Excitonic insulator emerging from semiconducting normal state in 1T-TiSe2
Abstract
A new state of matter, an excitonic insulator (EI) state, was predicted to emerge from Bose-Einstein condensation of electron-hole pairs. Some candidate materials were suggested but it has been elusive to confirm its existence. Recent works gave renewed support for the EI picture of the charge density wave (CDW) state below the critical temperature Tc ≈ 200 K of 1T-TiSe2. Yet, an important link to its establishment is to show that a majority fraction of the measured Tc indeed follows from the Coulomb interaction alone, while a quantitative match of the Tc may require assistance from the electron-lattice coupling. This will establish that the CDW is formed predominantly by the Coulomb interaction and help confirm the EI view for TiSe2. Here, we provide such calculations by solving the exciton gap equation with material specific electronic structures. We obtain, with no fitting parameters, Tc ≈ 135 27 K for the normal state gap of Eg ≈ 74 15 meV. It seems that the calculated Tc from Coulomb interaction gives a majority fraction of experimental Tc for recently determined values of Eg. The measured doping dependence of Tc was satisfactorily reproduced as well. Also in agreement with experiments are the same set of calculations of the photoemission spectroscopy and density of states. The semiconducting state above and EI below Tc together should give a coherent picture of 1T-TiSe2.