State of Art of Excitonic Insulator

Dear Colleagues, 

Predicted at the end of ’60, the excitonic insulator (EI) phase represents one of the most fascinating paradigms of condensed matter: a macroscopic quantum coherent state of electron–hole pairs (excitons), which spontaneously form and condense at thermodynamic equilibrium. An EI was expected at low temperature in semiconductors where the exciton binding energy exceeds the size of the electronic band gap. After decades of intense searching and implementation in artificial systems, e.g., bilayer quantum Hall systems, conclusive evidence for the realization of an EI phase in real materials is still elusive. The two most promising and extensively investigated candidates are the van der Waals layered crystals 1T-TiSe₂ and Ta₂NiSe₅. However, the coexistence of excitonic correlation and structural instability in these materials has posed doubts regarding the fundamental mechanisms driving the band gap opening. In an attempt to disentangle the two effects on their characteristic time and energy scales, time-resolved ARPES and transient optical spectroscopy addressing the out-of-equilibrium system response to an ultrashort light perturbation have provided valuable insights. Recently, the ground-state symmetries and the nature of collective modes in the excitonic ordered phase of Ta₂NiSe₅ have been objects of theoretical studies through ab initio DFT calculation and Hartree–Fock mean-field modelling, as well as experimental investigations by steady-state and out-of-equilibrium Raman and luminescence spectroscopy. To date, the actual realization of an EI state is just beginning to be explored, and the envisioned opportunity to explore many-boson phenomena in an EI system triggers intense research efforts among different communities. By collecting research on the key components in the field of crystal growth, condensed matter theory, and equilibrium and out-of-equilibrium spectroscopy, the present Special Issue “State of the Art of Excitonic Insulators” aims to become a status report on the progress achieved and the timely challenges faced in grasping the EI physics in actual van der Waals layered systems.

Dr. Selene Mor
Dr. Igor Yurkevich
Dr. Qiang Zhang
Guest Editors

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• inorganic crystal growth of new excitonic insulator candidates
• layered semiconductors, 2D materials, electron–hole bilayers
• electron correlations
• collective modes
• excitonic fluctuations and structural instabilities
• photo-induced phenomena and non-equilibrium physics
• steady-state and time-resolved ARPES
• transient optical spectroscopy
• (time-resolved) Raman spectroscopy
• DFT calculations and mean-field theory