WORKSHOP: Excitonic and competing orders in low-dimensional materials  

Pisa Modena - 21.06.2021 - WORKSHOP: Excitonic and competing orders in low-dimensional materials

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 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.

[1] Keldysh, L. V. & Kopaev, Yu. V. Possible instability of the semimetallic state against Coulomb interaction. Fiz. Tverd. Tela., 6, 2791 (1964) [Sov. Phys. Solid State 6, 2219 (1965)].
[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] Du, L. et al. Evidence for a topological excitonic insulator in InAs/GaSb bilayers. Nat. Comm. 8, 1971 (2017).
[5] Varsano, D. et al. Carbon nanotubes as excitonic insulators. Nat. Comm. 8, 1461 (2017).
[6] Li, J. I. A. et al. Excitonic superfluid phase in double bilayer graphene. Nat. Phys. 13, 751 (2017).
[7] Liu, X. et al. Quantum Hall drag of exciton condensate in graphene. Nat. Phys. 13, 746 (2017).
[8] Kogar, A. et al. Signatures of exciton condensation in a transition metal dichalcogenide. Science 358, 1314 (2017).
[9] Lu, Y. F. et al. Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5. Nat. Comm. 8, 14408 (2017).
[10] Werdehausen, D. et al. Coherent order parameter oscillations in the ground state of the excitonic insulator Ta2NiSe5. Sci. Adv. 4, aap8652 (2018).
[11] Zhu, Z. et al. Graphite in 90 T: evidence for strong-coupling excitonic pairing. Phys. Rev. X 9, 011058 (2019).
[12] Li, Z. et al. Possible excitonic insulating phase in quantum-confined Sb nanoflakes. Nano Lett. (2019), in press.
[13] Jia, Z.-Y. et al. Direct visualization of a two-dimensional topological insulator in the single layer 1T’- WTe2. Phys. Rev. B 96, 041108 (2017).
[14] Fei, Z. et al. Edge conduction in monolayer WTe2. Nat. Phys. 13, 677 (2017).
[15] Song, Y.-H. et al. Observation of Coulomb gap in the quantum spin Hall candidate single layer 1T’-WTe2. Nat. Comm. 9, 4071 (2018).
[16] Xue, F. & MacDonald, A. H. Time-reversal symmetry-breaking nematic insulators near quantum spin Hall phase transitions. Phys. Rev. Lett. 120, 186802 (2018).
[17] Hu, Y., Venderbos, J. W. F. & Kane, C. L. Fractional excitonic insulator. Phys. Rev. Lett. 121, 126601 (2018).
[18] Zhu, Q. et al. Gate tuning from exciton superfluid to quantum anomalous Hall in van der Waals heterobilayer. Sci. Adv. 5, eaau6120 (2019).
[19] Varsano, D. et al. A monolayer transition metal dichalcogenide as a topological excitonic insulator. arXiv:1906.07971.
[20] Fei, Z. et al. Ferroelectric switching of a two-dimensional metal. Nature 560, 336 (2018).
[21] Sajadi, E. et al. Gate-induced superconductivity in a monolayer topological insulator. Science 362, 922 (2018).
[22] Fatemi, V. et al. Electrically tunable low-density superconductivity in a monolayer topological insulator. Science 362, 926 (2018).
[23] Chi Z. et al. Superconductivity in pristine 2Ha – MoS2 at ultrahigh pressure. Phys. Rev. Lett. 120, 037002 (2018).

As the number of attendees is limited, if you are interested in this workshop please apply by June 6.
We would appreciate if you could provide us with the essential info concerning your current research activity and position. Accepted participants are encouraged to present a poster.

Elisa Molinari (Uni Modena e Reggio Emilia & CNR - Istituto Nanoscienze)
Massimo Rontani (Cnr-Nano)

An online meeting, hosted by CECAM-HQ
Additional sponsor: Psi-k



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