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Signatures of exciton condensation in a transition metal dichalcogenide

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Science  08 Dec 2017:
Vol. 358, Issue 6368, pp. 1314-1317
DOI: 10.1126/science.aam6432

Probing an excitonic condensate

Excitons—bound states of electrons and holes in solids—are expected to form a Bose condensate at sufficiently low temperatures. Excitonic condensation has been studied in systems such as quantum Hall bilayers where physical separation between electrons and holes enables a longer lifetime for their bound states. Kogar et al. observed excitons condensing in the three-dimensional semimetal 1T-TiSe2. In such systems, distinguishing exciton condensation from other types of order is tricky. To do so, the authors used momentum-resolved electron energy-loss spectroscopy, a technique developed to probe electronic collective excitations. The energy needed to excite an electronic mode became negligible at a finite momentum, signifying the formation of a condensate.

Science, this issue p. 1314