Article
Chemistry, Physical
Cheng Shen, Patrick J. Ledwith, Kenji Watanabe, Takashi Taniguchi, Eslam Khalaf, Ashvin Vishwanath, Dmitri K. Efetov
Summary: Magic-angle twisted trilayer graphene (MATTG) has flat electronic bands and exhibits correlated quantum phases. A spectroscopy technique is demonstrated in this work to dissociate intertwined bands and quantify the energy gaps and Chern numbers C of the correlated states in MATTG. Hard correlated gaps with C = 0 at integer moiré unit cell fillings and charge density wave states originating from van Hove singularities at fractional fillings are uncovered. Displacement-field-driven first-order phase transitions at charge neutrality and v = 2 are also demonstrated. Overall, these properties establish a diverse electrically tunable phase diagram of MATTG.
Article
Physics, Multidisciplinary
Aline Ramires, Jose L. Lado
Summary: Twisted trilayer graphene can emulate heavy fermion physics by hosting extended and localized modes that can be controlled by interlayer bias. In the presence of interactions, local moments are developed and Kondo coupled to coexisting extended states. By electrically controlling the exchange between local moments, the system can transition from a magnetic to heavy fermion regime, allowing exploration of a generalized Doniach phase diagram.
PHYSICAL REVIEW LETTERS
(2021)
Review
Physics, Applied
Pierre A. Pantaleon, Alejandro Jimeno-Pozo, Hector Sainz-Cruz, Phong Vo Tien, Tommaso Cea, Francisco Guinea
Summary: This Perspective article summarizes the study of superconductivity and correlated phases in non-twisted graphene bilayers and trilayers. The authors provide an overview of the recent experimental findings and analyze the theoretical models that aim to explain them. Superconductivity and cascades of correlated phases have been discovered in Bernal bilayer and rhombohedral trilayer graphene.
NATURE REVIEWS PHYSICS
(2023)
Editorial Material
Chemistry, Physical
Matthew Yankowitz
Summary: A new spectroscopic technique exploits overlapping electronic bands to investigate the highly correlated states of magic-angle twisted trilayer graphene.
Article
Multidisciplinary Sciences
Simon Turkel, Joshua Swann, Ziyan Zhu, Maine Christos, K. Watanabe, T. Taniguchi, Subir Sachdev, Mathias S. Scheurer, Efthimios Kaxiras, Cory R. Dean, Abhay N. Pasupathy
Summary: The magic-angle twisted trilayer graphene has shown a potential for engineering strongly correlated flat bands. Using low-temperature scanning tunneling microscopy, researchers have observed a strong reconstruction of the moire lattice in real trilayer samples, leading to the formation of localized twist-angle faults. These localized regions exhibit different electronic structures compared to the background regions, resulting in a doping-dependent, spatially granular electronic landscape.
Article
Materials Science, Multidisciplinary
Yuncheng Mao, Daniele Guerci, Christophe Mora
Summary: Stacking three monolayers of graphene with a twist creates two moire patterns, resulting in a moire-of-moire structure at larger distances. A low-energy theory is developed to describe the spectrum beyond the moire length scale, which includes one Dirac cone at the FM point and two weakly gapped points at KM and K'M in each valley of the underlying graphene. The model exhibits spatially varying velocities and small gaps in the moire-of-moire unit cell, and the resulting spectrum is protected by time-reversal and twofold-rotation symmetries.
Article
Physics, Multidisciplinary
Wei Qin, Allan H. MacDonald
Summary: Recent research has shown that superconductivity in magic-angle twisted trilayer graphene can survive in in-plane magnetic fields well beyond the Pauli limit, unlike magic-angle twisted bilayer graphene. The difference is attributed to the symmetry and relative displacements present in trilayers, which are not under experimental control at present. An gate electric field can break the symmetry and limit the in-plane critical magnetic field.
PHYSICAL REVIEW LETTERS
(2021)
Article
Multidisciplinary Sciences
Le Liu, Shihao Zhang, Yanbang Chu, Cheng Shen, Yuan Huang, Yalong Yuan, Jinpeng Tian, Jian Tang, Yiru Ji, Rong Yang, Kenji Watanabe, Takashi Taniguchi, Dongxia Shi, Jianpeng Liu, Wei Yang, Guangyu Zhang
Summary: This study observes the emergence of new correlated insulators from spin-polarized states to valley-polarized states in twisted double bilayer graphene. The results demonstrate a potential method to achieve isospin control and obtain new phases of matter in twisted multilayer systems.
NATURE COMMUNICATIONS
(2022)
Article
Multidisciplinary Sciences
Peter Rickhaus, Folkert K. de Vries, Jihang Zhu, Elias Portoles, Giulia Zheng, Michele Masseroni, Annika Kurzmann, Takashi Taniguchi, Kenji Watanabe, Allan H. MacDonald, Thomas Ihn, Klaus Ensslin
Summary: This study discovered a correlated electron-hole state in double-bilayer graphene twisted to 2.37 degrees, where moire states retain much of their isolated bilayer character. This allows the generation of an energetic overlap between narrow isolated electron and hole bands with good nesting properties, leading to the formation of ordered states with reconstructed Fermi surfaces consistent with a density-wave state that can be tuned without introducing chemical dopants.
Article
Multidisciplinary Sciences
Hyunjin Kim, Youngjoon Choi, Cyprian Lewandowski, Alex Thomson, Yiran Zhang, Robert Polski, Kenji Watanabe, Takashi Taniguchi, Jason Alicea, Stevan Nadj-Perge
Summary: Magic-angle twisted trilayer graphene (MATTG) is a moire material that exhibits strong electronic correlations and unconventional superconductivity. In this study, high-resolution scanning tunnelling microscopy and spectroscopy are used to investigate MATTG. Extensive regions of atomic reconstruction with mirror-symmetric stacking are observed, showing symmetry-breaking electronic transitions and doping-dependent band-structure deformations. Superconductivity is observed as pronounced dips in the tunnelling conductance at the Fermi level, accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed conductance evolution with doping suggests a transition from a gapped superconductor to a nodal superconductor, and the presence of peak-dip-hump structures indicates strong coupling to bosonic modes of MATTG.
Article
Multidisciplinary Sciences
Dorri Halbertal, Simon Turkel, Christopher J. Ciccarino, Jonas B. Hauck, Nathan Finney, Valerie Hsieh, Kenji Watanabe, Takashi Taniguchi, James Hone, Cory Dean, Prineha Narang, Abhay N. Pasupathy, Dante M. Kennes, D. N. Basov
Summary: The electronic and structural properties of atomically thin materials can be tuned by assembling them with an interlayer twist. In this study, the authors reveal surprising optical and electronic contrast in twisted double trilayer graphene and analyze the energetics of the unconventional lattice relaxation process. They demonstrate that this relaxation is non-local, leading to an energy gain in one domain of the moire lattice that is paid for by a relaxation in the other.
NATURE COMMUNICATIONS
(2022)
Article
Materials Science, Multidisciplinary
Chao Lei, Lukas Linhart, Wei Qin, Florian Libisch, Allan H. MacDonald
Summary: Researchers constructed a continuum model to study twisted trilayer graphene and found that mirror symmetry plays a key role in electronic properties. They calculated characteristics such as the Drude weight and weak-field Hall conductivity, and evaluated the influence of accidental lateral stacking shifts on electronic properties in experimental devices. They also discussed the possible importance of these shifts for superconductivity in twisted trilayers.
Article
Physics, Multidisciplinary
Maine Christos, Subir Sachdev, Mathias S. Scheurer
Summary: Motivated by recent experiments in twisted-trilayer graphene, this study investigates the effects of interactions and superconductivity in this system close to the magic angle. The researchers identify ground states at different filling fractions and construct a phase diagram. They also study the superconducting properties of these states and find triplet pairing dominance in the experimentally relevant regime. The results have implications for future experiments and theoretical work in this field.
Article
Materials Science, Multidisciplinary
Fang Xie, Nicolas Regnault, Dumitru Calugaru, B. Andrei Bernevig, Biao Lian
Summary: The study uses Hartree-Fock mean field approach to investigate the Hamiltonian of magic-angle twisted symmetric trilayer graphene, revealing different phases at different fillings and offering insights into the electron interactions in the system.
Article
Chemistry, Multidisciplinary
Xian Wang, Yingqi Cui, Li Zhang, Mingli Yang
Summary: The study demonstrates the tunable interlayer conductance of graphene by vertical electric field, controlled by interlayer stacking, distance, disc size, and field strength. Analysis of polarizability decomposition scheme reveals the significant role of the middle layer in shielding and promoting interlayer electron flow, with effects varying with twist angle and disc size.
Article
Chemistry, Multidisciplinary
Yi Wen, Shiang Fang, Matthew Coupin, Yang Lu, Colin Ophus, Efthimios Kaxiras, Jamie H. Warner
Summary: Four-dimensional (4D) scanning transmission electron microscopy was used to study the electric fields at the edges of 2D semiconducting monolayer MoS2. Sub-nanometer 1D features in the 2D electric field maps were observed at the outermost region along zigzag edges and also along nanowire MoS-terminated MoS2 edges. The presence of low Z number atoms terminating many of the uniform edges was revealed through electric field reconstructions, along with integrated differential phase contrast reconstructions. Density functional theory calculations supported the formation of periodic 1D edge states and indicated the enhancement of electric field magnitude for some edge terminations. The experimentally observed electric fields at the edges were attributed to the absence of an opposing electric field from a nearest neighbor atom when the electron beam propagates through the 2D monolayer and interacts. These results highlight the potential of 4D-STEM in mapping the atomic scale structure and fluctuations of electric fields around edge atoms in 2D materials.
Article
Multidisciplinary Sciences
Simon Turkel, Joshua Swann, Ziyan Zhu, Maine Christos, K. Watanabe, T. Taniguchi, Subir Sachdev, Mathias S. Scheurer, Efthimios Kaxiras, Cory R. Dean, Abhay N. Pasupathy
Summary: The magic-angle twisted trilayer graphene has shown a potential for engineering strongly correlated flat bands. Using low-temperature scanning tunneling microscopy, researchers have observed a strong reconstruction of the moire lattice in real trilayer samples, leading to the formation of localized twist-angle faults. These localized regions exhibit different electronic structures compared to the background regions, resulting in a doping-dependent, spatially granular electronic landscape.
Article
Chemistry, Multidisciplinary
Yan Zhao, Kaiyue Jiang, Can Li, Yufeng Liu, Gucheng Zhu, Michele Pizzochero, Efthimios Kaxiras, Dandan Guan, Yaoyi Li, Hao Zheng, Canhua Liu, Jinfeng Jia, Mingpu Qin, Xiaodong Zhuang, Shiyong Wang
Summary: Individual quantum nanomagnets based on metal-free multi-porphyrin systems have been synthesized. The magnetic coupling between porphyrins was tuned by converting specific porphyrin units to their radical or biradical state. The resulting chains exhibit different magnetic properties, with gap excitation in S = 1/2 antiferromagnets and distinct end states in S = 1 antiferromagnets.
Article
Chemistry, Physical
Mingu Kang, Shiang Fang, Jonggyu Yoo, Brenden R. Ortiz, Yuzki M. Oey, Jonghyeok Choi, Sae Hee Ryu, Jimin Kim, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Efthimios Kaxiras, Joseph G. Checkelsky, Stephen D. Wilson, Jae-Hoon Park, Riccardo Comin
Summary: The authors use high-resolution angle-resolved photoemission spectroscopy to determine the microscopic structure of three-dimensional charge order in AV(3)Sb(5) (A = K, Rb, Cs) and its interplay with superconductivity. The observed difference in charge order structure between CsV3Sb5 and the other compounds potentially explains the double-dome superconductivity in CsV3(Sb,Sn)(5) and the suppression of T-c in KV3Sb5 and RbV3Sb5. These findings provide fresh insights into the phase diagram of AV(3)Sb(5).
Article
Virology
Georgios Neofotistos, Mattia Angeli, Marios Mattheakis, Efthimios Kaxiras
Summary: Using the SAIVR model, the study investigates the factors influencing COVID-19 vaccine rollout effectiveness and susceptibility to resurgent epidemics. The findings highlight the impact of different vaccination coverage levels and waning immunity on distinct patterns of resurgent COVID-19 epidemics.
Article
Chemistry, Multidisciplinary
Yeonchoo Cho, Gabriel R. Schleder, Daniel T. Larson, Elise Brutschea, Kyung-Eun Byun, Hongkun Park, Philip Kim, Efthimios Kaxiras
Summary: The researchers propose a solution to the metal-semiconductor contact resistance problem, called modulation doping, by placing a doping layer on the opposite side of the metal-semiconductor interface. By using first-principles calculations, they demonstrate that modulation doping can reduce the Schottky barrier height and contact resistance at the metal-semiconductor interface. The feasibility of this approach is demonstrated for single-layer tungsten diselenide and 2D MXene materials, and it can be generalized for other 2D semiconductors.
Article
Physics, Multidisciplinary
Nikita V. Tepliakov, Johannes Lischner, Efthimios Kaxiras, Arash A. Mostofi, Michele Pizzochero
Summary: In this study, a new perspective on the electronic structure of armchair graphene nanoribbons is presented using simple model Hamiltonians and ab initio calculations. The research demonstrates that the energy-gap opening in these nanoribbons is caused by the breaking of a hidden symmetry through long-ranged hopping of pi electrons and structural distortions at the edges. This hidden symmetry can be restored or manipulated through in-plane lattice strain, enabling continuous energy-gap tuning, the emergence of Dirac points at the Fermi level, and topological quantum phase transitions. This work establishes an original interpretation of the semiconducting properties of armchair graphene nanoribbons and provides guidelines for their rational electronic structure design.
PHYSICAL REVIEW LETTERS
(2023)
Article
Multidisciplinary Sciences
Xi Zhang, Wei Ren, Elliot Bell, Ziyan Zhu, Kan-Ting Tsai, Yujie Luo, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Mitchell Luskin, Ke Wang
Summary: Charge carriers in monolayer graphene can be manipulated using a graphene microcavity, which enables interference of electrons and enhances collimation efficiency.
NATURE COMMUNICATIONS
(2022)
Article
Chemistry, Multidisciplinary
Valerie Hsieh, Dorri Halbertal, Nathan R. . Finney, Ziyan Zhu, Eli Gerber, Michele Pizzochero, Emine Kucukbenli, Gabriel R. Schleder, Mattia Angeli, Kenji Watanabe, Takashi Taniguchi, Eun-Ah Kim, Efthimios Kaxiras, James Hone, Cory R. Dean, D. N. Basov
Summary: Twisted van der Waals multilayers are regarded as a rich platform for accessing novel electronic phases. This study proposes that naturally formed stacking domains due to relative twist between layers can act as an additional control knob. The researchers observe selective adhesion of metallic nanoparticles and liquid water at domains with specific stacking configurations and demonstrate the manipulation of nanoparticles can locally reconfigure the moire superlattice.
Article
Chemistry, Multidisciplinary
Nikita V. Tepliakov, Ruize Ma, Johannes Lischner, Efthimios Kaxiras, Arash A. Mostofi, Michele Pizzochero
Summary: In this study, it is predicted that the recently fabricated heterojunctions of zigzag nanoribbons embedded in two-dimensional hexagonal boron nitride exhibit half-semimetallic behavior, with opposite energy shifts of the states residing at the two edges while maintaining their intrinsic antiferromagnetic exchange coupling. These heterojunctions undergo an antiferromagnetic-to-ferrimagnetic phase transition upon doping, where the sign of the excess charge controls the spatial localization of the net magnetic moments. This research holds promise for the development of carbon-based spintronics.
Article
Physics, Multidisciplinary
Ziyan Zhu, Marios Mattheakis, Weiwei Pan, Efthimios Kaxiras
Summary: In this study, we introduce a deep neural network model called HubbardNet for variational determination of the ground-state and excited-state wave functions of the one-dimensional and two-dimensional Bose-Hubbard model. The model demonstrates excellent generalization ability and outperforms traditional methods in terms of computational efficiency and accuracy.
PHYSICAL REVIEW RESEARCH
(2023)
Article
Materials Science, Multidisciplinary
Ziyan Zhu, Stephen Carr, Qiong Ma, Efthimios Kaxiras
Summary: Unconventional ferroelectricity has been observed in AB bilayer graphene sandwiched by hexagonal boron nitride (hBN), providing a new platform for manipulating correlated phases in multilayered van der Waals heterostructures. A low-energy continuum model is proposed for AB bilayer graphene encapsulated by hBN or graphene layers, with independent twist angles. The study demonstrates that twist angle asymmetry leads to layer polarization in graphene/hBN heterostructures, and an out-of-plane displacement field can not only tune the layer polarization but also flatten the low-energy bands. The model is extended to quadrilayer graphene heterostructures, showing that the electronic structures can be similarly controlled by an external electric field.
Article
Materials Science, Multidisciplinary
Jonathan Z. Lu, Ziyan Zhu, Mattia Angeli, Daniel T. Larson, Efthimios Kaxiras
Summary: In this paper, we develop a low-energy continuum model to study phonons in twisted moire bilayers. Using a configuration space approach and density functional theory, we obtain interatomic force constants and compute phonon properties for different twist angles. Our results show that the frequencies of low-energy phonon modes are reordered as the twist angle changes, and the atomic displacement fields develop periodicity.
