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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 (Tc) does not significantly alter with electron‐doping concentrations, which is stark contrast with the declining Tc 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. [ABSTRACT FROM AUTHOR]

Octahedral symmetry is one of the parameters to tune the functional properties of complex oxides. VO 2 , a complex oxide with a 3 d 1 electronic system, exhibits an insulator-metal transition (IMT) near room temperature (∼68 °C), accompanying a change in the octahedral structure from asymmetrical to symmetrical. However, the role of octahedral symmetry in VO 2 on the IMT characteristics is unclear. Crystal and electronic structure analyses combined with density-functional-theory calculations showed the bandwidth-controlled IMT characteristics of monoclinic VO 2 with high octahedral symmetry. The expanded apical V-O length for a high octahedral symmetry of a VO 2 film increased the bandwidth of the conduction band by depressing V 3 d -O 2 p hybridization. As a result, the interdimer hopping energy increased and thereby decreased the IMT temperature, although the short V-V chain enhanced electron correlation. These findings suggest that octahedral symmetry can control the IMT characteristics of VO 2 by changing the orbital occupancy.