scholarly journals Correlated insulating and superconducting states in twisted bilayer graphene below the magic angle

2019 ◽  
Vol 5 (9) ◽  
pp. eaaw9770 ◽  
Author(s):  
Emilio Codecido ◽  
Qiyue Wang ◽  
Ryan Koester ◽  
Shi Che ◽  
Haidong Tian ◽  
...  
Keyword(s):  
Energy Gap ◽  
Twist Angle ◽  
High Energy ◽  
Bilayer Graphene ◽  
Unit Cell ◽  
Flat Band ◽  
Magic Angle ◽  
Strongly Correlated ◽  
New Information ◽  
Twisted Bilayer Graphene

The emergence of flat bands and correlated behaviors in “magic angle” twisted bilayer graphene (tBLG) has sparked tremendous interest, though its many aspects are under intense debate. Here we report observation of both superconductivity and the Mott-like insulating state in a tBLG device with a twist angle of ~0.93°, which is smaller than the magic angle by 15%. At an electron concentration of ±5 electrons/moiré unit cell, we observe a narrow resistance peak with an activation energy gap ~0.1 meV. This indicates additional correlated insulating state, and is consistent with theory predicting a high-energy flat band. At doping of ±12 electrons/moiré unit cell we observe resistance peaks arising from the Dirac points in the spectrum. Our results reveal that the “magic” range of tBLG is in fact larger than what is previously expected, and provide a wealth of new information to help decipher the strongly correlated phenomena observed in tBLG.

scholarly journals Flat band carrier confinement in magic-angle twisted bilayer graphene

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Nikhil Tilak ◽  
Xinyuan Lai ◽  
Shuang Wu ◽  
Zhenyuan Zhang ◽  
Mingyu Xu ◽  
...  
Keyword(s):  
Twist Angle ◽  
Bilayer Graphene ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Flat Band ◽  
Magic Angle ◽  
Energy Bands ◽  
Carrier Confinement ◽  
Correlated Electron ◽  
Twisted Bilayer Graphene

AbstractMagic-angle twisted bilayer graphene has emerged as a powerful platform for studying strongly correlated electron physics, owing to its almost dispersionless low-energy bands and the ability to tune the band filling by electrostatic gating. Techniques to control the twist angle between graphene layers have led to rapid experimental progress but improving sample quality is essential for separating the delicate correlated electron physics from disorder effects. Owing to the 2D nature of the system and the relatively low carrier density, the samples are highly susceptible to small doping inhomogeneity which can drastically modify the local potential landscape. This potential disorder is distinct from the twist angle variation which has been studied elsewhere. Here, by using low temperature scanning tunneling spectroscopy and planar tunneling junction measurements, we demonstrate that flat bands in twisted bilayer graphene can amplify small doping inhomogeneity that surprisingly leads to carrier confinement, which in graphene could previously only be realized in the presence of a strong magnetic field.

scholarly journals Flat band carrier confinement in magic-angle twisted bilayer graphene

2020 ◽  
Author(s):  
Nikhil Tilak ◽  
xinyuan lai ◽  
Shuang Wu ◽  
Zhenyuan Zhang ◽  
Mingyu Xu ◽  
...  
Keyword(s):  
Twist Angle ◽  
Bilayer Graphene ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Flat Band ◽  
Magic Angle ◽  
Energy Bands ◽  
Angle Variation ◽  
Carrier Confinement ◽  
Twisted Bilayer Graphene

Abstract Magic angle twisted bilayer graphene has emerged as a powerful platform for studying strongly correlated electron physics, owing to its almost dispersionless low-energy bands and the ability to tune the band filling by electrostatic gating. Techniques to control the twist angle between graphene layers have led to rapid experimental progress, but improving sample quality is essential for separating the delicate correlation physics from disorder effects. Owing to the 2D nature of the system and the relatively low carrier density, the samples are highly susceptible to small doping inhomogeneity which can drastically modify the local potential landscape. This potential disorder is distinct from the twist-angle variation which has been studied elsewhere. Understanding and mitigating the effects of such disorder is important. Here, we demonstrate using low temperature scanning tunneling spectroscopy and planar tunneling junction measurements, how flat bands in twisted bilayer graphene can amplify small doping inhomogeneity leading to carrier confinement, thus obscuring magic-angle physics.

scholarly journals Carrier transport theory for twisted bilayer graphene in the metallic regime

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Girish Sharma ◽  
Indra Yudhistira ◽  
Nilotpal Chakraborty ◽  
Derek Y. H. Ho ◽  
M. M. Al Ezzi ◽  
...  
Keyword(s):  
Experimental Data ◽  
Carrier Transport ◽  
Transport Theory ◽  
Phonon Scattering ◽  
Twist Angle ◽  
Quantitative Agreement ◽  
Bilayer Graphene ◽  
Magic Angle ◽  
Accurate Treatment ◽  
Twisted Bilayer Graphene

AbstractUnderstanding the normal-metal state transport in twisted bilayer graphene near magic angle is of fundamental importance as it provides insights into the mechanisms responsible for the observed strongly correlated insulating and superconducting phases. Here we provide a rigorous theory for phonon-dominated transport in twisted bilayer graphene describing its unusual signatures in the resistivity (including the variation with electron density, temperature, and twist angle) showing good quantitative agreement with recent experiments. We contrast this with the alternative Planckian dissipation mechanism that we show is incompatible with available experimental data. An accurate treatment of the electron-phonon scattering requires us to go well beyond the usual treatment, including both intraband and interband processes, considering the finite-temperature dynamical screening of the electron-phonon matrix element, and going beyond the linear Dirac dispersion. In addition to explaining the observations in currently available experimental data, we make concrete predictions that can be tested in ongoing experiments.

scholarly journals Electronic properties of twisted bilayer graphene in high-energy k ⋅p-Hamiltonian approximation

2020 ◽  
Vol 34 (19n20) ◽  
pp. 2040055
Author(s):  
H. V. Grushevskaya ◽  
G. G. Krylov ◽  
H.-Y. Choi ◽  
S. P. Kruchinin
Keyword(s):  
Band Structure ◽  
Rotation Angle ◽  
Energy Gap ◽  
High Energy ◽  
Bilayer Graphene ◽  
Monolayer Graphene ◽  
Abelian Gauge ◽  
Relativistic Approach ◽  
Energy Formula ◽  
Twisted Bilayer Graphene

A model of twisted bilayer graphene has been offered on the base of developed quasi-relativistic approach with high energy [Formula: see text]-Hamiltonian. Monolayer-graphene twist is accounted as a perturbation of monolayer-graphene Hamiltonian in such a way that at a given point of the Brillouin zone there exists an external non-Abelian gauge field of another monolayer. Majorana-like resonances have been revealed in the band structure of model at a magic rotation angle [Formula: see text]. The simulations have also shown that a superlattice energy gap existing at a rotation angle [Formula: see text] vanishes at a rotation angle [Formula: see text].

scholarly journals Observation of a flat band and bandgap in millimeter-scale twisted bilayer graphene

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Keiju Sato ◽  
Naoki Hayashi ◽  
Takahiro Ito ◽  
Noriyuki Masago ◽  
Makoto Takamura ◽  
...  
Keyword(s):  
Large Angle ◽  
Bilayer Graphene ◽  
Epitaxial Graphene ◽  
Photoemission Spectroscopy ◽  
Flat Band ◽  
Magic Angle ◽  
Graphene Layers ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Twisted Bilayer Graphene ◽  
Micrometer Scale

AbstractMagic-angle twisted bilayer graphene, consisting of two graphene layers stacked at a special angle, exhibits superconductivity due to the maximized density of states at the energy of the flat band. Generally, experiments on twisted bilayer graphene have been performed using micrometer-scale samples. Here we report the fabrication of twisted bilayer graphene with an area exceeding 3 × 5 mm2 by transferring epitaxial graphene onto another epitaxial graphene, and observation of a flat band and large bandgap using angle-resolved photoemission spectroscopy. Our results suggest that the substrate potential induces both the asymmetrical doping in large angle twisted bilayer graphene and the electron doped nature of the flat band in magic-angle twisted bilayer graphene.

scholarly journals Strongly correlated Chern insulators in magic-angle twisted bilayer graphene

Nature ◽  
2020 ◽  
Vol 588 (7839) ◽  
pp. 610-615
Author(s):  
Kevin P. Nuckolls ◽  
Myungchul Oh ◽  
Dillon Wong ◽  
Biao Lian ◽  
Kenji Watanabe ◽  
...  
Keyword(s):  
Bilayer Graphene ◽  
Magic Angle ◽  
Strongly Correlated ◽  
Chern Insulators ◽  
Twisted Bilayer Graphene

scholarly journals Entropic evidence for a Pomeranchuk effect in Magic Angle graphene

2020 ◽  
Author(s):  
Shahal Ilani ◽  
Asaf Rozen ◽  
Jeong Min Park ◽  
Uri Zondiner ◽  
Yuan Cao ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Electron Density ◽  
Solid Phase ◽  
Excess Entropy ◽  
Magnetic Moments ◽  
Bilayer Graphene ◽  
Unit Cell ◽  
Magic Angle ◽  
Twisted Bilayer Graphene ◽  
Electronic Entropy

Abstract In the 1950's, Pomeranchuk predicted that, counterintuitively, liquid 3He may solidify upon heating, due to a high excess spin entropy in the solid phase. Here, using both local and global electronic entropy and compressibility measurements, we show that an analogous effect occurs in magic angle twisted bilayer graphene. Near a filling of one electron per moiré unit cell, we observe a dramatic increase in the electronic entropy to about 1kB per unit cell. This large excess entropy is quenched by an in-plane magnetic field, pointing to its magnetic origin. A sharp jump in the compressibility as a function of the electron density, associated with a reset of the Fermi level back to near the Dirac point, marks a clear boundary between two phases. We map this jump as a function of electron density, temperature, and magnetic field. This reveals a phase diagram that is consistent with a Pomeranchuk-like temperature- and field-driven transition from a rather conventional metal to a correlated state with nearly-free magnetic moments. The correlated state features an unusual dichotomy between properties associated with itinerant electrons, such as the absence of a thermodynamic gap, metallicity, and a Dirac-like compressibility, and properties usually associated with localized moments, such as a large entropy and its disappearance with magnetic field. Moreover, the energy scales characterizing these two sets of properties are vastly different: whereas the compressibility jump onsets at T∼30K, the bandwidth of magnetic excitations is ∼3K or smaller. This dichotomy and the large separation of energy scales have key implications for the physics of correlated states in twisted bilayer graphene.

scholarly journals Tuning superconductivity in twisted bilayer graphene

Science ◽  
2019 ◽  
Vol 363 (6431) ◽  
pp. 1059-1064 ◽  
Author(s):  
Matthew Yankowitz ◽  
Shaowen Chen ◽  
Hryhoriy Polshyn ◽  
Yuxuan Zhang ◽  
K. Watanabe ◽  
...  
Keyword(s):  
Twist Angle ◽  
Interlayer Coupling ◽  
Interlayer Spacing ◽  
Bilayer Graphene ◽  
Superconducting Phase ◽  
Low Energy ◽  
Flat Band ◽  
Strong Correlations ◽  
Twisted Bilayer Graphene ◽  
Quantum Phenomena

Materials with flat electronic bands often exhibit exotic quantum phenomena owing to strong correlations. An isolated low-energy flat band can be induced in bilayer graphene by simply rotating the layers by 1.1°, resulting in the appearance of gate-tunable superconducting and correlated insulating phases. In this study, we demonstrate that in addition to the twist angle, the interlayer coupling can be varied to precisely tune these phases. We induce superconductivity at a twist angle larger than 1.1°—in which correlated phases are otherwise absent—by varying the interlayer spacing with hydrostatic pressure. Our low-disorder devices reveal details about the superconducting phase diagram and its relationship to the nearby insulator. Our results demonstrate twisted bilayer graphene to be a distinctively tunable platform for exploring correlated states.

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