A team of researchers comprised of Professor of Physics Peter Abbamonte and graduate students Anshul Kogar and Mindy Rak from University of Illinois at Urbana-Champaign, USA along with their colleagues at Illinois, University of California, Berkeley, and University of Amsterdam, have proven the existence of excitonium in non-doped crystals of the oft-analyzed transition metal dichalcogenide titanium diselenide (1T-TiSe2) [1,2]. This enigmatic new form of matter was first theorized by Harvard theoretical physicist Bert Halperin in the 1960s. Since then, physicists have been trying to demonstrate its existence.
Excitonium is a condensate made up of excitons which can exhibit macroscopic quantum phenomena like a superconductor, superfluid or insulating electronic crystal. When an electron at the electron crowded valence band in a semiconductor gets excited, it jumps over the energy gap to the otherwise empty conduction band leaving behind a “hole” in the valence band. The hole behaves as if it is a positively charged particle which can attract the escaped electron. Its particle-like nature is attributed to the collective behavior of surrounding crowd of electrons. The hole can thus pair with the escaped electron and form a composite boson particle termed as “exciton”.
Scientists due to unavailability of proper experimental tools until now were unable to distinguish between exciton condensation and the Peierls phases as both of them share the same symmetry and similar observables which is a superlattice and the opening of a single-particle energy gap. Abbamonte and his team have developed a novel technique called momentum-resolved electron energy-loss spectroscopy (M-EELS) which is more sensitive to valence band excitations than inelastic x-ray or neutron scattering techniques. With their new technique, they for the first time were able to observe a soft plasmon phase consisting of low energy boson particles i.e. the paired electrons and holes that emerged as the temperature of the material reached its critical value at 190 K. The observation of these soft-plasmons is the first ever evidence of the discovery of exciton condensation or excitonium in a three dimensional solid.
This important discovery would contribute much towards unfolding the mysteries of quantum mechanics which would help in studying macroscopic quantum phenomena. It might also help us in understanding the metal-insulator transition in band solids where excitonium is believed to play a role. Besides, it can offer numerous technological applications.
- Anshul Kogar, Melinda S. Rak, Sean Vig, Ali A. Husain, Felix Flicker, Young II Joe, Luc Venema, Greg J. MacDougall, Tai C. Chiang, Eduardo Fradkin, Jasper van Wezel, Peter Abbamonte. Signatures of exciton condensation in a transition metal dichalcogenide. Science 358 (6368), 1314 (2017). DOI: 10.1126/science.aam6432