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Mater. 3 (2021) 042006 https://doi.org/10.1088/2515-7639/abb74e Journal of Physics: Materials ROADMAP The 2021 quantum materials roadmap OPEN ACCESS Feliciano Giustino1,2, Jin Hong Lee3, Felix Trier3, Manuel Bibes3, 4 4 5 6,7 8 RECEIVED Stephen M Winter , Roser Valentí , Young-Woo Son , Louis Taillefer , Christoph Heil , 18 May 2020 Adriana I Figueroa9, Bernard Plaçais10, QuanSheng Wu11, Oleg V Yazyev11, Erik P A M Bakkers12, REVISED 13 14,15 16 17 18 5 August 2020 Jesper Nygård , Pol Forn-Díaz , Silvano De Franceschi , J W McIver , L E F Foa Torres , Tony Low19, Anshuman Kumar20, Regina Galceran9, Sergio O Valenzuela9,21, Marius V Costache9, ACCEPTED FOR PUBLICATION 22 23 24,25 24,25 10 September 2020 Aur´elien Manchon , Eun-Ah Kim , Gabriel R Schleder , Adalberto Fazzio and 9,21 PUBLISHED Stephan Roche 19 January 2021 1 Oden Institute for Computational Engineering and Sciences, The University of Texas at Austin, Austin, TX 78712, United States of America Original content from 2 Department of Physics, The University of Texas at Austin, Austin, TX 78712, United States of America this work may be used 3 Unit´e Mixte de Physique, CNRS, Thales, Universit´e, Paris-Saclay, 91767 Palaiseau, France under the terms of the 4 Creative Commons Institut für Theoretische Physik, Goethe-Universität Frankfurt, 60438 Frankfurt am Main, Germany Attribution 4.0 licence. 5 Korea Institute for Advanced Study, Seoul 02455, Republic of Korea 6 D´epartement de physique, Institut quantique, and RQMP, Universit´e de Sherbrooke, Sherbrooke, Qu´ebec, Canada Any further distribution 7 of this work must Canadian Institute for Advanced Research, Toronto, Ontario, Canada maintain attribution to 8 Institute of Theoretical and Computational Physics, Graz University of Technology, NAWI Graz, Graz 8010, Austria the author(s) and the title 9 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain of the work, journal 10 ´ ´ ´ ´ citation and DOI. Laboratoire de Physique de l’Ecole normale superieure, ENS, Universite PSL, CNRS, Sorbonne Universite, Universite de Paris, Paris, France 11 Institute of Physics, Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne CH-1015, Switzerland 12 Eindhoven University of Technology, 5600 MB, Eindhoven, Netherlands 13 Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark 14 Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), Bellaterra, Barcelona 08193, Spain 15 Qilimanjaro Quantum Tech, Barcelona, Spain 16 University Grenoble Alpes and CEA, IRIG/PHELIQS, Grenoble F-38000, France 17 Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany 18 Departamento de Física, Facultad de Ciencias Físicas y Matem´aticas, Universidad de Chile, Santiago, Chile 19 Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, United States of America 20 Physics Department, Indian Institute of Technology Bombay, Mumbai 400076, India 21 ICREA—Instituci´o Catalana de Recerca i Estudis Avançats, Barcelona 08010, Spain 22 Aix-Marseille Universit´e, CNRS, CINaM, Marseille, France 23 Department of Physics, Cornell University, Ithaca, NY 14850, United States of America 24 Federal University of ABC, Santo Andr´e, S~ao Paulo 09210-580, Brazil 25 Brazilian Nanotechnology National Laboratory (LNNano/CNPEM), Campinas, S~ao Paulo 13083-970, Brazil E-mail: [email protected] and [email protected] Keywords: quantum materials, materials science, condensed matter, device engineering, topological materials, superconductors, 2D materials, quantum technologies Abstract In recent years, the notion of ‘Quantum Materials’ has emerged as a powerful unifying concept across diverse fields of science and engineering, from condensed-matter and coldatom physics to materials science and quantum computing. Beyond traditional quantum materials such as unconventional superconductors, heavy fermions, and multiferroics, the field has significantly expanded to encompass topological quantum matter, two-dimensional materials and their van der Waals heterostructures, Moir´e materials, Floquet time crystals, as well as materials and devices for quantum computation with Majorana fermions. In this Roadmap collection we aim to capture a snapshot of the most recent developments in the field, and to identify outstanding challenges and emerging opportunities. The format of the Roadmap, whereby experts in each discipline share their viewpoint and articulate their vision for quantum materials, reflects the dynamic and multifaceted nature of this research area, and is meant to encourage exchanges and discussions © 2021 The Author(s). Published by IOP Publishing Ltd J. Phys. Mater. 3 (2021) 042006 F Giustino et al across traditional disciplinary boundaries. It is our hope that this collective vision will contribute to sparking new fascinating questions and activities at the intersection of materials science, condensed matter physics, device engineering, and quantum information, and to shaping a clearer landscape of quantum materials science as a new frontier of interdisciplinary scientific inquiry. We stress that this article is not meant to be a fully comprehensive review but rather an up-to-date snapshot of different areas of research on quantum materials with a minimal number of references focusing on the latest developments. 2 J. Phys. Mater. 3 (2021) 042006 F Giustino et al Contents 1. Introduction 4 2. Complex oxides 6 3. Quantum spin liquids 9 4. Twisted 2D layered crystals 12 5. Cuprate superconductors 15 6. Ultrathin layered superconductors 18 7. Topological insulators 21 8. Topological semimetals 24 9. Quantum materials for topological devices based on Majorana modes 27 10. Superconductor and semiconductor qubits 30 11. Non-equilibrium phenomena in quantum materials 36 12. 2D hyperbolic materials 39 13. Spin torque materials 42 14. Magnetic skyrmions 45 15. Machine learning using experimental quantum materials data 48 16. Machine learning and DFT simulations of quantum materials 50 References 52 3 J. Phys. Mater. 3 (2021) 042006 F Giustino et al 1. Introduction Feliciano Giustino1,2 and Stephan Roche3,4 1 Oden Institute for Computational Engineering and Sciences, The University of Texas at Austin, Austin, TX 78712, United States of America 2 Department of Physics, The University of Texas at Austin, Austin, TX 78712, United States of America 3 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain 4 ICREA—Institucio´ Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain In recent years, the notion of ‘Quantum Materials’ has emerged as a powerful unifying concept across diverse fields of science and engineering, from condensed-matter and cold-atom physics to materials science and quantum computing. Originally introduced to emphasize the exotic properties of unconventional superconductors, heavy-fermion systems, and multifunctional oxides, the definition of quantum materials has morphed into a much broader container that also encompasses research on topological properties, two-dimensional (2D) materials, and driven quantum systems. This convergence of diverse research areas is perhaps best exemplified by the coexistence of strong correlation, quantum criticality, and superconductivity in Moir´e materials such as twisted bilayer graphene (TBG) [1]. With this example in mind, it is natural to broadly define quantum materials as all those versatile materials platforms that allow us to explore emergent quantum phenomena as well as their potential uses in future technology. Apart from unconventional superconductivity and Kondo physics, one of the earliest realizations of emergent phenomena in novel materials platforms can be traced back to the discovery of graphene. This discovery has opened a new dimension to explore unconventional transport properties of massless Dirac fermions, originally predicted to only occur at unreachable high energy scales [2]. The subsequent search for Dirac physics in materials beyond graphene has brought into focus the role of intrinsic spin–orbit coupling (SOC) effects and led to the ground-breaking prediction of the existence of topological insulators (TIs) and superconductors [3–5], as well as the ensuing experimental discovery of the former class [6, 7]. Such a new class of quantum materials is characterized by topologically protected massless Dirac surface states at the edges of 2D materials (2DMs) or at the surfaces of their three-dimensional (3D) counterparts. These efforts were instrumental to the theoretical prediction and subsequent experimental discovery of Weyl fermions chalcogenide semimetals with their unique Fermi surface arcs [8, 9]. These and many other advances have clearly shown that symmetry-based topological concepts are ubiquitous in materials physics and offer unique opportunities to connect seemingly unrelated materials families and enable new discoveries. In addition to its central role in topological quantum materials, SOC underpins