State of Art of Excitonic Insulators and Topological Materials

Dear Colleagues, 

Predicted at the end of 1960, 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 the 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 multi-boson phenomena in an EI system is triggering intense research efforts among different communities.

Topological materials are characterized by properties, referred to as topological invariants, that can be connected to a continuous evolution in momentum of the electronic wavefunctions. Discovered at the end of 2000 in bismuth-based binary compounds, 3D topological insulators (TIs) insulate in bulk and conduct at the surface. More recently, topological Weyl and Dirac semimetals containing chalcogens and pnictogens have emerged as another class of topological materials where the bulk is semimetal, and the valence and conduction bands cross at near the Fermi level. While topology is a fundamental concept in condensed matter, only recently it has been merged with the occurrence of strong electron–hole correlation within a single quantum phase called topological EI. The first experimental evidence was found in 2D systems like InAs/GaSb quantum-well bilayers. Very recently, theory predicted the realization of a topological EI phase in flat bands of twisted Van der Waals heterostructures and spectroscopy studies reported fingerprints of this matter phase in 3D Ta2Pd3Te5 semimetals.

By collecting the key actors in the field of crystal growth, condensed matter theory, and equilibrium and out-of-equilibrium spectroscopy, the present Special Issue on “State of Art of Excitonic Insulators and Topological Materials” represents a status report of the progress achieved and the timely challenges towards grasping 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