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Thursday, 12 October 2017 at 3:15 pm

McCullough, First Floor Auditorium, Room #115

Host: Sean Hartnoll

Peter Abbamonte

University of Illinois Urbana

Bose condensation of excitons in TiSe2 measured with momentum-resolved EELS
Bose condensation has shaped our understanding of macroscopic quantum phenomena,
having been realized in superconductors, atomic gases, and liquid helium. Excitons are
bosons that have been predicted to condense into either a superfluid or an insulating
electronic crystal. But definitive evidence for a thermodynamically stable exciton
condensate has never been achieved. In this talk I will describe our use of momentumresolved
electron energy-loss spectroscopy (M-EELS) to study the valence plasmon in the
transition metal dichalcogenide semimetal, 1TTiSe2. Near the phase transition
temperature, TC = 190 K, the plasmon energy falls to zero at nonzero momentum,
indicating dynamical slowing down of plasma fluctuations and crystallization of the valence
electrons into an exciton condensate. At low temperature, the plasmon evolves into an
amplitude mode of this electronic crystal. Our study represents the first observation of a
soft plasmon in any material, the first definitive evidence for exciton condensation in a
three-dimensional solid, and the discovery of a new form of matter, “excitonium.”