Competing correlated states and abundant orbital magnetism in twisted monolayer-bilayer graphene

AbstractFlat band moiré superlattices have recently emerged as unique platforms for investigating the interplay between strong electronic correlations, nontrivial band topology, and multiple isospin ‘flavor’ symmetries. Twisted monolayer-bilayer graphene (tMBG) is an especially rich system owing to its low crystal symmetry and the tunability of its bandwidth and topology with an external electric field. Here, we find that orbital magnetism is abundant within the correlated phase diagram of tMBG, giving rise to the anomalous Hall effect in correlated metallic states nearby most odd integer fillings of the flat conduction band, as well as correlated Chern insulator states stabilized in an external magnetic field. The behavior of the states at zero field appears to be inconsistent with simple spin and valley polarization for the specific range of twist angles we investigate, and instead may plausibly result from an intervalley coherent (IVC) state with an order parameter that breaks time reversal symmetry. The application of a magnetic field further tunes the competition between correlated states, in some cases driving first-order topological phase transitions. Our results underscore the rich interplay between closely competing correlated ground states in tMBG, with possible implications for probing exotic IVC ordering.

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Bulk valley transport and Berry curvature spreading at the edge of flat bands

Nature Communications ◽

10.1038/s41467-020-19284-w ◽

2020 ◽

Vol 11 (1) ◽

Author(s):

Subhajit Sinha ◽

Pratap Chandra Adak ◽

R. S. Surya Kanthi ◽

Bheema Lingam Chittari ◽

L. D. Varma Sangani ◽

...

Keyword(s):

Hall Effect ◽

Recent Observation ◽

High Sensitivity ◽

Anomalous Hall Effect ◽

Bilayer Graphene ◽

Flat Band ◽

Berry Curvature ◽

Flat Bands ◽

Current Path ◽

Orbital Magnetism

Abstract 2D materials based superlattices have emerged as a promising platform to modulate band structure and its symmetries. In particular, moiré periodicity in twisted graphene systems produces flat Chern bands. The recent observation of anomalous Hall effect (AHE) and orbital magnetism in twisted bilayer graphene has been associated with spontaneous symmetry breaking of such Chern bands. However, the valley Hall state as a precursor of AHE state, when time-reversal symmetry is still protected, has not been observed. Our work probes this precursor state using the valley Hall effect. We show that broken inversion symmetry in twisted double bilayer graphene (TDBG) facilitates the generation of bulk valley current by reporting experimental evidence of nonlocal transport in a nearly flat band system. Despite the spread of Berry curvature hotspots and reduced quasiparticle velocities of the carriers in these flat bands, we observe large nonlocal voltage several micrometers away from the charge current path — this persists when the Fermi energy lies inside a gap with large Berry curvature. The high sensitivity of the nonlocal voltage to gate tunable carrier density and gap modulating perpendicular electric field makes TDBG an attractive platform for valley-twistronics based on flat bands.

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Spectroscopy of a tunable moiré system with a correlated and topological flat band

Nature Communications ◽

10.1038/s41467-021-23031-0 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Xiaomeng Liu ◽

Cheng-Li Chiu ◽

Jong Yeon Lee ◽

Gelareh Farahi ◽

Kenji Watanabe ◽

...

Keyword(s):

Band Structure ◽

Twist Angle ◽

Bilayer Graphene ◽

Scanning Tunneling Spectroscopy ◽

Scanning Tunneling ◽

Flat Band ◽

Correlated Electron ◽

Partial Filling ◽

Valley Polarization ◽

Spectroscopic Signatures

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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Reversibility of the Magnetocaloric Effect in the Bean-Rodbell Model

Magnetochemistry ◽

10.3390/magnetochemistry7050060 ◽

2021 ◽

Vol 7 (5) ◽

pp. 60

Author(s):

Luis M. Moreno-Ramírez ◽

Victorino Franco

Keyword(s):

Magnetic Field ◽

Thermal Hysteresis ◽

Entropy Change ◽

Second Order ◽

Zero Field ◽

First Order ◽

Magnetic Field Change ◽

Magnetocaloric Materials ◽

Moderate Magnetic Field ◽

The Magnetic Field

The applicability of magnetocaloric materials is limited by irreversibility. In this work, we evaluate the reversible magnetocaloric response associated with magnetoelastic transitions in the framework of the Bean-Rodbell model. This model allows the description of both second- and first-order magnetoelastic transitions by the modification of the η parameter (η<1 for second-order and η>1 for first-order ones). The response is quantified via the Temperature-averaged Entropy Change (TEC), which has been shown to be an easy and effective figure of merit for magnetocaloric materials. A strong magnetic field dependence of TEC is found for first-order transitions, having a significant increase when the magnetic field is large enough to overcome the thermal hysteresis of the material observed at zero field. This field value, as well as the magnetic field evolution of the transition temperature, strongly depend on the atomic magnetic moment of the material. For a moderate magnetic field change of 2 T, first-order transitions with η≈1.3−1.8 have better TEC than those corresponding to stronger first-order transitions and even second-order ones.

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Quantum anomalous Hall effect in intrinsic magnetic topological insulator MnBi2Te4

Science ◽

10.1126/science.aax8156 ◽

2020 ◽

Vol 367 (6480) ◽

pp. 895-900 ◽

Cited By ~ 121

Author(s):

Yujun Deng ◽

Yijun Yu ◽

Meng Zhu Shi ◽

Zhongxun Guo ◽

Zihan Xu ◽

...

Keyword(s):

Magnetic Field ◽

Quantum Transport ◽

Topological Insulator ◽

Time Reversal ◽

Magnetic Order ◽

Anomalous Hall Effect ◽

Time Reversal Symmetry ◽

Zero Field ◽

Exotic States ◽

Ferromagnetic Layers

In a magnetic topological insulator, nontrivial band topology combines with magnetic order to produce exotic states of matter, such as quantum anomalous Hall (QAH) insulators and axion insulators. In this work, we probe quantum transport in MnBi2Te4 thin flakes—a topological insulator with intrinsic magnetic order. In this layered van der Waals crystal, the ferromagnetic layers couple antiparallel to each other; atomically thin MnBi2Te4, however, becomes ferromagnetic when the sample has an odd number of septuple layers. We observe a zero-field QAH effect in a five–septuple-layer specimen at 1.4 kelvin, and an external magnetic field further raises the quantization temperature to 6.5 kelvin by aligning all layers ferromagnetically. The results establish MnBi2Te4 as an ideal arena for further exploring various topological phenomena with a spontaneously broken time-reversal symmetry.

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Spin-polarized Dirac cone, flat band and saddle point in kagome magnet YMn6Sn6

10.21203/rs.3.rs-114140/v1 ◽

2020 ◽

Author(s):

Man Li ◽

Qi Wang ◽

Guangwei Wang ◽

Zhihong Yuan ◽

Wenhua Song ◽

...

Keyword(s):

Saddle Point ◽

Hall Effect ◽

Anomalous Hall Effect ◽

Flat Band ◽

Dirac Fermion ◽

Kagome Lattice ◽

Dirac Cone ◽

Kagomé Lattice ◽

Orbital Magnetism ◽

Spin Polarized

Abstract Kagome-lattice of 3d-transition metals hosting Weyl/Dirac fermions and topological flat bands exhibit non-trivial topological characters and novel quantum phases, such as anomalous Hall eﬀect and fractional quantum Hall eﬀect. With consideration of spin-orbit coupling and electron correlation, several instabilities could be induced. The complete characters of the electronic structure of kagome lattice, i.e. the saddle point, Dirac-cone, and flat band, around the Fermi energy (EF) remain elusive in magnetic kagome materials. We present the first experimental observation of the complete features in ferromagnetic kagome layers of YMn6Sn6 helically coupled along the c-axis, by using angle-resolved photoemission spectroscopy and band structure calculations. We demonstrate a Dirac dispersion near EF arising from a spin-polarized orbital, which carries an intrinsic Berry curvature and contributes to the anomalous Hall eﬀect in transport measurements. In addition, a flat band and a saddle point with a high density of states and with orbital-selective characters near EF are observed. These multi-orbital kagome features could cause multi-orbital magnetism. The Dirac fermion, flat band and saddle point in the vicinity of EF open an opportunity in manipulating the topological properties in magnetic materials.

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Magic in twisted transition metal dichalcogenide bilayers

Nature Communications ◽

10.1038/s41467-021-27042-9 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Trithep Devakul ◽

Valentin Crépel ◽

Yang Zhang ◽

Liang Fu

Keyword(s):

Transition Metal ◽

Density Functional ◽

Anomalous Hall Effect ◽

Quantum Spin ◽

Flat Band ◽

Magic Angle ◽

Transition Metal Dichalcogenide ◽

Strongly Correlated Electron ◽

Hartree Fock ◽

The Rich

AbstractThe long-wavelength moiré superlattices in twisted 2D structures have emerged as a highly tunable platform for strongly correlated electron physics. We study the moiré bands in twisted transition metal dichalcogenide homobilayers, focusing on WSe2, at small twist angles using a combination of first principles density functional theory, continuum modeling, and Hartree-Fock approximation. We reveal the rich physics at small twist angles θ < 4∘, and identify a particular magic angle at which the top valence moiré band achieves almost perfect flatness. In the vicinity of this magic angle, we predict the realization of a generalized Kane-Mele model with a topological flat band, interaction-driven Haldane insulator, and Mott insulators at the filling of one hole per moiré unit cell. The combination of flat dispersion and uniformity of Berry curvature near the magic angle holds promise for realizing fractional quantum anomalous Hall effect at fractional filling. We also identify twist angles favorable for quantum spin Hall insulators and interaction-induced quantum anomalous Hall insulators at other integer fillings.

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Dirac cone, flat band and saddle point in kagome magnet YMn6Sn6

Nature Communications ◽

10.1038/s41467-021-23536-8 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Man Li ◽

Qi Wang ◽

Guangwei Wang ◽

Zhihong Yuan ◽

Wenhua Song ◽

...

Keyword(s):

Saddle Point ◽

Hall Effect ◽

Theoretical Calculations ◽

Anomalous Hall Effect ◽

Flat Band ◽

Dirac Fermion ◽

Dirac Fermions ◽

Dirac Cone ◽

Fractional Quantum Hall ◽

Orbital Magnetism

AbstractKagome-lattices of 3d-transition metals hosting Weyl/Dirac fermions and topological flat bands exhibit non-trivial topological characters and novel quantum phases, such as the anomalous Hall effect and fractional quantum Hall effect. With consideration of spin–orbit coupling and electron correlation, several instabilities could be induced. The typical characters of the electronic structure of a kagome lattice, i.e., the saddle point, Dirac-cone, and flat band, around the Fermi energy (EF) remain elusive in magnetic kagome materials. We present the experimental observation of the complete features in ferromagnetic kagome layers of YMn6Sn6 helically coupled along the c-axis, by using angle-resolved photoemission spectroscopy and band structure calculations. We demonstrate a Dirac dispersion near EF, which is predicted by spin-polarized theoretical calculations, carries an intrinsic Berry curvature and contributes to the anomalous Hall effect in transport measurements. In addition, a flat band and a saddle point with a high density of states near EF are observed. These multi-sets of kagome features are of orbital-selective origin and could cause multi-orbital magnetism. The Dirac fermion, flat band and saddle point in the vicinity of EF open an opportunity in manipulating the topological properties in magnetic materials.

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Competing zero-field Chern insulators in Superconducting Twisted Bilayer Graphene

10.21203/rs.3.rs-138250/v1 ◽

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.

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Fractional Chern insulators in magic-angle twisted bilayer graphene

Nature ◽

10.1038/s41586-021-04002-3 ◽

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.

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