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Bhowmik S, Ghosh A, Chandni U. Emergent phases in graphene flat bands. REPORTS ON PROGRESS IN PHYSICS. PHYSICAL SOCIETY (GREAT BRITAIN) 2024; 87:096401. [PMID: 39059412 DOI: 10.1088/1361-6633/ad67ed] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/01/2023] [Accepted: 07/26/2024] [Indexed: 07/28/2024]
Abstract
Electronic correlations in two-dimensional materials play a crucial role in stabilising emergent phases of matter. The realisation of correlation-driven phenomena in graphene has remained a longstanding goal, primarily due to the absence of strong electron-electron interactions within its low-energy bands. In this context, magic-angle twisted bilayer graphene has recently emerged as a novel platform featuring correlated phases favoured by the low-energy flat bands of the underlying moiré superlattice. Notably, the observation of correlated insulators and superconductivity, and the interplay between these phases have garnered significant attention. A wealth of correlated phases with unprecedented tunability was discovered subsequently, including orbital ferromagnetism, Chern insulators, strange metallicity, density waves, and nematicity. However, a comprehensive understanding of these closely competing phases remains elusive. The ability to controllably twist and stack multiple graphene layers has enabled the creation of a whole new family of moiré superlattices with myriad properties. Here, we review the progress and development achieved so far, encompassing the rich phase diagrams offered by these graphene-based moiré systems. Additionally, we discuss multiple phases recently observed in non-moiré multilayer graphene systems. Finally, we outline future opportunities and challenges for the exploration of hidden phases in this new generation of moiré materials.
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Affiliation(s)
- Saisab Bhowmik
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India
| | - Arindam Ghosh
- Department of Physics, Indian Institute of Science, Bangalore 560012, India
- Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore 560012, India
| | - U Chandni
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India
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2
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Datta S, Bhowmik S, Varshney H, Watanabe K, Taniguchi T, Agarwal A, Chandni U. Nonlinear Electrical Transport Unveils Fermi Surface Malleability in a Moiré Heterostructure. NANO LETTERS 2024; 24:9520-9527. [PMID: 39058474 DOI: 10.1021/acs.nanolett.4c01946] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 07/28/2024]
Abstract
Van Hove singularities enhance many-body interactions and induce collective states of matter ranging from superconductivity to magnetism. In magic-angle twisted bilayer graphene, van Hove singularities appear at low energies and are malleable with density, leading to a sequence of Lifshitz transitions and resets observable in Hall measurements. However, without a magnetic field, linear transport measurements have limited sensitivity to the band's topology. Here, we utilize nonlinear longitudinal and transverse transport measurements to probe these unique features in twisted bilayer graphene at zero magnetic field. We demonstrate that the nonlinear responses, induced by the Berry curvature dipole and extrinsic scattering processes, intricately map the Fermi surface reconstructions at various fillings. Importantly, our experiments highlight the intrinsic connection of these features with the moiré bands. Beyond corroborating the insights from linear Hall measurements, our findings establish nonlinear transport as a pivotal tool for probing band topology and correlated phenomena.
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Affiliation(s)
- Suvronil Datta
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India
| | - Saisab Bhowmik
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India
| | - Harsh Varshney
- Department of Physics, Indian Institute of Technology, Kanpur 208016, India
| | - Kenji Watanabe
- Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
| | - Takashi Taniguchi
- Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
| | - Amit Agarwal
- Department of Physics, Indian Institute of Technology, Kanpur 208016, India
| | - U Chandni
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India
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3
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Shilov AL, Kashchenko MA, Pantaleón Peralta PA, Wang Y, Kravtsov M, Kudriashov A, Zhan Z, Taniguchi T, Watanabe K, Slizovskiy S, Novoselov KS, Fal'ko VI, Guinea F, Bandurin DA. High-Mobility Compensated Semimetals, Orbital Magnetization, and Umklapp Scattering in Bilayer Graphene Moiré Superlattices. ACS NANO 2024; 18:11769-11777. [PMID: 38648369 DOI: 10.1021/acsnano.3c13212] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/25/2024]
Abstract
Twist-controlled moiré superlattices (MSs) have emerged as a versatile platform for realizing artificial systems with complex electronic spectra. The combination of Bernal-stacked bilayer graphene (BLG) and hexagonal boron nitride (hBN) can give rise to an interesting MS, which has recently featured a set of unexpected behaviors, such as unconventional ferroelectricity and the electronic ratchet effect. Yet, the understanding of the electronic properties of BLG/hBN MS has, at present, remained fairly limited. Here, we combine magneto-transport and low-energy sub-THz excitation to gain insights into the properties of this MS. We demonstrate that the alignment between BLG and hBN crystal lattices results in the emergence of compensated semimetals at some integer fillings of the moiré bands, separated by van Hove singularities where the Lifshitz transition occurs. A particularly pronounced semimetal develops when eight holes reside in the moiré unit cell, where coexisting high-mobility electron and hole systems feature strong magnetoresistance reaching 2350% already at B = 0.25 T. Next, by measuring the THz-driven Nernst effect in remote bands, we observe valley splitting, indicating an orbital magnetization characterized by a strongly enhanced effective gv-factor of 340. Finally, using THz photoresistance measurements, we show that the high-temperature conductivity of the BLG/hBN MS is limited by electron-electron umklapp processes. Our multifaceted analysis introduces THz-driven magnetotransport as a convenient tool to probe the band structure and interaction effects in van der Waals materials and provides a comprehensive understanding of the BLG/hBN MS.
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Affiliation(s)
- Artur L Shilov
- Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore
| | - Mikhail A Kashchenko
- Programmable Functional Materials Lab, Center for Neurophysics and Neuromorphic Technologies, Moscow 127495, Russia
| | | | - Yibo Wang
- Institute for Functional Intelligent Materials, National University of Singapore, Singapore 117575, Singapore
| | - Mikhail Kravtsov
- Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore
- Institute for Functional Intelligent Materials, National University of Singapore, Singapore 117575, Singapore
| | - Andrei Kudriashov
- Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore
- Institute for Functional Intelligent Materials, National University of Singapore, Singapore 117575, Singapore
| | - Zhen Zhan
- IMDEA Nanoscience, Faraday 9, Madrid 28015, Spain
| | - Takashi Taniguchi
- International Center for Materials Nanoarchitectonics, National Institute of Material Science, Tsukuba 305-0044, Japan
| | - Kenji Watanabe
- Research Center for Functional Materials, National Institute of Material Science, Tsukuba 305-0044, Japan
| | - Sergey Slizovskiy
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, U.K
| | - Kostya S Novoselov
- Institute for Functional Intelligent Materials, National University of Singapore, Singapore 117575, Singapore
| | - Vladimir I Fal'ko
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, U.K
| | - Francisco Guinea
- IMDEA Nanoscience, Faraday 9, Madrid 28015, Spain
- Donostia International Physics Center, Paseo Manuel de Lardizábal 4, San Sebastián 20018, Spain
| | - Denis A Bandurin
- Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore
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Ostovan A, Milowska KZ, García-Cervera CJ. A twist for tunable electronic and thermal transport properties of nanodevices. NANOSCALE 2024; 16:7504-7514. [PMID: 38466025 DOI: 10.1039/d4nr00058g] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 03/12/2024]
Abstract
Twisted graphene-layered materials with nonzero interlayer twist angles (θ) have recently become appealing, as they exhibit a range of attractive physical properties, which include a Mott insulating phase and superconductivity. In this study, we consider nanodevices constructed from zigzag graphene nanoribbons with a top rectangular benzenoid [6,3]-flake. Using density functional theory and a non-equilibrium Green's function approach, we explore how the electronic and thermal transport properties in such nanodevices can be tuned through a twist of the top flake by an angle 0° ≤ θ ≤ 8.8° for different stacking configurations. We found a strong dependency of the electronic structure on the stacking type, as well as on the twisting regime, specifically in AA-stacking devices. Electron and hole van Hove singularities (vHSs), which originate, respectively, from the flatness of the top of the valence band for the minor-spin component and the bottom of the conduction band for the major-spin component, are found very close to the Fermi level in the density of states and electronic transmission spectra of AA-stacking devices with a twist angle of 1.1°. We establish that these vHSs in AA-1.1° devices are stable at higher temperatures and, with the increased number of available states, lead to larger values of electron thermal conductivity and finally total thermal conductivity in AA-1.1°. Our work highlights the essential role of twisting and stacking for the fabrication of nanoscale charge and heat switches and spurs future studies of twisted layered structures.
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Affiliation(s)
- Azar Ostovan
- Mathematics Department, University of California, Santa Barbara, CA 93106, USA.
| | - Karolina Z Milowska
- CIC nanoGUNE, Tolosa Hiribidea 76, 20018 Donostia-San Sebastián, Spain
- Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain
| | - Carlos J García-Cervera
- Mathematics Department, University of California, Santa Barbara, CA 93106, USA.
- BCAM, Basque Center for Applied Mathematics, E48009 Bilbao, Basque Country, Spain
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Völkl T, Aharon-Steinberg A, Holder T, Alpern E, Banu N, Pariari AK, Myasoedov Y, Huber ME, Hücker M, Zeldov E. Demonstration and imaging of cryogenic magneto-thermoelectric cooling in a van der Waals semimetal. NATURE PHYSICS 2024; 20:976-983. [PMID: 38882521 PMCID: PMC11178502 DOI: 10.1038/s41567-024-02417-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 08/21/2023] [Accepted: 01/25/2024] [Indexed: 06/18/2024]
Abstract
Attaining viable thermoelectric cooling at cryogenic temperatures is of considerable fundamental and technological interest for electronics and quantum materials applications. In-device temperature control can provide more efficient and precise thermal environment management compared with conventional global cooling. The application of a current and perpendicular magnetic field gives rise to cooling by generating electron-hole pairs on one side of the sample and to heating due to their recombination on the opposite side, which is known as the Ettingshausen effect. Here we develop nanoscale cryogenic imaging of the magneto-thermoelectric effect and demonstrate absolute cooling and an Ettingshausen effect in exfoliated WTe2 Weyl semimetal flakes at liquid He temperatures. In contrast to bulk materials, the cooling is non-monotonic with respect to the magnetic field and device size. Our model of magneto-thermoelectricity in mesoscopic semimetal devices shows that the cooling efficiency and the induced temperature profiles are governed by the interplay between sample geometry, electron-hole recombination length, magnetic field, and flake and substrate heat conductivities. The observations open the way for the direct integration of microscopic thermoelectric cooling and for temperature landscape engineering in van der Waals devices.
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Affiliation(s)
- T Völkl
- Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel
| | - A Aharon-Steinberg
- Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel
| | - T Holder
- Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel
- School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel
| | - E Alpern
- Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel
| | - N Banu
- Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel
| | - A K Pariari
- Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel
| | - Y Myasoedov
- Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel
| | - M E Huber
- Departments of Physics and Electrical Engineering, University of Colorado Denver, Denver, CO USA
| | - M Hücker
- Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel
| | - E Zeldov
- Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel
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Parappurath A, Ghawri B, Bhowmik S, Singha A, Watanabe K, Taniguchi T, Ghosh A. Band structure sensitive photoresponse in twisted bilayer graphene proximitized with WSe 2. NANOSCALE 2023; 15:18818-18824. [PMID: 37962416 DOI: 10.1039/d3nr04864k] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/15/2023]
Abstract
The ability to tune the twist angle between different layers of two-dimensional (2D) materials has enabled the creation of electronic flat bands artificially, leading to exotic quantum phases. When a twisted blilayer of graphene (tBLG) is placed at the van der Waals proximity to a semiconducting layer of transition metal dichalcogenide (TMDC), such as WSe2, the emergent phases in the tBLG can fundamentally modify the functionality of such heterostructures. Here we have performed photoresponse measurements in few-layer-WSe2/tBLG heterostructure, where the mis-orientation angle of the tBLG layer was chosen to lie close to the magic angle of 1.1°. Our experiments show that the photoresponse is extremely sensitive to the band structure of tBLG and gets strongly suppressed when the Fermi energy was placed within the low-energy moiré bands. Photoresponse could however be recovered when Fermi energy exceeded the moiré band edge where it was dominated by the photogating effect due to transfer of charge between the tBLG and the WSe2 layers. Our observations suggest the possibility of the screening effects from moiré flat bands that strongly affect the charge transfer process at the WSe2/tBLG interface, which is further supported by time-resolved photo-resistance measurements.
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Affiliation(s)
- Aparna Parappurath
- Department of Physics, Indian Institute of Science, Bangalore, 560012, India.
| | - Bhaskar Ghawri
- Department of Physics, Indian Institute of Science, Bangalore, 560012, India.
| | - Saisab Bhowmik
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, 560012, India
| | - Arup Singha
- Department of Physics, Indian Institute of Science, Bangalore, 560012, India.
| | - K Watanabe
- Research Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
| | - T Taniguchi
- International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
| | - Arindam Ghosh
- Department of Physics, Indian Institute of Science, Bangalore, 560012, India.
- Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore 560 012, India
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Bhowmik S, Ghawri B, Park Y, Lee D, Datta S, Soni R, Watanabe K, Taniguchi T, Ghosh A, Jung J, Chandni U. Spin-orbit coupling-enhanced valley ordering of malleable bands in twisted bilayer graphene on WSe 2. Nat Commun 2023; 14:4055. [PMID: 37422470 DOI: 10.1038/s41467-023-39855-x] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2022] [Accepted: 06/26/2023] [Indexed: 07/10/2023] Open
Abstract
Recent experiments in magic-angle twisted bilayer graphene have revealed a wealth of novel electronic phases as a result of interaction-driven spin-valley flavour polarisation. In this work, we investigate correlated phases due to the combined effect of spin-orbit coupling-enhanced valley polarisation and the large density of states below half filling of the moiré band in twisted bilayer graphene coupled to tungsten diselenide. We observe an anomalous Hall effect, accompanied by a series of Lifshitz transitions that are highly tunable with carrier density and magnetic field. The magnetisation shows an abrupt change of sign near half-filling, confirming its orbital nature. While the Hall resistance is not quantised at zero magnetic fields-indicating a ground state with partial valley polarisation-perfect quantisation and complete valley polarisation are observed at finite fields. Our results illustrate that singularities in the flat bands in the presence of spin-orbit coupling can stabilise ordered phases even at non-integer moiré band fillings.
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Affiliation(s)
- Saisab Bhowmik
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, 560012, India.
| | - Bhaskar Ghawri
- Department of Physics, Indian Institute of Science, Bangalore, 560012, India
| | - Youngju Park
- Department of Physics, University of Seoul, Seoul, 02504, Korea
| | - Dongkyu Lee
- Department of Physics, University of Seoul, Seoul, 02504, Korea
- Department of Smart Cities, University of Seoul, Seoul, 02504, Korea
| | - Suvronil Datta
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, 560012, India
| | - Radhika Soni
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, 560012, India
| | - K Watanabe
- Research Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan
| | - T Taniguchi
- International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan
| | - Arindam Ghosh
- Department of Physics, Indian Institute of Science, Bangalore, 560012, India
- Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore, 560012, India
| | - Jeil Jung
- Department of Physics, University of Seoul, Seoul, 02504, Korea.
- Department of Smart Cities, University of Seoul, Seoul, 02504, Korea.
| | - U Chandni
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, 560012, India.
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Aggarwal D, Narula R, Ghosh S. A primer on twistronics: a massless Dirac fermion's journey to moiré patterns and flat bands in twisted bilayer graphene. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2023; 35:143001. [PMID: 36745922 DOI: 10.1088/1361-648x/acb984] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/07/2022] [Accepted: 02/06/2023] [Indexed: 06/18/2023]
Abstract
The recent discovery of superconductivity in magic-angle twisted bilayer graphene (TBLG) has sparked a renewed interest in the strongly-correlated physics ofsp2carbons, in stark contrast to preliminary investigations which were dominated by the one-body physics of the massless Dirac fermions. We thus provide a self-contained, theoretical perspective of the journey of graphene from its single-particle physics-dominated regime to the strongly-correlated physics of the flat bands. Beginning from the origin of the Dirac points in condensed matter systems, we discuss the effect of the superlattice on the Fermi velocity and Van Hove singularities in graphene and how it leads naturally to investigations of the moiré pattern in van der Waals heterostructures exemplified by graphene-hexagonal boron-nitride and TBLG. Subsequently, we illuminate the origin of flat bands in TBLG at the magic angles by elaborating on a broad range of prominent theoretical works in a pedagogical way while linking them to available experimental support, where appropriate. We conclude by providing a list of topics in the study of the electronic properties of TBLG not covered by this review but may readily be approached with the help of this primer.
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Affiliation(s)
| | - Rohit Narula
- Department of Physics, IIT Delhi, Hauz Khas, New Delhi, India
| | - Sankalpa Ghosh
- Department of Physics, IIT Delhi, Hauz Khas, New Delhi, India
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Mahapatra PS, Garg M, Ghawri B, Jayaraman A, Watanabe K, Taniguchi T, Ghosh A, Chandni U. Quantum Hall Interferometry in Triangular Domains of Marginally Twisted Bilayer Graphene. NANO LETTERS 2022; 22:5708-5714. [PMID: 35796713 DOI: 10.1021/acs.nanolett.2c00627] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
Quantum Hall (QH) interferometry provides an archetypal platform for the experimental realization of braiding statistics of fractional QH states. However, the complexity of observing fractional statistics requires phase coherence over the length of the interferometer, as well as suppression of Coulomb charging energy. Here, we demonstrate a new type of QH interferometer based on marginally twisted bilayer graphene (mtBLG), with a twist angle θ ≈ 0.16°. With the device operating in the QH regime, we observe distinct signatures of electronic Fabry-Pérot and Aharonov-Bohm oscillations of the magneto-thermopower in the density-magnetic field phase space, at Landau level filling factors ν = 4, 8. We find that QH interference effects are intrinsic to the triangular AB/BA domains in mtBLG that show diminished Coulomb charging effects. Our results demonstrate phase-coherent interference of QH edge modes without any additional gate-defined complex architecture, which may be beneficial in experimental realizations of non-Abelian braiding statistics.
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Affiliation(s)
| | - Manjari Garg
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India
| | - Bhaskar Ghawri
- Department of Physics, Indian Institute of Science, Bangalore 560012, India
| | - Aditya Jayaraman
- Department of Physics, Indian Institute of Science, Bangalore 560012, India
| | - Kenji Watanabe
- Research Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
| | - Takashi Taniguchi
- International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
| | - Arindam Ghosh
- Department of Physics, Indian Institute of Science, Bangalore 560012, India
- Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore 560012, India
| | - U Chandni
- Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India
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