In the last twelve months, the field of the excitonic insulator has moved extremely fast.
This research has its origin in a heretic prediction formulated more than 50 years ago by a group of visionary physicists, including Leonid Keldysh and Walter Kohn [1-3]: If a narrow-gap semiconductor, or a semimetal with slightly overlapping conduction and valence bands, failed to fully screen its intrinsic charge carriers, then excitons---electron-hole pairs bound together by Coulomb attraction---would spontaneously form. This would destabilize the ground state, leading to a reconstructed ‘excitonic insulator’---a condensate of excitons at thermodynamic equilibrium. This chimeric phase shares fascinating similarities with the Bardeen-Cooper-Schrieffer superconductor: a distinctive broken symmetry, inherited by the exciton character, and collective modes of purely electronic origin. Its observation was deterred for many decades by the trade-off between competing effects in the semiconductor: as the size of the energy gap decreases, favoring spontaneous exciton generation, the screening of the electron-hole interaction increases, suppressing the exciton binding energy. In the last two years, mounting evidence [4-12] has been accumulating in low-dimensional materials, as they combine optimal band structures, poor screening behavior, truly long-ranged interactions, and giant excitonic effects (see also the list of recent literature maintained at www.nano.cnr.it/index.php?mod=men&id=196). This was the topic of our first Cecam Workshop in September 2018.
The last year has witnessed mounting indications that the most promising excitonic insulator candidates are 2d materials. These candidates however exhibit also other kinds of order: a variety that includes topological insulators [4,13-19], ferroelectrics [19,20], unconventional superconductors [21-23], often depending on tiny variations of tunable parameters, such as doping, pressure, strain. This has introduced new urgent and far-reaching questions, concerning the role of excitonic correlations in a plethora of allegedly unrelated phenomena, whose interplay is just beginning to be explored. At the same time, the long-term challenge of establishing the excitonic insulator through the signatures of macroscopic quantum coherence is attracting renewed interest in this class of materials.
By collecting the key actors of theoretical and experimental research, who are spread among different communities, this Workshop aims at in-depth analysis of common themes and novel challenges, both theoretical and computational, to progress our understanding of interacting systems in low dimensions.