ₆ O ₁₇

Abstract

We report on an emerging symmetry axis in the magnetoresistance of bulk single crystals of quasi-one-dimensional Li_0.9Mo₆O₁₇ below <i>T</i> _min = 25 K, the temperature at which the electrical resistivity experiences a minimum. Detailed angle-dependent magnetoresistance sweeps reveal that this symmetry axis is induced by the development of a negative magnetoresistance, which is suppressed only for magnetic fields oriented along the poles of the MoO₆ octahedra that form the conducting chains. We show that this unusual negative magnetoresistance is consistent with the melting of dark excitons, composed of previously omitted orbitals within the <i>t</i> _2g manifold that order below <i>T</i> _min. The unveiled symmetry axis in directional magnetic fields not only provides evidence for the crystallization of these dark excitons but also sheds new light on the long-standing mystery of the metal-insulator transition in Li_0.9Mo₆O₁₇.