Strain-Induced Quantum Phase Transitions in Magic-Angle Graphene

Parker, Daniel E.; Soejima, Tomohiro; Hauschild, Johannes; Zaletel, Michael P.; Bultinck, Nick

doi:10.1103/physrevlett.127.027601

Cited by 86 publications

(53 citation statements)

References 55 publications

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“…In some domains, we find an average strain of the atomic lattice of up to ϵ = 0.4%. According to earlier theoretical work, such values are high enough to locally induce a quantum phase transition 8 .…”

Section: Resultsmentioning

confidence: 89%

Imaging moiré deformation and dynamics in twisted bilayer graphene

et al. 2022

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In ‘magic angle’ twisted bilayer graphene (TBG) a flat band forms, yielding correlated insulator behavior and superconductivity. In general, the moiré structure in TBG varies spatially, influencing the overall conductance properties of devices. Hence, to understand the wide variety of phase diagrams observed, a detailed understanding of local variations is needed. Here, we study spatial and temporal variations of the moiré pattern in TBG using aberration-corrected Low Energy Electron Microscopy (AC-LEEM). We find a smaller spatial variation than reported previously. Furthermore, we observe thermal fluctuations corresponding to collective atomic displacements over 70 pm on a timescale of seconds. Remarkably, no untwisting is found up to 600 ∘C. We conclude that thermal annealing can be used to decrease local disorder. Finally, we observe edge dislocations in the underlying atomic lattice, the moiré structure acting as a magnifying glass. These topological defects are anticipated to exhibit unique local electronic properties.

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Section: Resultsmentioning

confidence: 89%

Imaging moiré deformation and dynamics in twisted bilayer graphene

et al. 2022

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“…(Note that it has been argued that the effective value of w AA may be susceptible to renormalization towards the chiral limit 57 , κ = 0, due to the effects of remote bands; thus, tuning w AA may be viewed as phenomenologically modeling this downward renormalization 9 .) Sometimes, we will also incorporate heterostrain 20,58,59 with strength and a sublattice potential of strength ∆ 60 . In the absence of such single-particle perturbations, the point group is D 6 , which includes Ĉ2z , Ĉ3z and M (inplane rotation about the x-axis).…”

Section: A Interacting Bm Modelmentioning

confidence: 99%

“…Strain and substrate alignment represent two singleparticle perturbations that are deleterious to the paired skyrmions, making them less energetically favorable and harder to find within HF. Strain significantly increases the kinetic bandwidth and is known to substantially degrade the gap of the KIVC at charge neutrality, eventually leading to a symmetric semimetal 59 . As shown in Fig.…”

Section: B 2e-skyrmionsmentioning

confidence: 99%

“…This topological mechanism would not be effective in other moiré platforms that lack Ĉ2z -symmetry 22,[106][107][108][109] . Strain takes the system away from the strong-coupling regime and similarly disfavors skyrmions: for instance, the parent insulator has been predicted to give way to a symmetric semimetal at charge neutrality or an IKS at non-zero integer fillings 20,59 . Hence the general expectation from our work and from these experimental observations is is that skyrmion superconductivity is most likely to emerge in 'pristine' samples with minimal screening.…”

Section: Skyrmion Superconductivitymentioning

confidence: 99%

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Skyrmions in twisted bilayer graphene: stability, pairing, and crystallization

Kwan¹,

Wagner²,

Bultinck³

et al. 2021

Preprint

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We study the excitations that emerge upon doping the translationally-invariant correlated insulating states in magic angle twisted bilayer graphene at various integer filling factors ν. We identify parameter regimes where these are associated with skyrmion textures in the spin or pseudospin degrees of freedom, and explore both short-distance pairing effects and the formation of long-range ordered skyrmion crystals. We perform a comprehensive analysis of the pseudospin skyrmions that emerge upon doping insulators at even ν, delineating the regime in parameter space where these are the lowest-energy charged excitations by means of self-consistent Hartree-Fock calculations on the interacting Bistritzer-MacDonald model. We explicitly demonstrate the purely electron-mediated pairing of skyrmions, a key ingredient behind a recent proposal of skyrmion superconductivity. Building upon this, we construct hopping models to extract the effective masses of paired skyrmions, and discuss our findings and their implications for skyrmion superconductivity in relation to experiments, focusing on the dome-shaped dependence of the transition temperature on the twist angle. We also investigate the properties of spin skyrmions about the quantized anomalous Hall insulator at ν = +3. In both cases, we demonstrate the formation of robust spin/pseudospin skyrmion crystals upon doping to a finite density away from integer filling.

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“…The difference between insulating and semi-metallic behavior at charge neutrality may come from a strain-induced quantum phase transition. 26 Theoretically, different candidate states for a gap opening at charge neutrality in TBG have similar energies. [27][28][29][30][31][32][33] Among them, (sublattice-polarized) quantum valley Hall or intervalley coherent order have also been discussed to gap out the slow bands in TTG, based on extensions of the effective, exactly soluble models from TBG to TTG 34 .…”

mentioning

confidence: 99%

Interaction-induced velocity renormalization in magic angle twisted trilayer graphene

Claßen¹,

Pixley²,

König³

2021

Preprint

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Twistronics heterostructures provide a novel route to control the electronic single particle velocity and thereby to engineer strong effective interactions. Here we show that the reverse may also hold, i.e. that these interactions strongly renormalize the band structure. We demonstrate this mechanism for mirror-symmetric magic angle twisted trilayer graphene at charge neutrality and in the vicinity of a phase transition which can be described by an Ising Gross-Neveu critical point corresponding, e.g., to the onset of valley Hall or Hall order. While the non-interacting model displays massless Dirac excitations with strongly different velocities, we show that interaction corrections make them equal in the infrared. However, the RG flow of the velocities and of the coupling to the critical bosonic mode is strongly non-monotonic and dominated by the vicinity of a repulsive fixed point. We predict experimental consequences of this theory for tunneling and transport experiments and discuss the expected behavior at other quantum critical points, including those corresponding to intervalley coherent ordering.

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Strain-Induced Quantum Phase Transitions in Magic-Angle Graphene

Cited by 86 publications

References 55 publications

Imaging moiré deformation and dynamics in twisted bilayer graphene

Imaging moiré deformation and dynamics in twisted bilayer graphene

Skyrmions in twisted bilayer graphene: stability, pairing, and crystallization

Interaction-induced velocity renormalization in magic angle twisted trilayer graphene

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