Article
Multidisciplinary Sciences
Yonglong Xie, Andrew T. Pierce, Jeong Min Park, Daniel E. Parker, Eslam Khalaf, Patrick Ledwith, Yuan Cao, Seung Hwan Lee, Shaowen Chen, Patrick R. Forrester, Kenji Watanabe, Takashi Taniguchi, Ashvin Vishwanath, Pablo Jarillo-Herrero, Amir Yacoby
Summary: Fractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states and have been recently observed in magic-angle twisted BLG at low magnetic field. The appearance of these states at 5 T is accompanied by the disappearance of nearby topologically trivial charge density wave states.
Article
Multidisciplinary Sciences
Youngjoon Choi, Hyunjin Kim, Yang Peng, Alex Thomson, Cyprian Lewandowski, Robert Polski, Yiran Zhang, Harpreet Singh Arora, Kenji Watanabe, Takashi Taniguchi, Jason Alicea, Stevan Nadj-Perge
Summary: Magic-angle twisted bilayer graphene (MATBG) exhibits a variety of correlated phenomena, and new techniques introduced can determine the topological phases that emerge in MATBG in a finite magnetic field. These topological phases form only in a specific range of twist angles and are influenced by strong electronic interactions.
Article
Physics, Multidisciplinary
Dumitru Calugaru, Nicolas Regnault, Myungchul Oh, Kevin P. Nuckolls, Dillon Wong, Ryan L. Lee, Ali Yazdani, Oskar Vafek, B. Andrei Bernevig
Summary: This article analytically computes the scanning tunneling microscopy (STM) signatures of integer-filled correlated ground states of the magic angle twisted bilayer graphene (TBG) narrow bands and assesses the possibility of Kekule distortion (KD). The results show that coupling the two opposite graphene valleys does not always result in KD.
PHYSICAL REVIEW LETTERS
(2022)
Article
Materials Science, Multidisciplinary
Yajuan Cheng, Zheyong Fan, Tao Zhang, Masahiro Nomura, Sebastian Volz, Guimei Zhu, Baowen Li, Shiyun Xiong
Summary: We report a local minimum in thermal conductivity in twisted bilayer graphene (TBG) at the angle of 1.08 degrees, which corresponds to the 'magic angle' in the transition of several other reported properties. The thermal magic angle arises from the competition between the delocalization of atomic vibrational amplitudes and stresses on one hand, and the increased AA stacking density on the other hand. The manifestation of a magic angle, disclosing new thermal mechanisms at nanoscale, further uncovers the unique physics of two-dimensional materials.
MATERIALS TODAY PHYSICS
(2023)
Article
Multidisciplinary Sciences
Xiaoxue Liu, Zhi Wang, K. Watanabe, T. Taniguchi, Oskar Vafek, J. I. A. Li
Summary: This study introduces a new device geometry to continuously tune the strength of electron-electron Coulomb interaction within twisted bilayer graphene, revealing opposite effects of charge screening on insulating and superconducting states.
Article
Chemistry, Multidisciplinary
Giorgio Di Battista, Paul Seifert, Kenji Watanabe, Takashi Taniguchi, Kin Chung Fong, Alessandro Principi, Dmitri K. Efetov
Summary: In this study, the thermal properties of superconducting MATBG were revealed by monitoring its temperature dependent critical current. The temperature dependence of the electronic thermal conductance was extracted, laying the foundation for future thermal transport studies on this system.
Article
Chemistry, Multidisciplinary
Yijin Zhang, Keisuke Kamiya, Takato Yamamoto, Masato Sakano, Xiaohan Yang, Satoru Masubuchi, Shota Okazaki, Keisuke Shinokita, Tongmin Chen, Kohei Aso, Yukiko Yamada-Takamura, Yoshifumi Oshima, Kenji Watanabe, Takashi Taniguchi, Kazunari Matsuda, Takao Sasagawa, Kyoko Ishizaka, Tomoki Machida
Summary: The fabrication of artificial structures using a twisted van der Waals assembly has been a key technique for recent advancements in the research of two-dimensional materials, allowing for the modulation of physical properties through the control of symmetry.
Article
Materials Science, Multidisciplinary
Tiago C. Barbosa, Andreij C. Gadelha, Douglas A. A. Ohlberg, Kenji Watanabe, Takashi Taniguchi, Gilberto Medeiros-Ribeiro, Ado Jorio, Leonardo C. Campos
Summary: In this study, we investigated the Raman spectra of twisted bilayer graphene samples with different twist angles. The results showed that electron-phonon interaction affects the linewidth of the G band, particularly near the magic angle. The lineshape of the 2D band is determined by the crystal lattice and strain soliton regions.
Article
Physics, Condensed Matter
Thi-Nga Do, Po-Hsin Shih, Godfrey Gumbs
Summary: The magic-angle twisted bilayer graphene (MATBLG) with special flat bands exhibits exotic physical properties, but exploiting the engineering of these properties by external fields is still in its early stages. We show that MATBLG under an external magnetic field presents a distinctive magnetoplasmon dispersion, which can be significantly modified by transferred momentum and charge doping. Special pronounced single magnetoplasmon and horizontal single-particle excitation modes near charge neutrality exist along a wide range of transferred momentum. We provide an insightful discussion of these unique features based on the electronic excitation of Landau levels quantized from the flat bands and Landau damping. Additionally, charge doping leads to peculiar multiple strong-weight magnetoplasmons. These characteristics make MATBLG a favorable candidate for plasmonic devices and technology applications.
JOURNAL OF PHYSICS-CONDENSED MATTER
(2023)
Article
Physics, Multidisciplinary
Tao Yu, Dante M. Kennes, Angel Rubio, Michael A. Sentef
Summary: Recent measurements of resistivity in magic-angle twisted bilayer graphene near the superconducting transition temperature exhibit twofold anisotropy when changing the direction of an in-plane magnetic field. This was interpreted as evidence for exotic nematic superconductivity. However, experimental findings suggest that in-plane magnetic field can hybridize chiral superconducting order parameters to induce nematicity in the transport response.
PHYSICAL REVIEW LETTERS
(2021)
Article
Materials Science, Multidisciplinary
Christophe De Beule, Fernando Dominguez, Patrik Recher
Summary: A two-channel scattering model is constructed for investigating valley Hall states in a triangular network of graphene based on symmetry arguments. It is found that different chiral zigzag modes exhibit distinct resonances under specific conditions, revealing rich physical phenomena.
Article
Chemistry, Multidisciplinary
Xiaoqian Liu, Ran Peng, Zhaoru Sun, Jianpeng Liu
Summary: In this study, the phonon properties of magic-angle twisted bilayer graphene (TBG) were investigated using many-body classical potential and interatomic forces generated by a deep neural network. Various vibrational patterns and their effects on electronic structures were discovered.
Article
Physics, Multidisciplinary
Yves H. Kwan, Glenn Wagner, Nick Bultinck, Steven H. Simon, S. A. Parameswaran
Summary: In this study, we investigate the excitations that emerge upon doping the correlated insulating states in magic-angle twisted bilayer graphene. We identify excitations associated with skyrmion textures in the spin or pseudospin degrees of freedom and analyze their behavior using comprehensive Hartree-Fock calculations. We demonstrate the electron-mediated pairing of skyrmions, construct hopping models to extract the effective masses of paired skyrmions, and discuss the implications of our findings for skyrmion superconductivity.
Article
Materials Science, Multidisciplinary
Fuwei Yang, Bai Song
Summary: Twisted bilayer graphene (TBLG) is a versatile platform for studying various transport phenomena. In this study, it is found that thermal flow is suppressed near the magic angle, with greater suppression at lower temperatures and larger gaps. When the chemical potential lies in the energy gap, over 100-fold heat-flow variation can be achieved within a small twist angle. By reducing the electron scattering rate, the radiation spectrum near the magic angle dramatically narrows, leading to over 10,000-fold suppression.
MATERIALS TODAY PHYSICS
(2022)
Article
Materials Science, Multidisciplinary
Ajesh Kumar, Ming Xie, A. H. MacDonald
Summary: Insulating states of magic-angle twisted bilayer graphene exhibit a series of collective modes corresponding to local particle-hole excitations, with one mode strongly coupling to THz photons. These low-energy collective modes are accurately described by a model with a local SU(8) degree of freedom in each moire unit cell.
Article
Physics, Multidisciplinary
Junyeong Ahn, Guang-Yu Guo, Naoto Nagaosa
Article
Multidisciplinary Sciences
Junyeong Ahn, Naoto Nagaosa
Summary: The paper investigates the conditions for intrinsic optical excitations in clean multi-band superconductors, showing that inversion symmetry breaking, symmetry protection of Bogoliubov Fermi surfaces, or finite spin-orbit coupling can lead to such excitations. This finding opens up new possibilities for studying clean-limit superconductors through optical measurements.
NATURE COMMUNICATIONS
(2021)
Article
Physics, Multidisciplinary
Junyeong Ahn, Guang-Yu Guo, Naoto Nagaosa, Ashvin Vishwanath
Summary: This study establishes a general theory of Riemannian geometry for resonant optical processes by identifying transition dipole moment matrix elements as tangent vectors, showing that optical responses can generally be thought of as manifestations of the Riemannian geometry of quantum states.
Article
Multidisciplinary Sciences
Benjamin Schrunk, Yevhen Kushnirenko, Brinda Kuthanazhi, Junyeong Ahn, Lin-Lin Wang, Evan O'Leary, Kyungchan Lee, Andrew Eaton, Alexander Fedorov, Rui Lou, Vladimir Voroshnin, Oliver J. Clark, Jamie Sanchez-Barriga, Sergey L. Bud'ko, Robert-Jan Slager, Paul C. Canfield, Adam Kaminski
Summary: The Fermi surface plays a crucial role in material properties. Fermi arcs can be signatures of unusual electronic states and can be obtained by breaking specific symmetries. In this study, experimental evidence is presented showing the emergence of hole- and electron-like Fermi arcs below the Neel temperature in an antiferromagnetic state. This magnetic splitting effect is unique as it creates bands with opposing curvature that change with temperature.
Correction
Multidisciplinary Sciences
Benjamin Schrunk, Yevhen Kushnirenko, Brinda Kuthanazhi, Junyeong Ahn, Lin-Lin Wang, Evan O'Leary, Kyungchan Lee, Andrew Eaton, Alexander Fedorov, Rui Lou, Vladimir Voroshnin, Oliver J. Clark, Jaime Sanchez-Barriga, Sergey L. Bud'ko, Robert-Jan Slager, Paul C. Canfield, Adam Kaminski
Article
Multidisciplinary Sciences
Junyeong Ahn, Su-Yang Xu, Ashvin Vishwanath
Summary: This article introduces a theory of axion electrodynamics at general frequencies and defines the proper optical axion magneto-electric coupling. By calculations, significant Kerr effect is found in thin-film antiferromagnets, even at seemingly long wavelengths that lack the resolution to observe the spatial modulation of magnetism. The study reveals that the magneto-electric response of materials like MnBi2Te4 is dominated by the axion contribution even at optical frequencies.
NATURE COMMUNICATIONS
(2022)
Article
Chemistry, Physical
Jian-Xiang Qiu, Christian Tzschaschel, Junyeong Ahn, Anyuan Gao, Houchen Li, Xin-Yue Zhang, Barun Ghosh, Chaowei Hu, Yu-Xuan Wang, Yu-Fei Liu, Damien Berube, Thao Dinh, Zhenhao Gong, Shang-Wei Lien, Sheng-Chin Ho, Bahadur Singh, Kenji Watanabe, Takashi Taniguchi, David C. Bell, Hai-Zhou Lu, Arun Bansil, Hsin Lin, Tay-Rong Chang, Brian B. Zhou, Qiong Ma, Ashvin Vishwanath, Ni Ni, Su-Yang Xu
Summary: Using circularly polarized light, researchers have observed helicity-dependent optical control of fully compensated antiferromagnetic order in a topological axion insulator called MnBi2Te4. This optical control and circular dichroism are based on the optical axion electrodynamics and can potentially be used to control PT-symmetric antiferromagnets and create dissipationless circuits using topological edge states.
Article
Physics, Multidisciplinary
Lin-Lin Wang, Junyeong Ahn, Robert-Jan Slager, Yevhen Kushnirenko, Benjamin G. Ueland, Aashish Sapkota, Benjamin Schrunk, Brinda Kuthanazhi, Robert J. McQueeney, Paul C. Canfield, Adam Kaminski
Summary: Complex magnetic formations, known as 'multi-q' structures, can induce unconventional surface states in materials like NdBi with band inversion. The researchers used density functional theory calculations and experimental data to show that these structures can lead to the emergence of unconventional surface state pairs with specific features in the bulk gap.
COMMUNICATIONS PHYSICS
(2023)
Article
Materials Science, Multidisciplinary
Junyeong Ahn
Summary: In this study, we systematically characterize the enhanced nonlinear electric responses in topological semimetals due to the topological singularity of the Bloch states close to the Fermi level. We identify two characteristics that improve the nonlinear transports compared to conventional Dirac and Weyl fermions. The type-II spectrum leads to a logarithmic enhancement of nonlinear response functions with zero scaling dimension, and the anisotropic linear-quadratic dispersion increases the power of small-mu divergence for the nonlinear response tensors along the linearly dispersing direction. Our work reveals new experimental signatures of unconventional nodal points in topological semimetals and provides a guiding principle for giant nonlinear electric responses.
Article
Materials Science, Multidisciplinary
Junyeong Ahn, Naoto Nagaosa
Summary: Optical spectral weight transfer associated with the onset of superconductivity at high-energy scales has been observed in multiple systems, with debates on its origin ongoing but a consensus that it is due to strong correlation effects beyond the BCS theory. Additionally, another route to a nonzero spectral weight transfer based on the quantum geometry of the conduction band in multiband systems has been shown.
Article
Materials Science, Multidisciplinary
Junyeong Ahn, Bohm-Jung Yang
Summary: Unconventional Majorana states can emerge on the surface of three-dimensional topological superconductors protected by rotational symmetry, classified into three different categories. The spectral properties of these states, which go beyond conventional spin-1/2 fermions, are unique to topological superconductors and have no counterparts in topological insulators. Unconventional Majorana surface states can also be obtained in the superconducting phase of doped Z(2) topological insulators or Dirac semimetals with rotational symmetry.
Article
Physics, Multidisciplinary
Junyeong Ahn, Bohm-Jung Yang
PHYSICAL REVIEW RESEARCH
(2020)
Article
Materials Science, Multidisciplinary
Eunwoo Lee, Rokyeon Kim, Junyeong Ahn, Bohm-Jung Yang
NPJ QUANTUM MATERIALS
(2020)
Article
Materials Science, Multidisciplinary
Yoonseok Hwang, Junyeong Ahn, Bohm-Jung Yang
Article
Materials Science, Multidisciplinary
Junyeong Ahn, Bohm-Jung Yang