Cascade of electronic transitions in magic-angle twisted bilayer graphene

Wong, Dillon; Nuckolls, Kevin P.; Oh, Myung Hwan; Lian, Biao; Xie, Yonglong; Jeon, Sangjun; Watanabe, Kenji; Bernevig, B. Andrei; Yazdani, Ali

doi:10.1038/s41586-020-2339-0

Cited by 439 publications

(335 citation statements)

References 33 publications

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“…However, in doing so, addressing the role of electronic interaction has been a major challenge for theorists and experimentalists alike. Attempts to control the insulating or superconducting states by controlling interaction [8][9][10], or otherwise [9,[11][12][13][14][15][16][17][18][19], have led to a surprisingly large number of dissimilar phase diagrams of TBG. In fact, even the number of insulating regions and that of the superconducting domes in these samples (under almost equivalent external circumstances) has been widely different.…”

Section: Introductionmentioning

confidence: 99%

Transport across twist angle domains in moiré graphene

Padhi

Tiwari

Neupert

et al. 2020

Phys. Rev. Research

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Many experiments in twisted bilayer graphene (TBG) differ from each other in terms of the details of their phase diagrams. Few controllable aspects aside, this discrepancy is largely believed to arise from the presence of a varying degree of twist angle inhomogeneity across different samples. Real-space maps indeed reveal TBG devices splitting into several large domains of different twist angles. Motivated by these observations, we study the quantum mechanical tunneling across a domain wall (DW) that separates two such regions. We show that the tunneling of the moiré particles can be understood by the formation of an effective step potential at the DW. The height of this step potential is simply a measure of the difference in twist angles. These computations lead us to identify the global transport signatures for detecting and quantifying the local twist angle variations. In particular, using Landauer-Büttiker formalism, we compute single-channel conductance (dI/dV) and Fano factor for shot noise (ratio of noise power and mean current). A reduction in the low-energy conductance and a zero-bias sub-meV gap in the conductance are observed which scale with the twist angle difference. One of the key findings of our work is that transport in presence of twist angle inhomogeneity is "noisy," though sub-Poissonian. In particular, the differential Fano factor peaks near the van Hove energies corresponding to the domains in the sample. The location and the strength of the peak are simply a measure of the degree of twist angle inhomogeneity.

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Section: Introductionmentioning

confidence: 99%

Transport across twist angle domains in moiré graphene

Padhi

Tiwari

Neupert

et al. 2020

Phys. Rev. Research

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“…Finally, very recent experiments have shown that for certain fillings the population of the flat bands occurs through a sequence of phase transitions at which a single spin/valley flavor takes all the carriers and becomes completely empty/filled 40,41 . Such phase transitions should also be detectable via optical conductivity as they would lead to a redistribution of the total spectral weight as a function of doping and temperature.…”

Section: Discussionmentioning

confidence: 99%

Correlated states in magic angle twisted bilayer graphene under the optical conductivity scrutiny

Calderón

Bascones

2020

npj Quantum Mater.

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Moiré systems displaying flat bands have emerged as novel platforms to study correlated electron phenomena. Insulating and superconducting states appear upon doping magic angle twisted bilayer graphene (TBG), and there is evidence of correlation induced effects at the charge neutrality point (CNP) which could originate from spontaneous symmetry breaking. Our theoretical calculations show how optical conductivity measurements can distinguish different symmetry breaking states, and reveal the nature of the correlated states. In the specific case of nematic order, which breaks the discrete rotational symmetry of the lattice, we find that the Dirac cones are displaced, not only in momentum space but also in energy, inducing finite Drude weight at the CNP. We also show that the sign of the Drude weight anisotropy induced by a nematic order depends on the degree of lattice relaxation, the doping and the nature of the symmetry breaking.

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“…Moiré superlattices have emerged as platforms to attain strongly correlated phases of matter by controlling the stacking configuration between the layers [1][2][3]. In twisted bilayer graphene (TBG) samples, examples include superconducting, Mott-insulating [4][5][6][7][8], and ferromagnetic states [9,10]. In twisted transition-metal dichalcogenide heterostructures (TMDs), evidence for moiré excitons has been reported [11][12][13].…”

Section: Introductionmentioning

confidence: 99%

Floquet engineering of twisted double bilayer graphene

Rodriguez-Vega

Vogl

Fiete

2020

Phys. Rev. Research

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Motivated by the recent experimental realization of twisted double bilayer graphene (TDBG) samples, we study, both analytically and numerically, the effects of circularly polarized light propagating in free space and confined in a waveguide on the band structure and topological properties of these systems. These two complementary Floquet protocols allow us to selectively tune different parameters of the system by varying the intensity and light frequency. For the drive protocol in free space, in the high-frequency regime, we find that in TDBG with AB/BA stacking, we can selectively close the zone-center quasienergy gaps around one valley while increasing the gaps near the opposite valley by tuning the parameters of the drive. In TDBG with AB/AB stacking, a similar effect can be obtained upon the application of a perpendicular static electric field. Furthermore, we study the topological properties of the driven system in different settings, provide accurate effective Floquet Hamiltonians, and show that relatively strong drives can generate flat bands. On the other hand, longitudinal light confined in a waveguide couples to the components of the interlayer hopping that are perpendicular to the TDBG sheet, allowing for selective engineering of the bandwidth of Floquet zone-center quasienergy bands without breaking the symmetries of the static system.

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Cascade of electronic transitions in magic-angle twisted bilayer graphene

Cited by 439 publications

References 33 publications

Transport across twist angle domains in moiré graphene

Transport across twist angle domains in moiré graphene

Correlated states in magic angle twisted bilayer graphene under the optical conductivity scrutiny

Floquet engineering of twisted double bilayer graphene

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