Twistronics

Twistronics is the study of how the angle (the twist) between layers of two-dimensional materials can change their electrical properties.[1] Materials such as graphene have been shown to have vastly different electrical properties (ranging from non-conductive to superconductive) depending on the angle between the layers.[2][3]

History

In 2007, National University of Singapore physicist Antonio Castro Neto hypothesized that pressing two misaligned graphene sheets together might yield new electrical properties, and separately proposed that graphene might offer a route to superconductivity, but he did not combine the two ideas.[2] The theoretical basis of twistronics was further developed by a 2011 paper by Allan MacDonald that predicted that a specific "magic angle" would radically change the amount of energy a free electron would require to tunnel between two graphene sheets.

This hypothesis was confirmed by Pablo Jarillo-Herrero and his student Yuan Cao of MIT and colleagues from Harvard and the National Institute for Materials Science in Tsukuba, Japan, who verified in 2018 that superconductivity existed in bilayer graphene where one layer was rotated by an angle of 1.1° relative to the other, forming a Moiré pattern, at a temperature of 1.7 K (−271.45 °C; −456.61 °F).[4][5][6] Yuan Cao created two bilayer devices that acted as an insulator instead of a conductor under a magnetic field. Increasing the field strength turned the second device into a superconductor. Because the only variable is field strength, studying superconductivity in bilayer graphene is faster and more convenient than working with multi-element cuprate crystals.[2]

Experiments have also been done using combinations of graphene layers with other materials that form heterostructures in the form of atomically thin sheets that are held together by the weak Van der Waals force.[7] For example, a study published in Science in July 2019 found that with the addition of a boron nitride lattice between two graphene sheets, unique orbital ferromagnetic effects were produced at a 1.17° angle, which could be used to implement memory in quantum computers.[8] Further spectroscopic studies of twisted bilayer graphene revealed strong electron-electron correlations at the magic angle.[9]

Publication of these discoveries has generated a host of theoretical papers seeking to understand and explain the phenomena[10] as well as numerous experiments[3] using varying numbers of layers, twist angles and other materials.[2][11]

References

  1. Carr, Stephen; Massatt, Daniel; Fang, Shiang; Cazeaux, Paul; Luskin, Mitchell; Kaxiras, Efthimios (2017-02-17). "Twistronics: Manipulating the electronic properties of two-dimensional layered structures through their twist angle". Physical Review B. 95 (7). arXiv:1611.00649. doi:10.1103/PhysRevB.95.075420. ISSN 2469-9950.
  2. Freedman, David H. (2019-04-30). "How Twisted Graphene Became the Big Thing in Physics". Quanta Magazine. Retrieved 2019-05-05.
  3. Gibney, Elizabeth (2019-01-02). "How 'magic angle' graphene is stirring up physics". Nature. 565 (7737): 15–18. doi:10.1038/d41586-018-07848-2. PMID 30602751.
  4. Jarillo-Herrero, Pablo; Kaxiras, Efthimios; Taniguchi, Takashi; Watanabe, Kenji; Fang, Shiang; Fatemi, Valla; Cao, Yuan (2018-03-06). "Magic-angle graphene superlattices: a new platform for unconventional superconductivity". arXiv:1803.02342. doi:10.1038/nature26160. Cite journal requires |journal= (help)
  5. Cao, Yuan; Fatemi, Valla; Demir, Ahmet; Fang, Shiang; Tomarken, Spencer L.; Luo, Jason Y.; Sanchez-Yamagishi, Javier D.; Watanabe, Kenji; Taniguchi, Takashi (2018-04-01). "Correlated insulator behaviour at half-filling in magic-angle graphene superlattices". Nature. 556 (7699): 80–84. arXiv:1802.00553. Bibcode:2018Natur.556...80C. doi:10.1038/nature26154. ISSN 0028-0836.
  6. Wang, Brian (2018-03-07). "Graphene superlattices could be used for superconducting transistors". NextBigFuture.com. Retrieved 2019-05-03.
  7. "1 + 1 does not equal 2 for graphene-like 2-D materials". phys.org. Retrieved 2019-08-01.
  8. Than, Ker (2019-07-26). "Physicists discover new quantum trick for graphene: magnetism". phys.org. Retrieved 2019-07-27.
  9. Scheurer, Mathias S. (2019-07-31). "Spectroscopy of graphene with a magic twist". Nature. 572 (7767): 40–41. doi:10.1038/d41586-019-02285-1.
  10. Freedman, David H. (2019-05-28). "What's the Magic Behind Graphene's 'Magic' Angle?". Quanta Magazine. Retrieved 2019-05-28.
  11. "Experiments explore the mysteries of 'magic' angle superconductors". phys.org. 2019-07-31. Retrieved 2019-07-31.
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