Excitonic order in quantum materials: fingerprints, platforms and opportunities

TL;DR

This review discusses the theoretical foundations, experimental signatures, candidate materials, and future challenges of excitonic order in quantum materials, emphasizing its distinction from other insulating states and potential for novel quantum phases.

Contribution

It provides a comprehensive overview of excitonic insulators, integrating recent experimental advances, theoretical insights, and material platforms, highlighting new opportunities in the field.

Findings

Identification of experimental fingerprints of excitonic order

Strategies to distinguish excitonic states from competing phases

Survey of diverse candidate materials and platforms

Abstract

The exciton insulator (EI) is a unique many-body ground state of condensed, spontaneously formed excitons (electron-hole pairs) in equilibrium, distinct from conventional band or Mott insulators. Originally proposed over half a century ago, the concept has recently gained renewed experimental traction thanks to advances in spectroscopic resolution, ultrafast probes, and materials synthesis. In this Review, we outline the essential theoretical ingredients underpinning excitonic order and discuss how dimensionality, disorder and screening affect stability. We then examine the diverse experimental fingerprints of the excitonic state, with central focus on strategies to disentangle excitonic order from competing phases such as charge density waves, Mott insulating states, and hybridization-driven insulators, particularly in systems where non-trivial band topology plays a role. We survey the rapidly expanding family of candidate materials, from layered chalcogenides and correlated rare-earth compounds to artificial excitonic platforms and optically driven non-equilibrium condensates. Finally, we discuss the key challenges and emerging opportunities in the field, identifying the theoretical and experimental frontiers that promise to shape the next decade of research.