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Coherent order parameter oscillations in the ground state of the excitonic insulator Ta2NiSe5.

ABSTRACT: The excitonic insulator is an intriguing electronic phase of condensed excitons. A prominent candidate is the small bandgap semiconductor Ta2NiSe5, in which excitons are believed to undergo a Bose-Einstein condensation-like transition. However, direct experimental evidence for the existence of a coherent condensate in this material is still missing. A direct fingerprint of such a state would be the observation of its collective modes, which are equivalent to the Higgs and Goldstone modes in superconductors. We report evidence for the existence of a coherent amplitude response in the excitonic insulator phase of Ta2NiSe5. Using nonlinear excitations with short laser pulses, we identify a phonon-coupled state of the condensate that can be understood as a novel amplitude mode. The condensate density contribution substantiates the picture of an electronically driven phase transition and characterizes the transient order parameter of the excitonic insulator as a function of temperature and excitation density.

SUBMITTER: Werdehausen D 

PROVIDER: S-EPMC5938280 | biostudies-literature | 2018 Mar

REPOSITORIES: biostudies-literature

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Coherent order parameter oscillations in the ground state of the excitonic insulator Ta<sub>2</sub>NiSe<sub>5</sub>.

Werdehausen Daniel D   Takayama Tomohiro T   Höppner Marc M   Albrecht Gelon G   Rost Andreas W AW   Lu Yangfan Y   Manske Dirk D   Takagi Hidenori H   Kaiser Stefan S  

Science advances 20180323 3

The excitonic insulator is an intriguing electronic phase of condensed excitons. A prominent candidate is the small bandgap semiconductor Ta<sub>2</sub>NiSe<sub>5</sub>, in which excitons are believed to undergo a Bose-Einstein condensation-like transition. However, direct experimental evidence for the existence of a coherent condensate in this material is still missing. A direct fingerprint of such a state would be the observation of its collective modes, which are equivalent to the Higgs and G  ...[more]

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