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Genís Lleopart

Departament de Ciència de Materials i Química Física &
Institut de Química Teòrica i Computacional (IQTC), Universitat de Barcelona

 “Evidence for low-lying correlated gapped states in strained graphene and α-graphyne”

Abstract

Graphene is a 2D gapless Dirac semimetal with ultrahigh charge/spin mobilities. The absence of a semiconducting gap means that currents in graphene cannot be easily modulated, as required in devices (e.g. on/off switching in transistors). Top-down strategies to induce a gap in graphene by introducing defects, distortions, doping, etc. in the 2D material have been largely unsuccessful. A new class of magnetic organic materials called Extended graphenic (XT-graphenic) or Covalent Organic Frameworks (CORFs), which keep the graphene hexagonal symmetry but introduce linkers between α-C [1], have been proposed, showing to be gapless too at their ground-state and bring some light to the opening of its gap by mechanical deformations [2] (compressing and stretching) or applying external electromagnetic fields. Here we focus on the nature of the ground-state of bi-axial strained graphene and α-graphyne. The standard methodology to analyse electronic structure of materials, Density Functional Theory (DFT), have shown to be a useful tool, especially if one uses hybrid density functionals. However, the description of the system depends on the amount of Hartree-Fock Exchange (HFE), which can be tuned [3]. Although nowadays is well known what is the proper way to apply DFT for a rich set of systems (e.g. metals and inorganic systems), due to its hexagonal symmetry and the different electronic solutions in competition (Semimetallic, Antiferromagnetic and Quinoid/Dimerized), CORFs have highlighted some of the deficiencies of DFT and the need of a consistent method in order to reach the physically meaningful description. In addition to DFT calculations, Hubbard-type models will be used to rationalize the nature of low-energy electronic states and its dynamics [4]. Using accurate benchmark results for the latter, we show that the relative energetic stability of electronic states in this correlated 2D system can be accurately captured by DFT calculations using carefully tailored hybrid functionals to extract effective t and U Hubbard parameters. Our tuned DFT approach demonstrates that, while graphene maintains a SEM ground state up to moderate strains, in α-graphyne a low energy correlated Mott-like antiferromagnetic insulating (AFI) state emerges at low strains, which competes with the SEM state [5]. Accurate calculations show that strained graphene possesses correlated gapped states that are not recovered by GGA-based DFT. We show that an electronic analogy exists between this system and α-graphyne and that both are well-described by tuned hybrid (non-GGA) DFT.

References

[1] J. Adjizian, P. Briddon, B. Humbert, et al.  Nat. Commun. 2014, 5, 5842.

[2] S. Sorella, K. Seki, O. O. Brovko, T. Shirakawa, S. Miyakoshi, S. Yunoki and E. Tosatti,  Phys. Rev. Lett.  2018, 121, 066402.

[3] I. Alcón, F. Viñes, I. de P. R. Moreira and S. T. Bromley, Nat. Commun. 2017, 8, 1957.

[4] B. Dong, H. Guo, Z. Liu, T. Yang, P. Tao, S. Tang, R. Saito, and Z. Zhang, Carbon 2018, 131, 223.

[5] G. Lleopart, I. de P. R. Moreira and S. T. Bromley, submitted.