Condensed Matter Theory Seminar: Benjamin Remez, University of Cambridge, “Collective Phenomena in Excitonic Matter”

Abstract: Bound electron-hole pairs, known as excitons, let us realize a plethora of bosonic correlated phases in the solid state. In recent years, the ability to manipulate excitonic states of matter has advanced significantly thanks to novel materials, most notably transition metal chalcogenides such as Ta2NiSe5, TiSe2, and WS2/MoS2 heterobilayers, providing us new access to this rich phase diagram. We focus on the many-body physics unique to these symmetry-breaking excitonic phases and show that their low-energy and mesoscale properties are dominated by collective phenomena, both in and out of equilibrium. First, we present a theory of disorder in equilibrium excitonic insulators [1], and show that their Goldstone collective excitations are robust against impurity scattering. We demonstrate this is a universal feature of the symmetry classification of scattering channels, and which explains recently observed room temperature mesoscale ballistic transport in Ta2NiSe5 [2]. Subsequently, we reconsider the optical footprint of so-called “dark condensates” of moire excitons in twisted dichalcogenide bilayers [3]. We propose that these are instead “leaky condensates” that can emit thanks to strong exciton interactions and the spontaneous production of Bogoliubov collective modes. We demonstrate how this process generates a distinctive optical signature and dominates emission at low temperatures.
[1] B. Remez and N. R. Cooper, Effects of disorder on the transport of collective modes in an excitonic condensate, Phys. Rev. B 101, 235129 (2020).
[2] H. M. Bretscher, P. Andrich, Y. Murakami, D. Golež, B. Remez, et al., Imaging the coherent propagation of collective modes in the excitonic insulator Ta2NiSe5 at room temperature, Sci. Adv. 7, eabd6147 (2021).
[3] B. Remez and N. R. Cooper, Dark and leaky exciton condensates in transition metal dichalcogenide moire bilayers, arXiv:2110.07628.
Host: Meng Cheng
Please contact Taylor Dunnigan (taylor.dunnigan@yale.edu) for zoom connection information.