Detecting photoelectrons from spontaneously formed excitons

Abstract

Excitons, quasiparticles of electrons and holes bound by Coulombic attraction, are created transiently by light and play an important role in optoelectronics, photovoltaics and photosynthesis. While they are also predicted to form spontaneously in a small gap semiconductor or a semimetal, leading to a Bose-Einstein condensate at low temperature, their material realization has been elusive without any direct evidence. Here we detect the direct photoemission signal from spontaneously formed excitons in a debated excitonic insulator candidate Ta2NiSe5. Our symmetry-selective angle-resolved photoemission spectroscopy reveals a characteristic excitonic feature above the transition temperature, which provides detailed properties of excitons such as anisotropic Bohr radius. The present result evidences so called preformed excitons and guarantees the exciton insulator nature of Ta2NiSe5 at low temperature. Direct photoemission can be an important tool to characterize steady-state excitons.

Signatures of the exciton gas phase and its condensation in monolayer 1T-ZrTe2

“…Another distinct characteristic of excitonic condensate, in contrast to the conventional CDW state, is the existence of the exciton gas state in the BEC limit. However, due to the very limited candidate materials, the experimental observation of the exciton gas phase is still left to be explored 13 .…”

Ultrafast charge carrier and exciton dynamics in an excitonic insulator probed by time-resolved photoemission spectroscopy

“…Even so, recent theoretical [35,36] and experimental [37] studies pointed out that the lattice distortion coupled to the electronic instability is in fact needed to open the band gap. Finally, a recent ARPES study of the VB top of Ta 2 NiSe 5 above T C aimed at the extraction of the entwined photoemission (PE) signature of a precursor state of spontaneously formed excitons [38].…”

Anisotropic d–d Transition in Rutile TiO2

“…Light polarization-dependent photoelectron spectroscopy usually reveals the parity symmetry of the initial state, or a reflection symmetry with respect to the plane of incidence . However, parity symmetry usually comes with clear, characteristic peak emergence or extinction with different polarization, , rather than the systematic energy shift observed here. Meanwhile, breaking symmetry via the creation of the surface is bound to cause changes in electronic structure, most notably by the structural relaxation to vacuum, and hence causes surface states, systematic energy shift between bulk and surface features, and broken time reverse symmetry. − However, the experimental evidence presented above suggests that the observed energy shift between [110] and [11̅0] does not happen on the topmost surface, so that the structural relaxation at the vacuum interface is not the foremost reason.…”