Transparent mirror effect in twist-angle-disordered bilayer graphene

AbstractMoiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a moiré system with tunable flat bands. Here, we use gate-tuned scanning tunneling spectroscopy to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of electronic correlations and topology for its flat band. Our spectroscopic experiments are in agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands with a large effective g-factor.

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Twist-angle-dependent thermal conduction in single-crystalline bilayer graphene

Applied Physics Letters ◽

10.1063/5.0045386 ◽

2021 ◽

Vol 118 (19) ◽

pp. 193104

Author(s):

Shuo Han ◽

Xianhua Nie ◽

Shangzhi Gu ◽

Wenyu Liu ◽

Luchen Chen ◽

...

Keyword(s):

Thermal Conduction ◽

Twist Angle ◽

Bilayer Graphene ◽

Single Crystalline

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Electronic Properties of Twisted Bilayer Graphene in the Presence of a Magnetic Field

EPJ Web of Conferences ◽

10.1051/epjconf/202023303004 ◽

2020 ◽

Vol 233 ◽

pp. 03004

Author(s):

M.F.C. Martins Quintela ◽

J.C.C. Guerra ◽

S.M. João

Keyword(s):

Magnetic Field ◽

Electronic Properties ◽

Twist Angle ◽

Bilayer Graphene ◽

Polynomial Method ◽

Zero Energy ◽

Energy Bands ◽

Optical And Electronic Properties ◽

Twisted Bilayer Graphene ◽

Kernel Polynomial Method

In AA-stacked twisted bilayer graphene, the lower energy bands become completely flat when the twist angle passes through certain specific values: the so-called “magic angles”. The Dirac peak appears at zero energy due to the flattening of these bands when the twist angle is sufficiently small [1-3]. When a constant perpendicular magnetic field is applied, Landau levels start appearing as expected [5]. We used the Kernel Polynomial Method (KPM) [6] as implemented in KITE [7] to study the optical and electronic properties of these systems. The aim of this work is to analyze how the features of these quantities change with the twist angle in the presence of an uniform magnetic field.

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Theory of correlated insulating behaviour and spin-triplet superconductivity in twisted double bilayer graphene

Nature Communications ◽

10.1038/s41467-019-12981-1 ◽

2019 ◽

Vol 10 (1) ◽

Cited By ~ 41

Author(s):

Jong Yeon Lee ◽

Eslam Khalaf ◽

Shang Liu ◽

Xiaomeng Liu ◽

Zeyu Hao ◽

...

Keyword(s):

Displacement Field ◽

Twist Angle ◽

Vertical Displacement ◽

Bilayer Graphene ◽

Magic Angle ◽

Hartree Fock ◽

Flat Bands ◽

Half Filling ◽

Spin Triplet ◽

Experimental Findings

AbstractTwo graphene monolayers twisted by a small magic angle exhibit nearly flat bands, leading to correlated electronic states. Here we study a related but different system with reduced symmetry - twisted double bilayer graphene (TDBG), consisting of two Bernal stacked bilayer graphenes, twisted with respect to one another. Unlike the monolayer case, we show that isolated flat bands only appear on application of a vertical displacement field. We construct a phase diagram as a function of twist angle and displacement field, incorporating interactions via a Hartree-Fock approximation. At half-filling, ferromagnetic insulators are stabilized with valley Chern number $${C}_{{\rm{v}}}=\pm 2$$Cv=±2. Upon doping, ferromagnetic fluctuations are argued to lead to spin-triplet superconductivity from pairing between opposite valleys. We highlight a novel orbital effect arising from in-plane fields plays an important role in interpreting experiments. Combined with recent experimental findings, our results establish TDBG as a tunable platform to realize rare phases in conventional solids.

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Steering the current ﬂow in twisted bilayer graphene

Journal of Physics Materials ◽

10.1088/2515-7639/ac4ae0 ◽

2022 ◽

Author(s):

Jesús Arturo Sánchez-Sánchez ◽

Montserrat Navarro-Espino ◽

Yonatan Betancur Ocampo ◽

José Eduardo Barrios Vargas ◽

Thomas Stegmann

Keyword(s):

Current Flow ◽

Twist Angle ◽

Bottom Layer ◽

Bilayer Graphene ◽

Energy Bands ◽

Initial Direction ◽

Steering Angle ◽

Graphene Layers ◽

Ballistic Electron ◽

Twisted Bilayer Graphene

Abstract A nanoelectronic device made of twisted bilayer graphene (TBLG) is proposed to steer the direction of the current ﬂow. The ballistic electron current, injected at one edge of the bottom layer, can be guided predominantly to one of the lateral edges of the top layer. The current is steered to the opposite lateral edge, if either the twist angle is reversed or the electrons are injected in the valence band instead of the conduction band, making it possible to control the current ﬂow by electric gates. When both graphene layers are aligned, the current passes straight through the system without changing its initial direction. The observed steering angle exceeds well the twist angle and emerges for a broad range of experimentally accessible parameters. It is explained by the twist angle and the trigonal shape of the energy bands beyond the van Hove singularity due to the Moiré interference pattern. As the shape of the energy bands depends on the valley degree of freedom, the steered current is valley polarized. Our ﬁndings show how to control and manipulate the current ﬂow in TBLG. Technologically, they are of relevance for applications in twistronics and valleytronics.

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Carrier transport theory for twisted bilayer graphene in the metallic regime

Nature Communications ◽

10.1038/s41467-021-25864-1 ◽

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.

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

Science Advances ◽

10.1126/sciadv.aaw9770 ◽

2019 ◽

Vol 5 (9) ◽

pp. eaaw9770 ◽

Cited By ~ 35

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.

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Twist-angle two-dimensional superlattices and their application in (opto)electronics

Journal of Semiconductors ◽

10.1088/1674-4926/43/1/011001 ◽

2022 ◽

Vol 43 (1) ◽

pp. 011001

Author(s):

Kaiyao Xin ◽

Xingang Wang ◽

Kasper Grove-Rasmussen ◽

Zhongming Wei

Keyword(s):

Twist Angle ◽

Transition Metal Dichalcogenides ◽

Bilayer Graphene ◽

Magic Angle ◽

Two Dimensional ◽

Experimental Realization ◽

Theoretical Simulation ◽

Band Engineering ◽

Theoretical Predictions ◽

Metal Dichalcogenides

Abstract Twist-angle two-dimensional systems, such as twisted bilayer graphene, twisted bilayer transition metal dichalcogenides, twisted bilayer phosphorene and their multilayer van der Waals heterostructures, exhibit novel and tunable properties due to the formation of Moiré superlattice and modulated Moiré bands. The review presents a brief venation on the development of “twistronics” and subsequent applications based on band engineering by twisting. Theoretical predictions followed by experimental realization of magic-angle bilayer graphene ignited the flame of investigation on the new freedom degree, twist-angle, to adjust (opto)electrical behaviors. Then, the merging of Dirac cones and the presence of flat bands gave rise to enhanced light-matter interaction and gate-dependent electrical phases, respectively, leading to applications in photodetectors and superconductor electronic devices. At the same time, the increasing amount of theoretical simulation on extended twisted 2D materials like TMDs and BPs called for further experimental verification. Finally, recently discovered properties in twisted bilayer h-BN evidenced h-BN could be an ideal candidate for dielectric and ferroelectric devices. Hence, both the predictions and confirmed properties imply twist-angle two-dimensional superlattice is a group of promising candidates for next-generation (opto)electronics.

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Selectively enhanced photocurrent generation in twisted bilayer graphene with van Hove singularity

Nature Communications ◽

10.1038/ncomms10699 ◽

2016 ◽

Vol 7 (1) ◽

Cited By ~ 60

Author(s):

Jianbo Yin ◽

Huan Wang ◽

Han Peng ◽

Zhenjun Tan ◽

Lei Liao ◽

...

Keyword(s):

Twist Angle ◽

Bilayer Graphene ◽

Monolayer Graphene ◽

Valuable Insight ◽

Strong Light ◽

Van Hove Singularities ◽

Spatially Resolved ◽

Enhanced Sensitivity ◽

Photocurrent Generation ◽

Twisted Bilayer Graphene

Abstract Graphene with ultra-high carrier mobility and ultra-short photoresponse time has shown remarkable potential in ultrafast photodetection. However, the broad and weak optical absorption (∼2.3%) of monolayer graphene hinders its practical application in photodetectors with high responsivity and selectivity. Here we demonstrate that twisted bilayer graphene, a stack of two graphene monolayers with an interlayer twist angle, exhibits a strong light–matter interaction and selectively enhanced photocurrent generation. Such enhancement is attributed to the emergence of unique twist-angle-dependent van Hove singularities, which are directly revealed by spatially resolved angle-resolved photoemission spectroscopy. When the energy interval between the van Hove singularities of the conduction and valance bands matches the energy of incident photons, the photocurrent generated can be significantly enhanced (up to ∼80 times with the integration of plasmonic structures in our devices). These results provide valuable insight for designing graphene photodetectors with enhanced sensitivity for variable wavelength.

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Enhanced third-harmonic generation by manipulating the twist angle of bilayer graphene

Light Science & Applications ◽

10.1038/s41377-020-00459-5 ◽

2021 ◽

Vol 10 (1) ◽

Author(s):

Seongju Ha ◽

Nam Hun Park ◽

Hyeonkyeong Kim ◽

Jiseon Shin ◽

Jungseok Choi ◽

...

Keyword(s):

Harmonic Generation ◽

Incident Light ◽

Twist Angle ◽

Third Harmonic Generation ◽

Optical Nonlinearity ◽

Bilayer Graphene ◽

Monolayer Graphene ◽

Electronic Band ◽

Third Harmonic ◽

The Third

AbstractTwisted bilayer graphene (tBLG) has received substantial attention in various research fields due to its unconventional physical properties originating from Moiré superlattices. The electronic band structure in tBLG modified by interlayer interactions enables the emergence of low-energy van Hove singularities in the density of states, allowing the observation of intriguing features such as increased optical conductivity and photocurrent at visible or near-infrared wavelengths. Here, we show that the third-order optical nonlinearity can be considerably modified depending on the stacking angle in tBLG. The third-harmonic generation (THG) efficiency is found to significantly increase when the energy gap at the van Hove singularity matches the three-photon resonance of incident light. Further study on electrically tuneable optical nonlinearity reveals that the gate-controlled THG enhancement varies with the twist angle in tBLG, resulting in a THG enhanced up to 60 times compared to neutral monolayer graphene. Our results prove that the twist angle opens up a new way to control and increase the optical nonlinearity of tBLG, suggesting rotation-induced tuneable nonlinear optics in stacked two-dimensional material systems.

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