Excitonic insulator: New perspectives in long-range interacting systems  

Modena - 03.09.2018 - 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 [9], low-dimensional [10] and van der Waals [11-14] heterostructures, Dirac and Weyl materials [15-17], topological insulators [18]. 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

Massimo Rontani (CNR-NANO Modena, Italy, Italy)
Elisa Molinari (University of Modena and Reggio Emilia & CNR-NANO, Modena, Italy)



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