2026-11-12 ワシントン大学（UW）
ワシントン大学の研究チームは、マジックアングル・ツイスト二層グラフェン（MATBG）に機械的なひずみを加えることで、電気抵抗を大幅に変化させる新しい制御手法を開発した。MATBGは、厚さ原子1個分のグラフェンシート2枚を約1.1度回転させて重ねた材料であり、極低温で超伝導や磁性などの特異な量子状態を示す。研究チームは、電圧によって伸縮する装置に微小な試料を取り付け、原子格子に加えるひずみを連続的に制御することに成功した。その結果、ひずみに対する電気抵抗の変化率は通常の金属の100倍以上に達し、極低温ではさらに増幅されることを確認した。この現象は、電子状態が格子構造のわずかな変形に極めて敏感であることを示している。また、同様の巨大なひずみ応答は高温超伝導体など他の特殊な材料にも関連する可能性がある。本成果は、二次元量子材料の電子状態や超伝導発現機構の解明に新たな実験手段を提供し、将来の量子デバイスや新機能材料の開発につながると期待される。
Unstrained sheets of carbon atoms in the lefthand image produce a characteristic honeycomb moiré pattern when rotated relative to one another. When one of the sheets is strained relative to the other, as in the righthand image, new patterns and behaviors emerge. Photo: Matt Yankowitz
＜関連情報＞
- https://www.washington.edu/news/2026/11/12/magic-angle-graphene-strain/
- https://www.science.org/doi/10.1126/science.adz0597
- https://doi.org/10.1038/nature26160
魔法角ねじれ二層グラフェンにおける巨大なひずみ抵抗効果（Giant elastoresistance in magic-angle twisted bilayer graphene）
Xuetao Ma, Zhaoyu Liu, Jiaqi Cai, Kenji Watanabe, Takashi Taniguchi, Xiaodong Xu, Jiun-Haw Chu, Matthew Yankowitz
Science, 394(6825), 768–772:2026-11-12
DOI:10.1126/science.adz0597
Tii-summary
ねじれ二次元材料における強相関電子状態やトポロジカル相は、原子格子およびモアレ超格子の幾何学的構造に極めて敏感であり、ねじれ角、圧力、ひずみが重要な制御パラメータとなる。しかし、原子層デバイスに機械的ひずみを精密に加え、その電子物性への影響を系統的に調べることは困難だった。本研究では、圧電素子を用いてねじれ二層グラフェンに一軸方向のひずみを連続的に印加し、電気輸送特性を測定した。その結果、約1.1度の魔法角に近い試料では、ベルナル積層二層グラフェンと比較して顕著に大きなひずみ抵抗効果が観測され、ゲージファクターは通常の金属の100倍を超える値に達した。この応答は電子バンドの充填率と温度に依存し、特定の電子濃度領域では、温度低下に伴ってキュリー・ワイス型の発散的な振る舞いを示した。研究チームは、その微視的な起源として、電子ネマティック揺らぎや、アイソスピンモーメントの揺らぎに由来する電子エントロピーの増大などを議論している。本成果は、一軸ひずみがモアレ材料の強相関電子物理を調べる有効な実験手法であり、ねじれ角や圧力に加わる新たな力学的制御パラメータであることを示した。
English
Strongly correlated and topological phases in moiré materials are highly sensitive to lattice geometry at both atomic and superlattice length scales. Although twist angle, pressure, and strain provide effective tuning parameters, systematically investigating strain responses in atomically thin devices has been challenging. In this study, the researchers used a piezoelectric technique to continuously apply uniaxial strain to twisted bilayer graphene and measured its electrical transport properties. Near the magic angle of approximately 1.1°, the devices exhibited pronounced elastoresistance, substantially stronger than that observed in Bernal-stacked bilayer graphene. The gauge factor exceeded that of conventional metals by more than two orders of magnitude. This response depended on band filling and temperature, exhibiting a Curie-Weiss-like divergence in selected doping regimes as temperature decreased. The researchers discuss possible microscopic origins, including electronic nematic fluctuations and enhanced electronic entropy associated with fluctuating isospin moments. These findings establish uniaxial strain as a versatile experimental probe of correlated physics in moiré materials and demonstrate that mechanical strain can serve as an additional effective control parameter alongside twist angle and pressure.
English
Strongly correlated and topological phases in moiré materials are highly sensitive to lattice geometry at both atomic and superlattice length scales. Although twist angle, pressure, and strain provide effective tuning parameters, systematically investigating strain responses in atomically thin devices has been challenging. In this study, the researchers used a piezoelectric technique to continuously apply uniaxial strain to twisted bilayer graphene and measured its electrical transport properties. Near the magic angle of approximately 1.1°, the devices exhibited pronounced elastoresistance, substantially stronger than that observed in Bernal-stacked bilayer graphene. The gauge factor exceeded that of conventional metals by more than two orders of magnitude. This response depended on band filling and temperature, exhibiting a Curie-Weiss-like divergence in selected doping regimes as temperature decreased. The researchers discuss possible microscopic origins, including electronic nematic fluctuations and enhanced electronic entropy associated with fluctuating isospin moments. These findings establish uniaxial strain as a versatile experimental probe of correlated physics in moiré materials and demonstrate that mechanical strain can serve as an additional effective control parameter alongside twist angle and pressure.
魔法角グラフェン超格子における非従来型超伝導（Unconventional superconductivity in magic-angle graphene superlattices）
Yuan Cao, Valla Fatemi, Shiang Fang, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Pablo Jarillo-Herrero
Nature, 556(7699), 43–50:2018-03-05
DOI：10.1038/nature26160
Tii-summary
高温超伝導体などに見られる非従来型超伝導は、電子間の強い相互作用によって生じると考えられているが、その発現機構には未解明の部分が多い。本研究では、2枚のグラフェンシートを約1.1度の特定の角度（魔法角）で重ねたねじれ二層グラフェンを作製し、その電子輸送特性を調べた。この構造では、モアレ超格子の形成によってフェルミエネルギー近傍に平坦な電子バンドが生じ、電子間相互作用が強まることで、バンドの半充填状態に相関絶縁状態が形成される。研究チームは、ゲート電圧によってキャリア濃度を変化させ、この絶縁状態から離れた領域で、最大1.7 Kの臨界温度を持つゼロ抵抗状態を観測し、非従来型超伝導の発現を実証した。また、温度とキャリア濃度の相図には、銅酸化物高温超伝導体に類似したドーム状の超伝導領域が現れた。さらに、電気抵抗の量子振動から、相関絶縁状態の近傍に小さなフェルミ面が存在することを確認した。本成果は、炭素のみから構成される二次元材料で超伝導を実現し、ねじれ角や電界によって電子状態を精密に制御できる新しい量子物質研究の基盤を確立したものであり、高温超伝導や量子スピン液体の発現機構解明につながる重要な成果である。
English
Unconventional superconductivity, observed in strongly correlated materials such as high-temperature superconductors, remains poorly understood. In this study, the researchers investigated the electronic transport properties of twisted bilayer graphene, created by stacking two graphene sheets with a relative twist angle of approximately 1.1°, known as the magic angle. The resulting moiré superlattice produces flat electronic bands near the Fermi energy, enhancing electron-electron interactions and generating correlated insulating states at half-filling. By electrostatically tuning the carrier density away from these insulating states, the researchers observed tunable zero-resistance states with a superconducting critical temperature of up to 1.7 K, demonstrating unconventional superconductivity. The temperature-carrier-density phase diagram exhibited dome-shaped superconducting regions resembling those found in copper-oxide high-temperature superconductors. Furthermore, quantum oscillations in longitudinal resistance revealed small Fermi surfaces near the correlated insulating states. This study established a purely carbon-based, two-dimensional superconducting system whose electronic properties can be precisely controlled through twist angle and electrostatic gating. The findings provide a versatile platform for investigating strongly correlated quantum phenomena and may offer important insights into the mechanisms underlying high-temperature superconductivity and quantum spin liquids.
English
Unconventional superconductivity, observed in strongly correlated materials such as high-temperature superconductors, remains poorly understood. In this study, the researchers investigated the electronic transport properties of twisted bilayer graphene, created by stacking two graphene sheets with a relative twist angle of approximately 1.1°, known as the magic angle. The resulting moiré superlattice produces flat electronic bands near the Fermi energy, enhancing electron-electron interactions and generating correlated insulating states at half-filling. By electrostatically tuning the carrier density away from these insulating states, the researchers observed tunable zero-resistance states with a superconducting critical temperature of up to 1.7 K, demonstrating unconventional superconductivity. The temperature-carrier-density phase diagram exhibited dome-shaped superconducting regions resembling those found in copper-oxide high-temperature superconductors. Furthermore, quantum oscillations in longitudinal resistance revealed small Fermi surfaces near the correlated insulating states. This study established a purely carbon-based, two-dimensional superconducting system whose electronic properties can be precisely controlled through twist angle and electrostatic gating. The findings provide a versatile platform for investigating strongly correlated quantum phenomena and may offer important insights into the mechanisms underlying high-temperature superconductivity and quantum spin liquids.
