Robust Excitonic‐Insulating States in Cu‐Substituted Ta₂NiSe₅

Abstract

Excitonic-insulating phases can be realized in semimetals or narrow bandgap semiconductors where the bandgap is smaller than the excitonbinding energy (E ex ). [2,3] Since the theoretical suggestion of excitonic insulator in 1960s, [1][2][3] recent experimental studies have successfully demonstrated evidence of the excitonic-insulating phases in real crystal systems. [7][8][9][10][11][12][13][14][15][16] Among them, Ta 2 NiSe 5 has been investigated intensively because of its relatively higher excitonic transition temperature (T c ) at ≈326 K. [13,14] Ta 2 NiSe 5 consists of three layers with alternating chemically bonded Ta and Ni atoms sandwiched by two Se layers that are further stacked by van der Waals interaction along the (020) direction. [13][14][15] An orthorhombic structure, space group of Cmcm, with a direct bandgap at Γ point in the Brillouin zone is energetically stable at high temperature above T c . [11][12][13][14][15] The exciton-binding energy (E ex ) is larger than the bandgap of orthorhombic phase. The excitonic-insulating phase transition occurs with structural transformation into monoclinic (space group C2/c) when T is lower than T c . [15] The excitonicinsulating transition in Ta 2 NiSe 5 is mainly driven by strong Coulomb interaction between electron and hole and, moreover, does not involve charge density wave (CDW), [11][12][13][14][15] which is well contrasted to emergence of CDW in 1T-TiSe 2 excitonic Excitonic insulators exhibit intriguing quantum phases that further attract numerous interests in engineering the electrical and optical properties of Ta 2 NiSe 5 . However, tuning the electronic properties such as spin-orbit coupling strength and orbital repulsion via pressure in Ta 2 NiSe 5 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 Ta 2 NiSe 5 are demonstrated. The electron doping is conducted by substituting Cu into Ni site (Ta 2 Ni 1-x Cu x Se 5 ). The majority carrier of pristine sample is a hole-type and is converted to electrontype 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 Ta 2 NiSe 5 . The findings of invariant excitonic-insulating states in n-type Cu-substituted Ta 2 NiSe 5 can be utilized for further electronic device applications by using excitons.