High Temperature Excitonic Insulator and Possible Superfluid Based on Two-Dimensional Diamond

Recent research on excitonic insulator has progressed mainly based on narrow bandgap semiconductor or semimetal. Herein, we realize excitonic insulator based on two-dimensional (2D) wide band gap diamond with transition temperature as high as 220K. The resistance rises dramatically by more than three orders, which can be explained by the Bose-Einstein condensation (BEC) of excitons. While cooling down below transition temperature, the wavelength of the bounded exciton caused by boron and nitrogen centers becomes highly overlapped, leading to BEC process. Furthermore, the variable hopping mechanism is confirmed in agreement with the exciton transport process. When temperature drops down further, a sudden drop of resistance over three orders was observed around 60K, possibly due to the formation of excitonic superfluid resulting from highly overlap of wavelength of the large density bounded excitons at lower temperature. This study provides evidences for excitonic insulator and possible superfluid phase based on wide bandgap semiconductor.