Lattice fluctuation and strong electron-phonon coupling in Ta2NiSe5

Graphene lattice on blue gradient
September 26, 2022

Monday, September 26, 2022 from 2:30-3:30 p.m.
Location: 325 Physics South Hall & Zoom
Speaker: Cheng Chen, LBNL/University of Oxford

Abstract: Analogous to Cooper pair condensation in superconductors, when electron-hole pairs (excitons) condense, an excitonic insulator is born. Creation of an exciton condensate has been demonstrated in bilayer junctions, but remains challenging in bulk systems due to either fast recombination or an often inseparable lattice instability. Using angle-resolved photoemission spectroscopy and hard x-ray scattering, we examine the nature of the broken symmetry in a leading ‘excitonic insulator’ candidate - van der Waals chalcogenide Ta2NiSe5. Strong lattice fluctuation induced pseudogap state, diverging structural susceptibility, and negative electronic compressibility all point to a lattice instability origin over a true exciton condensation in this system. With this example, we also demonstrate how model calculations can be paired with nonthermal tuning methods to estimate microscopic interaction parameters, help delineate multiple degrees of freedom in "intertwined" phase transitions.

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