scholarly journals Electric field–tunable superconductivity in alternating-twist magic-angle trilayer graphene

Science ◽  
2021 ◽  
Vol 371 (6534) ◽  
pp. 1133-1138 ◽  
Author(s):  
Zeyu Hao ◽  
A. M. Zimmerman ◽  
Patrick Ledwith ◽  
Eslam Khalaf ◽  
Danial Haie Najafabadi ◽  
...  
Keyword(s):  
Displacement Field ◽  
Weak Coupling ◽  
Twist Angle ◽  
Magic Angle ◽  
Van Hove Singularity ◽  
Displacement Fields ◽  
Graphene Layers ◽  
Vdw Heterostructure ◽  
Quantum Phenomena ◽  
Trilayer Graphene

Engineering moiré superlattices by twisting layers in van der Waals (vdW) heterostructures has uncovered a wide array of quantum phenomena. We constructed a vdW heterostructure that consists of three graphene layers stacked with alternating twist angles ±θ. At the average twist angle θ ~ 1.56°, a theoretically predicted “magic angle” for the formation of flat electron bands, we observed displacement field–tunable superconductivity with a maximum critical temperature of 2.1 kelvin. By tuning the doping level and displacement field, we found that superconducting regimes occur in conjunction with flavor polarization of moiré bands and are bounded by a van Hove singularity (vHS) at high displacement fields. Our findings display inconsistencies with a weak coupling description, suggesting that the observed moiré superconductivity has an unconventional nature.

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

2019 ◽  
Vol 10 (1) ◽  
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.

scholarly journals Magic of high-order van Hove singularity

2019 ◽  
Vol 10 (1) ◽  
Author(s):  
Noah F. Q. Yuan ◽  
Hiroki Isobe ◽  
Liang Fu
Keyword(s):  
Density Of States ◽  
Twist Angle ◽  
Saddle Points ◽  
Two Dimensions ◽  
High Order ◽  
Van Hove Singularity ◽  
Vertical Electric Field ◽  
New Type ◽  
Field Correlation ◽  
Trilayer Graphene

AbstractThe van Hove singularity in density of states generally exists in periodic systems due to the presence of saddle points of energy dispersion in momentum space. We introduce a new type of van Hove singularity in two dimensions, resulting from high-order saddle points and exhibiting power-law divergent density of states. We show that high-order van Hove singularity can be generally achieved by tuning the band structure with a single parameter in moiré superlattices, such as twisted bilayer graphene by tuning twist angle or applying pressure, and trilayer graphene by applying vertical electric field. Correlation effects from high-order van Hove singularity near Fermi level are also discussed.

scholarly journals Phonons and Quantum Criticality Revealed by Temperature Linear Resistivity in Twisted Double Bilayer Graphene

2021 ◽  
Author(s):  
Yanbang Chu ◽  
Le Liu ◽  
Cheng Shen ◽  
Jinpeng Tian ◽  
Jian Tang ◽  
...  
Keyword(s):  
Displacement Field ◽  
Single Particle ◽  
Carrier Density ◽  
Phonon Scattering ◽  
Twist Angle ◽  
Bilayer Graphene ◽  
Quantum Criticality ◽  
Van Hove Singularity ◽  
Scattering Model ◽  
Wide Range

Abstract Twisted double bilayer graphene (TDBG) is an electric-field-tunable moiré system, exhibiting electron correlated states and related temperature linear (T-linear) resistivity. The displacement field provides a new knob to in-situ tune the relative strength of electron interactions in TDBG, yielding not only a rich phase diagram but also the ability to investigate each phase individually. Here, we report a study of carrier density (n), displacement field (D) and twist angle (θ) dependence of T-linear resistivity in TDBG. For a large twist angle (θ > 1.5°) where correlated insulating states are absent, we observe a T-linear resistivity (order of 10Ω/K) over a wide range of carrier density and its slope decreases with increasing of n before reaching the van Hove singularity, in agreement with acoustic phonon scattering model. The slope of T-linear resistivity is non-monotonically dependent on displacement field, with a single peak structure closely connected to single-particle van Hove Singularity (vHS) in TDBG. For an optimal twist angle of ~ 1.23° in the presence of correlated states, the slope of T-linear resistivity is found maximum at the boundary of the correlated halo regime (order of 100Ω/K), resulting a ‘M’ shape displacement field dependence. The observation is beyond the phonon scattering model from single particle picture, and instead it suggests a strange metal behavior. We interpret the observation as a result of symmetry-breaking instability developed at quantum critical points where electron degeneracy changes. Our results demonstrate that TDBG is an ideal system to study the interplay between phonon and quantum criticality, and might help to map out the evolution of the order parameters for the ground states.

Tuning the electrical properties of exfoliated graphene layers using deep ultraviolet irradiation

2014 ◽  
Vol 2 (27) ◽  
pp. 5404-5410 ◽  
Author(s):  
M. Z. Iqbal ◽  
M. F. Khan ◽  
M. W. Iqbal ◽  
Jonghwa Eom
Keyword(s):  
Electrical Properties ◽  
Ultraviolet Irradiation ◽  
Single Layer ◽  
Single Layer Graphene ◽  
Graphene Bilayer ◽  
Graphene Layers ◽  
Exfoliated Graphene ◽  
Deep Ultraviolet ◽  
Trilayer Graphene ◽  
Unique Band

Deep ultraviolet irradiation tunes the electronic properties of mechanically exfoliated single-layer graphene, bilayer graphene, and trilayer graphene while maintaining their unique band structure and electrical properties.

The Study in the Displacement Field near Crack of the Plexiglass by Frequency Domination Method of the White Speckle

2006 ◽  
Vol 306-308 ◽  
pp. 357-362 ◽  
Author(s):  
Xin Hua Ji ◽  
Fang Yu Xu ◽  
Jin Long Chen ◽  
Yu Wen Qin
Keyword(s):  
Light Source ◽  
Displacement Field ◽  
Vibration Isolation ◽  
Laser Speckle ◽  
Displacement Fields ◽  
Fracture Properties ◽  
Normal Environment

The fracture properties of Plexiglass bright the attentions of the researchers as it is the import material used in aero-planes industry The white speckle technique could obtain displacement fields nondestructively under the normal environment. Compare to the laser speckle method there are no interference light source and the vibration isolation needed. In the paper the principle of the technique is described and the displacement field near crack and SIF are measured. The results show that the technique is very suitable to the application in industry.

scholarly journals Gate-tunable charge carrier electrocaloric effect in trilayer graphene

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Natalia Cortés ◽  
Oscar Negrete ◽  
Francisco J. Peña ◽  
Patricio Vargas
Keyword(s):  
Charge Carrier ◽  
Tight Binding ◽  
Field Potential ◽  
Thermal Response ◽  
The Novel ◽  
Layered Systems ◽  
Electrocaloric Effect ◽  
Electronic Density ◽  
Graphene Layers ◽  
Trilayer Graphene

AbstractThe electrocaloric (EC) effect is the change in temperature and entropy of a material driven by the application of an electric field. Our tight-binding calculations linked to Fermi statistics, show that the EC effect can be produced in trilayer graphene (TLG) structures connected to a heat source, triggered by changes in the electronic density of states (DOS) at the Fermi level when external gate fields are applied on the outer graphene layers. We demonstrate that entropy changes are sensitive to the stacking arrangement in TLG systems. The AAA-stacked TLG presents an inverse EC response (cooling) regardless of the temperature value and gate field potential strength, whereas the EC effect in ABC-stacked TLG remains direct (heating) above room temperature. We reveal otherwise the TLG with Bernal-ABA stacking generates both the direct and inverse EC response within the same sample, associated with gate-dependent electronic transitions of thermally excited charge carriers from the valence band to the conduction band in the band structure. The novel charge carrier electrocaloric effect we propose in quantum layered systems may bring a wide variety of prototype van der Waals materials that could be used as versatile platforms to controlling the thermal response in nanodevices.

scholarly journals Steering the current flow in twisted bilayer graphene

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.

scholarly journals The Reconstruction of Displacement Fields of Defects in Crystals from Electron Micrographs

1969 ◽  
Vol 22 (3) ◽  
pp. 345 ◽  
Author(s):  
AK Head
Keyword(s):  
Electron Micrographs ◽  
Displacement Field ◽  
Defects In Crystals ◽  
Displacement Fields ◽  
Beam Diffraction

It is shown that the theorem of Part I, namely, that there is a unique reversible connection between displacement fields and electron micrographs for the case of two-beam diffraction and analytic displacement fields, can be extended to many-beam diffraction conditions. The case of a systematic set of diffracting vectors is parallel to the two-beam case with a unique reversible connection between one component of the displacement field and one micrograph. In the general many-beam case there is a unique reversible connection between the vector displacement field and three micrographs.

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.

Sign in / Sign up

Export Citation Format

Share Document