scholarly journals Chiral magic-angle twisted bilayer graphene in a magnetic field: Landau level correspondence, exact wave functions, and fractional Chern insulators

2021 ◽  
Vol 104 (12) ◽  
Author(s):  
Yarden Sheffer ◽  
Ady Stern
Keyword(s):  
Magnetic Field ◽  
Landau Level ◽  
Bilayer Graphene ◽  
Wave Functions ◽  
Magic Angle ◽  
Chern Insulators ◽  
Twisted Bilayer Graphene

scholarly journals Competing zero-field Chern insulators in Superconducting Twisted Bilayer Graphene

2021 ◽  
Author(s):  
Dmitri Efetov ◽  
Petr Stepanov ◽  
Ming Xie ◽  
Takashi Taniguchi ◽  
Kenji Watanabe ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Bilayer Graphene ◽  
Chern Number ◽  
Zero Magnetic Field ◽  
Magic Angle ◽  
Zero Field ◽  
Superconducting Phases ◽  
Chern Insulators ◽  
Twisted Bilayer Graphene ◽  
The Creation

Abstract The discovery of magic angle twisted bilayer graphene (MATBG) has unveiled a rich variety of superconducting, magnetic and topologically nontrivial phases. The existence of all these phases in one material, and their tunability, has opened new pathways for the creation of unusual gate tunable junctions. However, the required conditions for their creation – gate induced transitions between phases in zero magnetic field – have so far not been achieved. Here, we report on the first experimental demonstration of a device that is both a zero-field Chern insulator and a superconductor. The Chern insulator occurs near moiré cell filling factor = +1 in a hBN non-aligned MATBG device and manifests itself via an anomalous Hall effect. The insulator has Chern number C= ±1 and a relatively high Curie temperature of Tc ≈ 4.5 K. Gate tuning away from this state exposes strong superconducting phases with critical temperatures of up to Tc ≈ 3.5 K. In a perpendicular magnetic field above B > 0.5 T we observe a transition of the = +1 Chern insulator from Chern number C = ±1 to C = 3, characterized by a quantized Hall plateau with Ryx = h/3e2. These observations show that interaction-induced symmetry breaking in MATBG leads to zero-field ground states that include almost degenerate and closely competing Chern insulators, and that states with larger Chern numbers couple most strongly to the B-field. By providing the first demonstration of a system that allows gate-induced transitions between magnetic and superconducting phases, our observations mark a major milestone in the creation of a new generation of quantum electronics.

scholarly journals Symmetry-broken Chern insulators and Rashba-like Landau-level crossings in magic-angle bilayer graphene

2021 ◽  
Author(s):  
Ipsita Das ◽  
Xiaobo Lu ◽  
Jonah Herzog-Arbeitman ◽  
Zhi-Da Song ◽  
Kenji Watanabe ◽  
...  
Keyword(s):  
Landau Level ◽  
Bilayer Graphene ◽  
Magic Angle ◽  
Level Crossings ◽  
Chern Insulators

scholarly journals Unconventional sequence of correlated Chern insulators in magic-angle twisted bilayer graphene

2021 ◽  
Author(s):  
Andrew T. Pierce ◽  
Yonglong Xie ◽  
Jeong Min Park ◽  
Eslam Khalaf ◽  
Seung Hwan Lee ◽  
...  
Keyword(s):  
Bilayer Graphene ◽  
Magic Angle ◽  
Chern Insulators ◽  
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 Chern insulators, van Hove singularities and topological flat bands in magic-angle twisted bilayer graphene

2021 ◽  
Vol 20 (4) ◽  
pp. 488-494 ◽  
Author(s):  
Shuang Wu ◽  
Zhenyuan Zhang ◽  
K. Watanabe ◽  
T. Taniguchi ◽  
Eva Y. Andrei
Keyword(s):  
Bilayer Graphene ◽  
Magic Angle ◽  
Van Hove Singularities ◽  
Flat Bands ◽  
Chern Insulators ◽  
Twisted Bilayer Graphene

scholarly journals Fractional Chern insulators in magic-angle twisted bilayer graphene

Nature ◽  
2021 ◽  
Vol 600 (7889) ◽  
pp. 439-443
Author(s):  
Yonglong Xie ◽  
Andrew T. Pierce ◽  
Jeong Min Park ◽  
Daniel E. Parker ◽  
Eslam Khalaf ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Critical Role ◽  
Density Wave ◽  
Bilayer Graphene ◽  
Zero Magnetic Field ◽  
Magic Angle ◽  
Zero Field ◽  
Quantum Geometry ◽  
Chern Insulators

AbstractFractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states that may provide a new avenue towards manipulating non-Abelian excitations. Early theoretical studies1–7 have predicted their existence in systems with flat Chern bands and highlighted the critical role of a particular quantum geometry. However, FCI states have been observed only in Bernal-stacked bilayer graphene (BLG) aligned with hexagonal boron nitride (hBN)8, in which a very large magnetic field is responsible for the existence of the Chern bands, precluding the realization of FCIs at zero field. By contrast, magic-angle twisted BLG9–12 supports flat Chern bands at zero magnetic field13–17, and therefore offers a promising route towards stabilizing zero-field FCIs. Here we report the observation of eight FCI states at low magnetic field in magic-angle twisted BLG enabled by high-resolution local compressibility measurements. The first of these states emerge at 5 T, and their appearance is accompanied by the simultaneous disappearance of nearby topologically trivial charge density wave states. We demonstrate that, unlike the case of the BLG/hBN platform, the principal role of the weak magnetic field is merely to redistribute the Berry curvature of the native Chern bands and thereby realize a quantum geometry favourable for the emergence of FCIs. Our findings strongly suggest that FCIs may be realized at zero magnetic field and pave the way for the exploration and manipulation of anyonic excitations in flat moiré Chern bands.

scholarly journals Gate-defined Josephson junctions in magic-angle twisted bilayer graphene

2021 ◽  
Author(s):  
Folkert K. de Vries ◽  
Elías Portolés ◽  
Giulia Zheng ◽  
Takashi Taniguchi ◽  
Kenji Watanabe ◽  
...  
Keyword(s):  
Josephson Junctions ◽  
Bilayer Graphene ◽  
Magic Angle ◽  
Twisted Bilayer Graphene
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