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Evidence for unconventional superconductivity in twisted bilayer graphene
Superconductivity · Submitted 28 Sep 2021
abstract compiled · 2026-09-23 · no language-model review
The authors' main claim These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.
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The abstract, sentence by sentence
The emergence of superconductivity with doping from correlated insulators in magic-angle twisted bilayer graphene (MATBG) has raised the intriguing possibility that its pairing mechanism is distinct from that of conventional superconductors, as described by the Bardeen-Cooper-Schrieffer (BCS) theory. While there is now ample evidence for strong electronic correlations in MATBG, recent studies have claimed that unlike correlated insulators, superconductivity persists even when these interactions are partially screened. This suggests that the pairing in MATBG might be conventional in nature, a consequence of the large density of states of its nearly flat bands, perhaps phonon-mediated as in BCS superconductors. Here we combine tunneling and Andreev reflection spectroscopy with the scanning tunneling microscope (STM) to observe several key experimental signatures for unconventional superconductivity in MATBG. We show that the tunneling spectra below the transition temperature $T_c$ are inconsistent with that of a conventional s-wave superconductor, but rather resemble that of a nodal superconductor with an anisotropic pairing mechanism. We observe a large discrepancy between the tunneling energy gap $\Delta_T$, which far exceeds the mean-field BCS ratio (with $2\Delta_T/k_BT_c \sim 25$) and the energy gap $\Delta_{AR}$ extracted from Andreev reflection spectroscopy ($2\Delta_{AR}/k_BT_c \sim 6$). The tunneling gap persists even when superconductivity is suppressed, indicating its emergence from a pseudogap phase, with a suppressed density of states at the Fermi level. Moreover, the pseudogap state and superconductivity are both absent when MATBG is aligned with the hexagonal boron nitride (hBN) underneath. These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The emergence of superconductivity with doping from correlated insulators in magic-angle twisted bilayer graphene (MATBG) has raised the intriguing possibility that its pairing mechanism is distinct from that of conventional superconductors, as described by the Bardeen-Cooper-Schrieffer (BCS) theory. | background 1.00 | 0.04 | 0.04 | hedged possibility 0.96 |
| s1 | While there is now ample evidence for strong electronic correlations in MATBG, recent studies have claimed that unlike correlated insulators, superconductivity persists even when these interactions are partially screened. | background 0.94 | 0.04 | 0.02 | asserted 0.88 |
| s2 | This suggests that the pairing in MATBG might be conventional in nature, a consequence of the large density of states of its nearly flat bands, perhaps phonon-mediated as in BCS superconductors. | background 0.93 | 0.04 | 0.02 | hedged possibility 0.88 |
| s3 | Here we combine tunneling and Andreev reflection spectroscopy with the scanning tunneling microscope (STM) to observe several key experimental signatures for unconventional superconductivity in MATBG. | method 0.61 | 0.97 | 0.06 | asserted 0.99 |
| s4 | We show that the tunneling spectra below the transition temperature $T_c$ are inconsistent with that of a conventional s-wave superconductor, but rather resemble that of a nodal superconductor with an anisotropic pairing mechanism. | result 0.99 | 0.96 | 0.25 | asserted 1.00 |
| s5 | We observe a large discrepancy between the tunneling energy gap $\Delta_T$, which far exceeds the mean-field BCS ratio (with $2\Delta_T/k_BT_c \sim 25$) and the energy gap $\Delta_{AR}$ extracted from Andreev reflection spectroscopy ($2\Delta_{AR}/k_BT_c \sim 6$). | result 1.00 | 0.94 | 0.08 | asserted 1.00 |
| s6 | The tunneling gap persists even when superconductivity is suppressed, indicating its emergence from a pseudogap phase, with a suppressed density of states at the Fermi level. | result 0.65 | 0.90 | 0.06 | asserted 0.63 |
| s7 | Moreover, the pseudogap state and superconductivity are both absent when MATBG is aligned with the hexagonal boron nitride (hBN) underneath. | result 1.00 | 0.87 | 0.04 | asserted 1.00 |
| s8 | These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG. | interpretation 1.00 | 0.83 | 0.93 | asserted 0.89 |
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The paper
Original abstract
The emergence of superconductivity with doping from correlated insulators in magic-angle twisted bilayer graphene (MATBG) has raised the intriguing possibility that its pairing mechanism is distinct from that of conventional superconductors, as described by the Bardeen-Cooper-Schrieffer (BCS) theory. While there is now ample evidence for strong electronic correlations in MATBG, recent studies have claimed that unlike correlated insulators, superconductivity persists even when these interactions are partially screened. This suggests that the pairing in MATBG might be conventional in nature, a consequence of the large density of states of its nearly flat bands, perhaps phonon-mediated as in BCS superconductors. Here we combine tunneling and Andreev reflection spectroscopy with the scanning tunneling microscope (STM) to observe several key experimental signatures for unconventional superconductivity in MATBG. We show that the tunneling spectra below the transition temperature $T_c$ are inconsistent with that of a conventional s-wave superconductor, but rather resemble that of a nodal superconductor with an anisotropic pairing mechanism. We observe a large discrepancy between the tunneling energy gap $\Delta_T$, which far exceeds the mean-field BCS ratio (with $2\Delta_T/k_BT_c \sim 25$) and the energy gap $\Delta_{AR}$ extracted from Andreev reflection spectroscopy ($2\Delta_{AR}/k_BT_c \sim 6$). The tunneling gap persists even when superconductivity is suppressed, indicating its emergence from a pseudogap phase, with a suppressed density of states at the Fermi level. Moreover, the pseudogap state and superconductivity are both absent when MATBG is aligned with the hexagonal boron nitride (hBN) underneath. These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.
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How the literature uses it
11 arXiv papers cite this in 16 sentences
Papers citing it · 13 sentences without a confident label
Compares with it 1 paper
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Parallel spectroscopic measurements on twisted bilayer graphene revealed similar phenomenology [ref] —including nodal tunneling spectra, giant gap-to-$T_C$ ratios, and pseudogap physics with anomalous resilience to temperature and magnetic fields—suggesting a common origin of superconductivity in bilayers and trilayers.
Spectroscopic Signatures of Strong Correlations and Unconventional Superconductivity in Twisted Trilayer Graphene · Introduction · 0.97
Cites it as support 1 paper
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When combined with other recent experiments [ref] , these observations affirm the unconventional nature of superconductivity within the entire class of graphene moiré systems.
Ascendance of Superconductivity in Magic-Angle Graphene Multilayers · Introduction · 0.97
Cites it as background 1 paper
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More attention has been attracted after the discovery of unconventional superconductivity, orbital magnetism and correlated insulating phases [ref] in TBG systems at the magic angle, around 1.1$^∘$.
Truncated atomic plane wave method for the subband structure calculations of Moir\'e systems · Introduction · 0.80
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