Collective modes of the excitonic condensate in 1T-TiSe2
When light is absorbed by a semiconductor, it typically does so by promoting an electron into the conduction band from the valence band, where it leaves a positively charged hole. Under the right circumstances, the electron and hole--which have opposite charge--will form a bound state rather like a hydrogen atom. Such bound states are called "excitons," and are routinely observed in absorption spectra of materials like silicon or germanium.
In the 1960s, a Soviet physicist named Leonid Keldysh pointed out that, if the band gap of a semiconductor were sufficiently small, the energy of an exciton could be negative. In this situation, excitons would spontaneously proliferate, forming an exotic, many-body phase called an "excitonic insulator," which is a macroscopic condensate of electron-hole pairs. For the last 50 years, physicists have searched for an excitonic insulator in nature. Despite many promising candidates, proof of the existence of such a phase has never been found. The reason is that its tell-tale signature—an electronic “soft mode” with finite momentum—could not be detected with any experimental technique.
In this talk I will present the first definitive evidence for the existence of an excitonic insulator, which we have observed in the transition metal dichalcogenide 1T-TiSe2 using a new, meV-resolved electron energy-loss scattering (M-EELS) technique. I will show that, while the prevailing electronic mode at room temperature is a conventional plasmon, near the onset temperature of CDW order it disperses to zero energy at finite momentum, signifying the formation of an excitonic state. At lower temperatures, the excitation hardens becomes an amplitude mode of the electron-hole condensate. Our study represents the first observation of a soft electronic mode, and the first unambiguous evidence for the existence of an excitonic insulator in any material.
Hosted by Steve Anlage.