Article
Physics, Multidisciplinary
Nitin Prasad, G. William Burg, Kenji Watanabe, Takashi Taniguchi, Leonard F. Register, Emanuel Tutuc
Summary: The article introduces a tunneling spectroscopy technique in a double bilayer graphene heterostructure, utilizing momentum-conserving tunneling as an energy filter for tunneling carriers and allowing measurement of the quasiparticle state broadening at well-defined energies. The experimental results demonstrate that the broadening increases linearly with excited state energy relative to the Fermi level and has weak dependence on temperature, while in-plane magnetotunneling experiments reveal a high degree of rotational alignment between the graphene bilayers and an absence of momentum randomizing processes.
PHYSICAL REVIEW LETTERS
(2021)
Article
Physics, Multidisciplinary
Jonah Herzog-Arbeitman, Aaron Chew, Dmitri K. Efetov, Andrei Bernevig
Summary: This Letter focuses on the study of magic angle twisted bilayer graphene (TBG) at 2 pi flux. By using a newly developed gauge-invariant formalism, the exact single-particle band structure and topology are determined. It is found that the characteristic TBG flat bands reemerge at 2 pi flux, but they split and acquire Chern number +1 due to the magnetic field breaking C2zT. In addition, reentrant correlated insulating states driven by the Coulomb interaction at integer fillings are discovered, and characteristic Landau fans from their excitation spectrum are predicted.
PHYSICAL REVIEW LETTERS
(2022)
Article
Materials Science, Multidisciplinary
M. S. Mrudul, Gopal Dixit
Summary: The study shows significant differences in high-harmonic generation between monolayer and bilayer graphene, with the interlayer coupling and contributions from intraband and interband transitions playing important roles. Additionally, interesting polarization and ellipticity dependencies are observed in the total harmonic spectrum of monolayer and bilayer graphene.
Article
Materials Science, Multidisciplinary
Hasan M. Abdullah, Abderrahim El Mouhafid, Gokaran Shukla, Udo Schwingenschlogl
Summary: One of the key features of graphene is the chirality of the electrons. Introduction of an external perturbation can change the chirality and significantly modify the quantum transport. This study explores the tunneling in AB-stacked bilayer graphene in the presence of an in-plane pseudomagnetic field and an out-of-plane electric field.
Article
Materials Science, Multidisciplinary
Ma Zhou
Summary: The plasmon dispersion of biased twisted bilayer graphene shows local classical behavior at long wavelengths, unlike conventional 2DEG. In BTBG, the plasmon energy is independent of carrier concentration and Fermi energy as opposed to the density-dependent plasmon energy in 2DEG.
Article
Materials Science, Multidisciplinary
Shu-Chang Zhao, Lu Gao, Qiang Cheng, Qing-Feng Sun
Summary: This study investigates the crossed Andreev reflection and nonlocal transport in proximitized graphene/superconductor/proximitized graphene junctions with pseudospin staggered potential and intrinsic spin-orbit coupling. When the intrinsic spin-orbit couplings in the left and right graphene have opposite signs, the crossed Andreev reflection, coexisting with the local Andreev reflection and elastic cotunneling, can be completely eliminated for electrons with specific spin-valley indices. A perfect crossed Andreev reflection with a probability of 100% is achieved in the space of incident angle and electron energy. The crossed conductance and its dependence on superconductor length are also investigated. The energy ranges for the crossed Andreev reflection, without the local Andreev reflection and elastic cotunneling, are clarified for different magnitudes of the pseudospin potential and spin-orbit coupling. The spin-valley index of electrons responsible for the perfect crossed Andreev reflection can be controlled by changing the sign of the intrinsic spin-orbit coupling or applying different biases to the left and right graphene. These results are useful for designing flexible and highly efficient Cooper pair splitters based on spin-valley degree of freedom.
Article
Chemistry, Physical
Sun-Woo Kim, Sunam Jeon, Moon Jip Park, Youngkuk Kim
Summary: The energy spectrum of twisted bilayer graphene (TBG) exhibits recursive higher-order topological properties known as higher-order topological insulator (HOTI) phases. These HOTI phases, characterized by localized corner states, are replicas of the original HOTIs and maintain the self-similarity of the Hofstadter spectrum. It is shown that TBG exhibits exact flux translational symmetry at all commensurate angles, and the original HOTI phase re-emerges at a half-flux periodicity, preserving the effective twofold rotation. Multiple replicas of the original HOTIs are also found for fluxes without protecting symmetries, featuring localized corner states and edge-localized real-space topological markers. These replica HOTIs result from different interaction scales in TBG, including intralayer and interlayer couplings. The topological aspect of the Hofstadter butterflies observed in this study highlights the symmetry-protected topology in quantum fractals.
NPJ COMPUTATIONAL MATERIALS
(2023)
Article
Materials Science, Multidisciplinary
Jule-Katharina Schrepfer, Szu-Chao Chen, Ming-Hao Liu, Klaus Richter, Martina Hentschel
Summary: The study explores electron optics phenomena in graphene by controlling Dirac fermionic resonant states and emission characteristics through adjusting microcavity shapes. By introducing a back-gate voltage in graphene devices, different effective refractive indices and corresponding Fresnel laws can be simulated. Single-layer and double-layer graphene cavities exhibit distinct tunneling effects, leading to varied dwell times and emission characteristics, with single-layer graphene systems showing strong confinement of emitted charge carriers resembling a lensing effect.
Article
Physics, Multidisciplinary
Yu Saito, Jingyuan Ge, Louk Rademaker, Kenji Watanabe, Takashi Taniguchi, Dmitry A. Abanin, Andrea F. Young
Summary: A small twist between two layers of graphene can lead to flat band structures, which can form ferromagnetic Chern insulators when the moire superlattice interacts with a magnetic field. In twisted bilayer graphene, Coulomb interactions and magnetic field effects give rise to new quantum states.
Article
Materials Science, Multidisciplinary
Jonah Herzog-Arbeitman, Aaron Chew, B. Andrei Bernevig
Summary: Bloch's theorem is crucial in topological band theory, but is broken by a perpendicular magnetic field, posing challenges in studying topological systems in strong flux. Moire materials have made this problem relevant in experiments, and this paper focuses on finding a solution. The authors construct a mathematical framework and analyze the behaviors of systems under 2n flux using a simple square lattice model and the Bistritzer-MacDonald Hamiltonian.
Article
Physics, Multidisciplinary
Jiachen Yu, Benjamin A. Foutty, Zhaoyu Han, Mark E. Barber, Yoni Schattner, Kenji Watanabe, Takashi Taniguchi, Philip Phillips, Zhi-Xun Shen, Steven A. Kivelson, Benjamin E. Feldman
Summary: This study examines the phase diagram of isospin polarization in magic-angle twisted bilayer graphene and discovers multiple many-body quantum phases and topological states in narrow energy bands, providing insights into the transitions between these states and their interplay with interactions and symmetry breaking.
Editorial Material
Chemistry, Multidisciplinary
Kanudha Sharda
Summary: The study describes a scalable method to synthesize twisted bilayer graphene, resulting in growth over the largest lateral surface areas reported so far, which has attracted significant interest in the field.
Article
Materials Science, Multidisciplinary
Xiang Hu, Timo Hyart, Dmitry I. Pikulin, Enrico Rossi
Summary: Flat-band systems provide a promising platform for studying exotic collective ground states. In this study, using double-twisted bilayer graphene heterostructures as an example, the stability of the exciton condensate (EC) phase is investigated. It is found that for realistic interaction strengths, the EC phase is favored over other phases when the bilayers have opposite doping. The quantum metric of the Bloch wave functions plays a critical role in stabilizing the EC phase.
Article
Materials Science, Multidisciplinary
Ivan Amelio, N. D. Drummond, Eugene Demler, Richard Schmidt, Atac Imamoglu
Summary: Recent progress in two-dimensional materials and their moire heterostructures has led to new possibilities for the realization of ground-state excitonic insulators. In this study, we propose and analyze a scheme where an intralayer exciton is screened by excitations out of the excitonic insulator to form interlayer polarons. We make predictions about the excitonic insulator excitation spectrum and show how changing the interlayer exciton binding energy modifies the optical spectra.
Article
Materials Science, Multidisciplinary
Patrick Wilhelm, Thomas C. Lang, Andreas M. Laeuchli
Summary: An extensive study was conducted on interaction-driven insulators in spin- and valley-polarized moire flat bands of twisted bilayer graphene, revealing fractional Chern insulator phases and competing charge-density-wave phases. Analysis at different parameters highlighted the competition between these phases, with kinetic energy and Coulomb interaction strength playing crucial roles. The ground-state selection was intuitively understood to be driven by the interplay between single-particle and interaction-induced hole dispersion with the Berry curvature of the Chern bands. The resulting phase diagram showed remarkable agreement with experimental findings, extending the relevance of the results beyond graphene-based materials.
Article
Materials Science, Multidisciplinary
Tong Su, Ching Hua Lee, San-Dong Guo, Guangzhao Wang, Wee-Liat Ong, Liemao Cao, Weiwei Zhao, Shengyuan A. Yang, Yee Sin Ang
Summary: Janus niobium oxydihalide (NbOXY) is a multifunctional anisotropic semiconductor with exceptional properties in piezoelectricity, photocatalysis, optoelectronics, and sustainable energy applications. It exhibits stable and flexible monolayers with a band gap in the visible light regime. NbOXY has an exceptionally high anisotropic carrier mobility and large deformation potential, making it suitable for strain sensing applications. It also demonstrates strong piezoelectricity in both in-plane and out-of-plane directions, as well as strong optical absorption and linear optical dichroism in the visible-to-ultraviolet regime. Its compatibility with water splitting and high absorption peaks in the deep UV regime make it a promising material for solar-to-hydrogen conversion. Our findings highlight the exceptional potential of NbOXY for various functional device applications.
MATERIALS TODAY PHYSICS
(2023)
Article
Physics, Applied
Haiyu Meng, Jeremy Lim, Lingling Wang, Ching Hua Lee, Yee Sin Ang
Summary: In this paper, we propose a mechanism to continuously reconfigure a resonator using a phase-change material, enabling efficient spectra modulation. By incorporating a phase-change material VO2 substrate into a photonic crystal, the system behaves as a one-port device with near-perfect absorption and a two-port device with high transmission when VO2 is in different phases. The switching operation of the proposed device can be expanded into the mid-infrared regime, and it is compatible with wide-angle operation and robust against structural distortion.
APPLIED PHYSICS LETTERS
(2023)
Article
Physics, Applied
Wei Jie Chan, L. K. Ang, Yee Sin Ang
Summary: Two-dimensional semi-Dirac systems undergo topological phase transitions between insulating, semi-Dirac, and band inversion phases under external modulation. The behavior of quantum transport and shot noise signatures during these transitions is still unknown.
APPLIED PHYSICS LETTERS
(2023)
Article
Engineering, Electrical & Electronic
Chun Yun Kee, Yee Sin Ang, Er-Ping Li, L. K. Ang
Summary: This paper revises the Mark-Helfrich law for ultrathin trap-filled dielectrics, proposes a new scaling of current line density, and emphasizes the difference in current flow between strip contact and edge contact geometries. The developed model is important for the characterization of ultrathin dielectrics used in various electronic applications.
IEEE TRANSACTIONS ON ELECTRON DEVICES
(2023)
Article
Physics, Applied
Zeyu Zhang, Zelong Peng, Peng Jiang, Yee Sin Ang, Chao Zhang, Zhongshui Ma
Summary: This study explores the performance of 2D materials and their heterostructures in energy conversion applications. The Thomson effect, an important process in thermoelectric devices, is found to improve the performance of thermionic refrigeration. However, the overall coefficient of performance is reduced by the Thomson effect.
JOURNAL OF APPLIED PHYSICS
(2023)
Article
Physics, Applied
Tong Su, Yueyan Li, Qianqian Wang, Weiwei Zhao, Liemao Cao, Yee Sin Ang
Summary: Recent experiments have demonstrated that semimetal bismuth (Bi) is an excellent electrical contact to monolayer MoS2 with ultralow contact resistance. However, the contact physics of other semimetal/monolayer-semiconductor systems remains largely unexplored. In this study, we investigate the electrical contact properties between six two-dimensional transition metal dichalcogenides (TMDCs) and two representative semimetals, Bi and antimony (Sb), using density functional theory. We find that weak metalization occurs at the semimetal/TMDC interfaces, generating semimetal-induced gap states (SMIGSs) below the conduction band minimum and effectively reducing the n-type Schottky barrier height. We propose a modified Schottky-Mott rule that considers SMIGS, interface dipole potential, and Fermi level shifting, which improves the agreement with density functional theory simulations of the barrier height. We also show that Sb contacts have lower tunneling-specific resistivity than Bi contacts, indicating better charge injection efficiency. Our findings suggest that Bi and Sb are promising companion electrode materials for advancing 2D semiconductor device technology.
JOURNAL OF PHYSICS D-APPLIED PHYSICS
(2023)
Article
Chemistry, Multidisciplinary
Son-Tung Nguyen, Cuong Q. Nguyen, Yee Sin Ang, Nguyen Van Hoang, Nguyen Manh Hung, Chuong Nguyen
Summary: van der Waals heterostructures offer a powerful platform for manipulating the electronic properties of two-dimensional materials and exploring exotic physical phenomena. In this study, we created a graphene/BSe heterostructure and investigated its electronic characteristics and the tunability of its contact types under electric fields. Our findings demonstrate that the graphene/BSe heterostructure is stable and forms a p-type Schottky contact with high carrier mobility, making it a promising candidate for future Schottky field-effect transistors. Additionally, applying an electric field allows for the control and manipulation of the heterostructure's electronic properties, including the transition from a p-type to an n-type Schottky contact and from a Schottky to an ohmic contact. These results provide a rational basis for the design of energy-efficient and tunable heterostructure devices based on the graphene/BSe heterostructure.
Article
Physics, Fluids & Plasmas
L. K. Ang, Yee Sin Ang, Ching Hua Lee
Summary: We present the theory of out-of-plane (or vertical) electron thermal-field emission from two-dimensional (2D) semimetals. We show that the current-voltage-temperature characteristic is well captured by a universal scaling relation applicable for broad classes of 2D semimetals. Our findings reveal that band topologies in two spatial dimension are indistinguishable from each other and bear no special signature in electron emission characteristics. These findings provide theoretical foundations for the design of 2D-material-based vacuum nanoelectronics.
PHYSICS OF PLASMAS
(2023)
Article
Chemistry, Physical
Jin Yang, Ankit Jain, Liwu Fan, Yee Sin Ang, Hanying Li, Wee-Liat Ong
Summary: The pressure-enhanced electronic properties of metal halide perovskites are beneficial for photovoltaic and thermoelectric applications. However, the thermal conductivity behavior under pressure has not been well-studied. By using the phonon Wigner transport equation, we found that in the cubic phase of methylammonium lead triiodide, the dominant coherence-channel thermal conductivity remains unchanged with increasing pressure. This anomalous behavior is attributed to the straightening of octahedra and shortening of the lattice constant under pressure.
CHEMISTRY OF MATERIALS
(2023)
Article
Thermodynamics
Mingli Wang, Jiafen Ruan, Fang Tao, Jiayi Zhu, Yaoyuan Zhang, Yi Xiong, Xin Zhang, Yee Sin Ang
Summary: This study proposes a technically feasible approach to recover waste heat generated by PEMFCs through integrating liquid-state thermocells (LTCs) with PEMFCs for cogeneration of electric power. The results show that the hybrid system achieves higher power density and conversion efficiency compared to a single PEMFC. Optimizing some parameters can further improve the performance of the hybrid system.
ENERGY CONVERSION AND MANAGEMENT
(2023)
Article
Physics, Fluids & Plasmas
Wei Jie Chan, Cherq Chua, Yee Sin Ang, Lay Kee Ang
Summary: This article reviews recent experimental studies on field emission in two-dimensional materials and summarizes and compares their emission characteristics. The importance of establishing consistent physics-based models to understand field emission from these quantum materials is emphasized, and possible future research directions are highlighted.
IEEE TRANSACTIONS ON PLASMA SCIENCE
(2023)
Article
Physics, Condensed Matter
Xiao-Shu Guo, San-Dong Guo, Shuo-Ning Si, Kai Cheng, Ke Wang, Yee Sin Ang
Summary: Janus ScXY (X & NOTEQUAL; Y = Cl, Br and I) monolayers are predicted to be piezoelectric ferromagnetic semiconductors with dynamical, mechanical and thermal stabilities. They exhibit an in-plane easy axis of magnetization and have no spontaneous valley polarization. By doping with holes, the direction of magnetization can be tuned, leading to spontaneous valley polarization. These findings could be used to design piezoelectric and valleytronic devices.
JOURNAL OF PHYSICS-CONDENSED MATTER
(2023)
Article
Materials Science, Multidisciplinary
Muzamil Shah, Muhammad Qasim Mehmood, Yee Sin Ang, Muhammad Zubair, Yehia Massoud
Summary: We investigated the magneto-optical properties of an ultrathin Floquet topological insulator (FTI) in the terahertz frequency regime under an external perpendicular magnetic field. By treating circularly polarized off-resonant light as an external perturbation, we introduced a mass gap at the Dirac cone in the FTI, resulting in massive Dirac fermions in the surface state. Through tuning the optical field energy, we observed electronic phase transitions between the trivial insulator state and the band insulator state in the FTI thin-film system. Using Kubo formalism, we calculated the conductivities and found that they are influenced by the off-resonant optical field, magnetic field, and chemical potentials. Additionally, we demonstrated giant Kerr and Faraday rotations in the FTI thin film, which can be controlled via magnetic and off-resonant optical fields.
Article
Materials Science, Multidisciplinary
Jun Fang, Chao Zhang, Zhongshui Ma
Summary: We investigate the microscopic pairing mechanisms of superconductivity in doped Weyl semimetals. Depending on the nature of electron scattering, there are three possible superconducting pairings. We calculate the condensation energy required for electron pairing.
Article
Materials Science, Multidisciplinary
Liemao Cao, Xiaohui Deng, Zhen-kun Tang, Rui Tan, Yee Sin Ang
Summary: We investigate the interface properties between WSi2N4 and Mo2B, O-modified Mo2B, and OH-modified Mo2B nanosheets. We find that WSi2N4 and Mo2B form n-type Schottky contacts, while functionalizing Mo2B with O and OH leads to the formation of both n-type and p-type ohmic contacts with WSi2N4. Additionally, we demonstrate the emergence of quasi-ohmic contact with ultralow lateral Schottky barrier and zero vertical interfacial tunneling barriers in Mo2B(OH)2-contacted WSi2N4.
JOURNAL OF MATERIALS CHEMISTRY C
(2024)
