Excitonic insulator: New perspectives in long-range interacting systems
Location: CECAM-HQ-EPFL, Lausanne, Switzerland
Fifty years ago a few outstanding physicists, including Leonid Keldysh and the Nobel prize recipient Walter Kohn, put forward a heretic paradigm of a strongly correlated insulator [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’ that would exhibit a distinctive broken symmetry, inherited by the exciton character, as well as peculiar collective modes of purely electronic origin. Intriguingly, the excitonic insulator, which shares similarities with the Bardeen-Cooper-Schrieffer superconducting ground state, could display unusual macroscopic quantum coherence effects [4-8]. So far, the observation of this phase has been elusive. The crux of the matter is the trade-off between competing effects in the semiconductor: as the size of the energy gap decreases, favouring spontaneous exciton generation, the screening of the electron-hole interaction increases, suppressing the exciton binding energy.
Very recently, novel low-dimensional systems and quantum devices seem to renew the promise of the excitonic insulator, as they combine optimal band structures, poor screening behavior, truly long-ranged interactions, and giant excitonic effects. These include systems as diverse as carbon nanotubes , low-dimensional  and van der Waals [11-14] heterostructures, Dirac and Weyl materials [15-17], topological insulators . 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 challenges, both theoretical and computational, to establish a roadmap to the excitonic insulator.
An additional list of recent works on the excitonic insulator is maintained at Cnr-Nano website
Oral presentations are by invitation only. Abstract submissions are open until 15 June 2018 through this website (please include title and abstract as well as your name, affiliation, position, field of research). The program timetable will cover three full working days, including a poster session with short poster presentation talks. All participants will have their workshop fees waived.
Elisa Molinari (Uni Modena e Reggio Emilia & CNR - Istituto Nanoscienze) - Organiser & speaker
Massimo Rontani (Cnr-Nano) - Organiser & speaker