Ultralow electric-field-driven metal-insulator transition in two-dimensional topological nodal-line states of CaAs₃

In the vicinity of a topological quantum phase transition between direct-gap semiconducting and nodal-line semi-metallic (NLSM) phases, the interplay between band topology and electron correlations can drive instabilities toward exotic many-body states with novel electronic properties. Despite extensive theoretical predictions, experimentally identifying such correlated phases of NLSM state remains elusive. Here, we report that a direct-gap semiconductor CaAs₃, close to a topological quantum phase transition, hosts unconventional two-dimensional (2D) NLSM states, exhibiting unique temperature- and electric-field-driven metal-insulator transitions. At low temperatures, where bulk conduction is fully suppressed, a 2D metallic state emerges and subsequently turns into an insulating phase. This insulating phase exhibits anomalous Shubnikov–de Haas oscillations with unusual temperature dependence and undergoes an insulator-to-metal transition at an exceptionally small electric field strength of ~10⁻³ V/cm, the lowest ever reported. These unique electronic properties are unprecedented yet consistent with the excitonic insulating phase of NLSM states, which offers a promising platform for exploring novel functionalities of correlated NLSM states near a topological quantum phase transition.