Excitonic insulator: New perspectives in long-range interacting systems

September 3, 2018 to September 5, 2018
Location : CECAM-HQ-EPFL, Lausanne, Switzerland
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  • Massimo Rontani (CNR-NANO, Modena, Italy)
  • Elisa Molinari (University of Modena and Reggio Emilia & CNR-NANO, Modena, Italy)



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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


Abstract submission

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.



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[2] Jèrome, D., Rice, T. M. & Kohn, W. Excitonic insulator. Phys. Rev. 158, 462 (1967).
[3] Halperin, B. I. & Rice, T. M. The excitonic state at the semiconductor-semimetal transition. Solid State Phys. 21, 115 (1968).
[4] Lozovik, Y. E. & Yudson, V. I. A new mechanism for superconductivity: pairing between spatially separated electrons and holes. Zh. Eksp. i Teor. Fiz. 71, 738 (1976). [Sov. Phys.–JETP 44, 389 (1976)].
[5] Portengen, T., Oestreich, T. & Sham, L. J. Theory of electronic ferroelectricity. Phys. Rev. B 54, 17452 (1996).
[6] Balatsky, A. V., Joglekar, Y. N. & Littlewood, P. B. Dipolar superfluidity in electron-hole bilayer systems. Phys. Rev. Lett. 93, 266801 (2004).
[7] Rontani, M. & Sham, L. J. Coherent transport in a homojunction between an excitonic insulator and semimetal. Phys. Rev. Lett. 94, 186404 (2005).
[8] Su, J. & MacDonald, A. H. How to make a bilayer exciton condensate flow. Nature Phys. 4, 799 (2008).
[9] Varsano, D. et al. Carbon nanotubes as excitonic insulators. Nat. Comm. 8, 1461 (2017).
[10] Nandi, A. et al. Exciton condensation and perfect Coulomb drag. Nature 488, 481 (2012).
[11] Kogar, A. et al. Signatures of exciton condensation in a transition metal dichalcogeneide. Science 358, 1314 (2017).
[12] Rodin, A. S. & Castro Neto, A. H. Excitonic collapse in semiconducting transition-metal dichalcogenides. Phys. Rev. B 88, 195437 (2013).
[13] Fogler, M. M., Butov, L. V. & Novoselov, K. S. High-temperature superfluidity with indirect excitons in van der Waals heterostructures. Nat. Comm. 5, 4555 (2014).
[14] Stroucken, T. & Koch, S. W. Optically bright p-excitons indicating strong Coulomb coupling in transition-metal dichalcogenides. J. Phys.: Cond. Matter 27, 345003 (2015).
[15] Khveshchenko, D. V. Ghost excitonic insulator transition in layered graphite. Phys. Rev. Lett. 87, 246802 (2001).
[16] Drut, J. E. & Laende, T. A. Is graphene in vacuum an insulator? Phys. Rev. Lett. 102, 026802 (2009).
[17] Min, H., Bistritzer, R., Su, J. & MacDonald, A. H. Room-temperature superfluidity in graphene bilayers. Phys. Rev. B 78, 121401(R) (2008).
[18] Seradjeh, B., Moore, J. E. & and Franz, M. Exciton Condensation and Charge Fractionalization in a Topological Insulator Film. Phys. Rev. Lett. 103, 066402 (2009).