Dynamics of Excitonic Insulators

Electron-hole attraction leads to a gap-opening transition driven by excitons
Enhancement of Raman response at low energies reveals the excitonic soft mode near Tc
Sulfur substitution suppresses the excitonic instability (blue) while the phase transition (black) persists at large x due to lattice instability

Attraction between electrons in metals drives superconductivity – an example of a macroscopic coherent state. In semiconductors, electrons and holes may sustain macroscopic coherence due to Coulomb attraction in another state – the excitonic insulator. With only a few candidate materials known, many of its features can be obscured by structural effects, making the excitonic insulator state challenging to identify. The interpretation and analysis I developed for polarization-resolved Raman spectroscopy allowed to reveal critical softening of an excitonic collective mode driving the transition in Ta2NiSe5, and coherent superposition of band states at the gap edge, analogous to coherence factors in a superconductor.

More recently, we explored the phase diagram of Ta2Ni(Se1-xSx)5 and found an excitonic quantum phase transition masked by the onset of a ferroelastic structural instability at lower temperatures.

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