Robust Excitonic-Insulating States in Cu-Substituted Ta2NiSe5

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 (Ta2Ni1-xCuxSe5). 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.