Unexpected stabilization of a single-q charge density wave in pristine 1T-TiSe2

Over the past decade, studies revisiting the classical charge-density-wave system 1T-TiSe₂, long understood to host the 2 × 2 triple-q order with equivalent subcomponents, have suggested that these subcomponents may not be identical, challenging a 50-year-old paradigm. Here, we demonstrate not only that these subcomponents are non-identical but also that they develop sequentially rather than simultaneously. By tracking folded-band features, angle-resolved photoemission spectroscopy reveals that a unidirectional 2 × 1 single-q order stabilizes first, while the other two components emerge sequentially at lower temperatures, completing the triple-q phase. Our free-energy analysis explains this hierarchy through repulsive coupling among the three subcomponents, as supported by first-principles calculations incorporating phonon anharmonicity. Consequently, the three subcomponents are inequivalent in both phase and amplitude. These results reconcile reported symmetry breaking and differing characteristic temperatures and establish a framework in which ostensibly simple, high-symmetry charge-density order emerges from temperature-dependent competition and cooperation among inequivalent subcomponents. The authors show that the subcomponents of the charge density wave in 1T-TiSe₂ are unequal and set in one by one, beginning with an unexpected unidirectional state.