Discovery of an Electronic Goldstone mode in the (Putative) Exitonic Insulator, 1T-TiSe2

Condensed Matter Seminars

Speaker: Peter Abbamonte, University of Illinois
Date & Time: May 27, 2016, 2:00 - 3:00
Location: AMPEL 311
Local Contact: George Sawatzky
Intended Audience: Graduate

1T-TiSe2 is a layered, transition metal dichalcogenide that exhibits a competition between superconducting and charge density wave (CDW) ground states, mimicking many aspects of the phase diagram of high temperature superconductors. Because of its nearly inverted band structure, the CDW phase has often been interpreted as an “excitonic insulator” ground state formed via spontaneous proliferation of excitons with finite momentum. Some skeptics have argued, however, that the CDW is a conventional Peierls phase driven by electron-phonon interactions, setting off a (partly semantic) debate about the relative importance of lattice and Coulomb effects in this system.

In this talk I will report the discovery, using momentum-resolved M-EELS, of a new type of electronic collective mode in TiSe2. Previously misinterpreted as a plasmon excitation in infrared experiments, I will show that this excitation is in fact the top of the dispersion branch of the electronic Goldstone (or “phason”) mode of the CDW phase, and can be thought of as the amplitude mode of the electron-hole condensate. I will discuss, in the context of the interaction between this mode and the acoustic phonons, the best way to define an excitonic insulator in a real material in which the lattice does not necessarily stay put.