Promotion of superconductivity in magic-angle graphene multilayers

Zhang, Yiran; Polski, Robert; Lewandowski, Cyprian; Thomson, Alex; Peng, Yang; Choi, Youngjoon; Kim, Hyunjin; Watanabe, Kenji; Taniguchi, Takashi; Alicea, Jason; Oppen, Felix von; Refael, Gil; Nadj-Perge, Stevan

doi:10.1126/science.abn8585

Cited by 78 publications

(31 citation statements)

References 70 publications

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“…44 This is also possibly responsible for the gapped states at displacement field that is smaller than 𝐷𝐷 𝑐𝑐 (𝜈𝜈) for integer filling 𝜈𝜈 = 2 reported previously. 45 Such candidate heavy fermion state in MATTG requires further studies on localized magnetic moments in flat band at these integer fillings.…”

Section: 𝑫𝑫-Induced Phase Transitionsmentioning

confidence: 99%

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Dirac spectroscopy of strongly correlated phases in twisted trilayer graphene

et al. 2022

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Mirror-symmetric magic-angle twisted trilayer graphene (MATTG) hosts flat electronic bands close to zero energy, and has been recently shown to exhibit abundant correlated quantum phases with flexible electrical tunability. [1][2][3] However studying these phases proved challenging as these are obscured by intertwined Dirac bands. In this work, we demonstrate a novel spectroscopy technique, that allows to quantify the energy gaps and Chern numbers of the correlated states in MATTG by driving band crossings between Dirac cone Landau levels and the energy gaps in the flat bands. We uncover hard correlated gaps with Chern numbers of C = 0 at integer moiré unit cell fillings of ν = 2 and 3 and reveal novel charge density wave states originating from van Hove singularities at fractional fillings of ν = 5/3 and 11/3. In addition, we demonstrate the existence of displacement field driven first-order phase transitions at charge neutrality and half fillings of the moiré unit cell ν = ± 2, which is consistent with a theoretical strong-coupling analysis, implying the breaking of the C 2 T symmetry. Overall these properties establish the diverse and electrically tunable phase diagram of MATTG and provide an avenue for investigating electronic quantum phases and strong correlations in multiple-band moiré systems.

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Section: 𝑫𝑫-Induced Phase Transitionsmentioning

confidence: 99%

“…Our work helps to bridge between flat band correlated insulator and nearby unconventional superconductor, and provides an avenue to quantitatively study flat band correlated insulators in other multiband moiré systems. 36,45,46…”

Section: 𝑫𝑫-Induced Phase Transitionsmentioning

confidence: 99%

Dirac spectroscopy of strongly correlated phases in twisted trilayer graphene

et al. 2022

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“…These materials are very thin, up to a single atomic layer, and have a non-singlet superconducting order parameter facilitated by their multi-orbital band structure and electronic correlations. The most notable examples are few-layer transition metal dichalcogenides, presumably hosting so-called Ising superconductivity [6][7][8], and twisted graphene multilayers [9][10][11].…”

Section: Introductionmentioning

confidence: 99%

Inducing Superconductivity in Bilayer Graphene by Alleviation of the Stoner Blockade

Shavit¹,

Oreg²

2023

Preprint

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External magnetic fields conventionally suppress superconductivity, both by orbital and paramagnetic effects. A recent experiment [1] has shown that in a Bernal stacked bilayer graphene system, the opposite occursa finite critical magnetic field is necessary to observe superconducting features occurring in the vicinity of a magnetic phase transition. We propose an extraordinary electronic-correlation-driven mechanism by which this anomalous superconductivity manifests. Specifically, the electrons tend to avoid band occupations near high density of states regions due to their mutual repulsion. Considering the nature of spontaneous symmetry breaking involved, we dub this avoidance Stoner blockade. We show how a magnetic field softens this blockade, allowing weak superconductivity to take place, consistent with experimental findings. Our principle prediction is that a small reduction of the Coulomb repulsion would result in sizable superconductivity gains, both in achieving higher critical temperatures and expanding the superconducting regime. Within the theory we present, magnetic field and spin-orbit coupling of the Ising type have a similar effect on the Bernal stacked bilayer graphene system, elucidating the emergence of superconductivity when the system is proximitized to a WSe2 substrate. We further demonstrate in this paper the sensitivity of superconductivity to disorder in the proposed scenario. We find that a disorder that does not violate Anderson's theorem may still induce a reduction of Tc through its effect on the density of states, establishing the delicate nature of the Bernal bilayer graphene superconductor.

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“…In the search for high temperature superconductors, more and more evidence points to two-dimensional (2D) structures as good candidates, but their theoretical understanding remains limited, because many of these materials have a complex structure. Specifically, multilayer graphene has been put forward as a promising testbed for unconventional superconductivity [1][2][3][4][5][6]. When the graphene layers are twisted by a certain 'magic' angle, the emerging moiré superlattice allows for a superconducting phase that can be tuned with electric fields.…”

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confidence: 99%

Topological two-band electron-hole superconductors with $d$-wave symmetry: a possible application to magic-angle twisted trilayer graphene

Loon¹,

Melo²

2023

Preprint

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We discuss a two-band model for two-dimensional superconductors with electron and hole bands separated by an energy gap and singlet d-wave pairing in each band. This model exhibits a Vshaped to U-shaped transition in the density of the states of the superconductor, which is similar to that found in recent tunneling experiments in magic-angle twisted trilayer graphene [1]. This qualitative difference in behavior occurs when the electron and hole chemical potentials change, leading to topological quantum phase transitions between gapless and gapful d-wave superconducting states, due to the annihilation of chiral Dirac fermions at the critical points. Lastly, we show that direct thermodynamic evidence of these topological quantum phase transitions can be found in measurements of the compressibility, which exhibits logarithmic singularities at the transition points.

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Promotion of superconductivity in magic-angle graphene multilayers

Cited by 78 publications

References 70 publications

Dirac spectroscopy of strongly correlated phases in twisted trilayer graphene

Dirac spectroscopy of strongly correlated phases in twisted trilayer graphene

Inducing Superconductivity in Bilayer Graphene by Alleviation of the Stoner Blockade

Topological two-band electron-hole superconductors with $d$-wave symmetry: a possible application to magic-angle twisted trilayer graphene

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