Quantum anomalous Hall octet driven by orbital magnetism in bilayer graphene

Geisenhof, Fabian R.; Winterer, Felix; Seiler, A.; Lenz, Jakob; Xu, Tianyi; Zhang, Fan; Weitz, R. Thomas

doi:10.1038/s41586-021-03849-w

Cited by 52 publications

(33 citation statements)

References 42 publications

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“…Two devices are discussed exemplarily in the following: D1-DW of the former and D2 (which has been also investigated in ref. 27 .) of the latter type.…”

Section: Resultsmentioning

confidence: 98%

“…When a uniform electric field is applied to a bilayer graphene flake with a DW, topologically protected valley-helical states emerge along the dislocation, surrounded by insulating bulk 12 , 14 , 19 . Critically for the present work, stacking domain walls can have much richer interplay with spontaneous symmetry breaking in bilayer graphene 20 – 27 compared to artificially created ones, as not being forced by applied bias to have charge imbalance between layers. The interplay between stacking domain walls and spontaneous symmetry breaking is of peculiar interest in the presence of a quantising magnetic field, since bilayer graphene exhibits a very rich phase diagram owing to the eightfold degeneracy of the zero-energy Landau levels 28 – 30 (coming from two valleys, two orbital Landau level indices, and two spins – neglecting Zeeman splitting).…”

Section: Introductionmentioning

confidence: 99%

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Interplay between topological valley and quantum Hall edge transport

et al. 2022

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An established way of realising topologically protected states in a two-dimensional electron gas is by applying a perpendicular magnetic field thus creating quantum Hall edge channels. In electrostatically gapped bilayer graphene intriguingly, even in the absence of a magnetic field, topologically protected electronic states can emerge at naturally occurring stacking domain walls. While individually both types of topologically protected states have been investigated, their intriguing interplay remains poorly understood. Here, we focus on the interplay between topological domain wall states and quantum Hall edge transport within the eight-fold degenerate zeroth Landau level of high-quality suspended bilayer graphene. We find that the two-terminal conductance remains approximately constant for low magnetic fields throughout the distinct quantum Hall states since the conduction channels are traded between domain wall and device edges. For high magnetic fields, however, we observe evidence of transport suppression at the domain wall, which can be attributed to the emergence of spectral minigaps. This indicates that stacking domain walls potentially do not correspond to a topological domain wall in the order parameter.

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“…Two devices are discussed exemplarily in the following: D1-DW of the former and D2 (which has been also investigated in ref. 27 .) of the latter type.…”

Section: Resultsmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

Interplay between topological valley and quantum Hall edge transport

et al. 2022

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“…Few-layer graphene with rhombohedral (R)-stacking also generates highly degenerate flat bands in zero field but without the requirement of a moiré superlattice [5,[17][18][19][20]. Bernal-stacked bilayer graphene, equivalent to R-stacking, is the simplest member of this family of structures, however, to date reports of symmetry-broken states in bilayer graphene have generally required large magnetic fields [21][22][23][24] or involved subtle gapped states at charge neutrality [25][26][27][28][29][30]. Intriguingly, R-stacked (ABC) trilayer graphene was recently shown to exhibit spontaneous itinerant spin and valley polarization at zero magnetic field, displaying a series of phase transitions, hysteresis, anomalous Hall, and superconductivity at millikelvin temperatures in carefully prepared samples [31,32].…”

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confidence: 99%

Cascade of isospin phase transitions in Bernal bilayer graphene at zero magnetic field

de la Barrera,

Aronson,

Zheng

et al. 2021

Preprint

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Emergent phenomena arising from the collective behavior of electrons is generally expected when Coulomb interactions dominate over the kinetic energy, as in delocalized quasiparticles in highly degenerate flat bands. Bernal-stacked bilayer graphene intrinsically supports a pair of flat bands predicted to host a variety of spontaneous broken-symmetry states arising from van Hove singularities and a four-fold spin-valley (isospin) degeneracy [1][2][3][4][5][6]. Here, we show that ultra-clean samples of bilayer graphene display a cascade of symmetry-broken states with spontaneous and spin and valley ordering at zero magnetic field. Using capacitive sensing in a dual-gated geometry, we tune the carrier density and electric displacement field independently to explore the phase space of transitions and probe the character of the isospin order. Itinerant ferromagnetic states emerge near the conduction and valence band edges with complete spin and valley polarization and a high degree of displacement field tunability. At larger hole densities, two-fold degenerate quantum oscillations manifest in an additional broken symmetry state that is enhanced by the application of an in-plane magnetic field. Both types of symmetry-broken states display enhanced layer polarization at low temperatures, suggesting a coupling to the layer pseudospin degree of freedom in the electronic wavefunctions [1,6]. Notably, the zero-field spontaneous symmetry breaking reported here emerges in the absence of a moiré superlattice and is intrinsic to natural graphene bilayers. Thus, we demonstrate that the tunable bands of bilayer graphene represent a related, but distinct approach to produce flat band collective behavior, complementary to engineered moiré structures.

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“…For example, the anomalous Hall effect (AHE) is a striking experimental signature that emerges when spontaneous spin or valley polarization breaks time reversal symmetry in bands with finite Berry curvature [14]. AHE reflecting orbital ferromagnetism has been reported in multi-layer vdW heterostructures such as twisted bilayer graphene aligned with hexagonal boron nitride (hBN) [15,16], and in naturally-occurring structures like Bernal-stacked (AB) bilayer graphene [17].…”

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confidence: 99%

Spontaneous time-reversal symmetry breaking in twisted double bilayer graphene

Kuiri¹,

Coleman²,

Gao³

et al. 2022

Preprint

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Twisted double bilayer graphene (tDBG) comprises two Bernal-stacked bilayer graphene sheets with a twist between them. Gate voltages applied to top and back gates of a tDBG device tune both the flatness and topology of the electronic bands, enabling an unusual level of experimental control. Broken spin/valley symmetry metallic states have been observed in tDBG devices with twist angles ∼ 1.2-1.3 • , but the topologies and order parameters of these states have remained unclear. We report the observation of an anomalous Hall effect in the correlated metal state of tDBG, with hysteresis loops spanning 100s of mT in out-of-plane magnetic field (B ⊥ ) that demonstrate spontaneously broken time-reversal symmetry. The B ⊥ hysteresis persists for in-plane fields up to several Tesla, suggesting valley (orbital) ferromagnetism. At the same time, the resistivity is strongly affected by even mT-scale values of in-plane magnetic field, pointing to spin-valley coupling or to a direct orbital coupling between in-plane field and the valley degree of freedom.

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Quantum anomalous Hall octet driven by orbital magnetism in bilayer graphene

Cited by 52 publications

References 42 publications

Interplay between topological valley and quantum Hall edge transport

Interplay between topological valley and quantum Hall edge transport

Cascade of isospin phase transitions in Bernal bilayer graphene at zero magnetic field

Spontaneous time-reversal symmetry breaking in twisted double bilayer graphene

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