Robust Excitonic‐Insulating States in Cu‐Substituted Ta₂NiSe₅

Abstract Excitonic insulators exhibit intriguing quantum phases that further attract numerous interests in engineering the electrical and optical properties of Ta₂NiSe₅. However, tuning the electronic properties such as spin‐orbit coupling strength and orbital repulsion via pressure in Ta₂NiSe₅ are always accompanied with electron‐hole pair breaking, which is a bottleneck for further applications. Here, the robust excitonic‐insulating states invariant with electron‐doping concentrations in Ta₂NiSe₅ are demonstrated. The electron doping is conducted by substituting Cu into Ni site (Ta₂Ni_1‐xCu_xSe₅). The majority carrier of pristine sample is a hole‐type and is converted to electron‐type with a doping concentration over x = 0.01, whose carrier density can be controlled by varying the Cu concentration. The excitonic transition temperature (T_c) does not significantly alter with electron‐doping concentrations, which is stark contrast with the declining T_c as the hole‐type dopant of Fe or Co increases. The optical conductivity data also demonstrate the invariant excitonic‐insulating states in Cu‐doped Ta₂NiSe₅. The findings of invariant excitonic‐insulating states in n‐type Cu‐substituted Ta₂NiSe₅ can be utilized for further electronic device applications by using excitons.