Hofstadter’s butterfly and the fractal quantum Hall effect in moiré superlattices
Cory Dean,Lei Wang,Patrick Maher,Carlos Forsythe,Fereshte Ghahari,Yuanda Gao,Jyoti Katoch,Masa Ishigami,Pilkyung Moon,Mikito Koshino,Takashi Taniguchi,Kenji Watanabe,Kenneth L. Shepard,James Hone,Philip Kim +14 more
Reads0
Chats0
TLDR
It is demonstrated that moiré superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a periodic modulation with ideal length scales of the order of ten nanometres, enabling unprecedented experimental access to the fractal spectrum.Abstract:
Moire superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a periodic potential modulation on a length scale ideally suited to studying the fractal features of the Hofstadter energy spectrum in large magnetic fields. In 1976 Douglas Hofstadter predicted that electrons in a lattice subjected to electrostatic and magnetic fields would show a characteristic energy spectrum determined by the interplay between two quantizing fields. The expected spectrum would feature a repeating butterfly-shaped motif, known as Hofstadter's butterfly. The experimental realization of the phenomenon has proved difficult because of the problem of producing a sufficiently disorder-free superlattice where the length scales for magnetic and electric field can truly compete with each other. Now that goal has been achieved — twice. Two groups working independently produced superlattices by placing ultraclean graphene (Ponomarenko et al.) or bilayer graphene (Kim et al.) on a hexagonal boron nitride substrate and crystallographically aligning the films at a precise angle to produce moire pattern superstructures. Electronic transport measurements on the moire superlattices provide clear evidence for Hofstadter's spectrum. The demonstrated experimental access to a fractal spectrum offers opportunities for the study of complex chaotic effects in a tunable quantum system. Electrons moving through a spatially periodic lattice potential develop a quantized energy spectrum consisting of discrete Bloch bands. In two dimensions, electrons moving through a magnetic field also develop a quantized energy spectrum, consisting of highly degenerate Landau energy levels. When subject to both a magnetic field and a periodic electrostatic potential, two-dimensional systems of electrons exhibit a self-similar recursive energy spectrum1. Known as Hofstadter’s butterfly, this complex spectrum results from an interplay between the characteristic lengths associated with the two quantizing fields1,2,3,4,5,6,7,8,9,10, and is one of the first quantum fractals discovered in physics. In the decades since its prediction, experimental attempts to study this effect have been limited by difficulties in reconciling the two length scales. Typical atomic lattices (with periodicities of less than one nanometre) require unfeasibly large magnetic fields to reach the commensurability condition, and in artificially engineered structures (with periodicities greater than about 100 nanometres) the corresponding fields are too small to overcome disorder completely11,12,13,14,15,16,17. Here we demonstrate that moire superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a periodic modulation with ideal length scales of the order of ten nanometres, enabling unprecedented experimental access to the fractal spectrum. We confirm that quantum Hall features associated with the fractal gaps are described by two integer topological quantum numbers, and report evidence of their recursive structure. Observation of a Hofstadter spectrum in bilayer graphene means that it is possible to investigate emergent behaviour within a fractal energy landscape in a system with tunable internal degrees of freedom.read more
Citations
More filters
Journal ArticleDOI
Van der Waals heterostructures
TL;DR: With steady improvement in fabrication techniques and using graphene’s springboard, van der Waals heterostructures should develop into a large field of their own.
Journal ArticleDOI
Unconventional superconductivity in magic-angle graphene superlattices
Yuan Cao,Valla Fatemi,Shiang Fang,Kenji Watanabe,Takashi Taniguchi,Efthimios Kaxiras,Pablo Jarillo-Herrero +6 more
TL;DR: The realization of intrinsic unconventional superconductivity is reported—which cannot be explained by weak electron–phonon interactions—in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle.
Journal ArticleDOI
2D materials and van der Waals heterostructures
TL;DR: Two-dimensional heterostructures with extended range of functionalities yields a range of possible applications, and spectrum reconstruction in graphene interacting with hBN allowed several groups to study the Hofstadter butterfly effect and topological currents in such a system.
Journal ArticleDOI
Correlated insulator behaviour at half-filling in magic-angle graphene superlattices
Yuan Cao,Valla Fatemi,Ahmet Demir,Shiang Fang,Spencer Tomarken,Jason Luo,Javier Sanchez-Yamagishi,Kenji Watanabe,Takashi Taniguchi,Efthimios Kaxiras,Raymond Ashoori,Pablo Jarillo-Herrero +11 more
TL;DR: It is shown experimentally that when this angle is close to the ‘magic’ angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling, and these flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons.
Journal ArticleDOI
One-dimensional electrical contact to a two-dimensional material.
Lei Wang,Inanc Meric,Pinshane Y. Huang,Qun Gao,Yuanda Gao,Helen Tran,Takashi Taniguchi,Kenji Watanabe,Luis M. Campos,David A. Muller,Jing Guo,Philip Kim,James Hone,Kenneth L. Shepard,Cory Dean,Cory Dean +15 more
TL;DR: In graphene heterostructures, the edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2D materials, and enables high electronic performance, including low-temperature ballistic transport over distances longer than 15 micrometers, and room-tem temperature mobility comparable to the theoretical phonon-scattering limit.
References
More filters
Journal ArticleDOI
Boron nitride substrates for high-quality graphene electronics
Cory Dean,Andrea Young,Inanc Meric,Changgu Lee,Lei Wang,Sebastian Sorgenfrei,Kenji Watanabe,Takashi Taniguchi,Philip Kim,Kenneth L. Shepard,James Hone +10 more
TL;DR: Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2).
Journal ArticleDOI
New Method for High-Accuracy Determination of the Fine-Structure Constant Based on Quantized Hall Resistance
TL;DR: In this article, the Hall voltage of a two-dimensional electron gas, realized with a silicon metal-oxide-semiconductor field effect transistor, was measured and it was shown that the Hall resistance at particular, experimentally well-defined surface carrier concentrations has fixed values which depend only on the fine-structure constant and speed of light, and is insensitive to the geometry of the device.
Journal ArticleDOI
Quantized Hall conductance in a two-dimensional periodic potential
TL;DR: In this article, the Hall conductance of a two-dimensional electron gas has been studied in a uniform magnetic field and a periodic substrate potential, where the Kubo formula is written in a form that makes apparent the quantization when the Fermi energy lies in a gap.
Journal ArticleDOI
Energy levels and wave functions of Bloch electrons in rational and irrational magnetic fields
TL;DR: In this paper, an effective single-band Hamiltonian representing a crystal electron in a uniform magnetic field is constructed from the tight-binding form of a Bloch band by replacing the operator of the Schr\"odinger equation with a matrix method, and the graph of the spectrum over a wide range of "rational" fields is plotted.
Journal ArticleDOI
Substrate-induced band gap in graphene on hexagonal boron nitride: Ab initio density functional calculations
Gianluca Giovannetti,Gianluca Giovannetti,Petr Khomyakov,Geert Brocks,Paul J. Kelly,Jeroen van den Brink,Jeroen van den Brink +6 more
TL;DR: In this article, the electronic structure of a graphene sheet on top of a hexagonal boron nitride (h-BN) substrate was determined using ab initio density functional calculations.