Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H12: Devices from 2D Materials -- TheoryFocus
|
Hide Abstracts |
Sponsoring Units: DMP Chair: Yang Gao, Carnegie Mellon University Room: BCEC 153A |
Tuesday, March 5, 2019 2:30PM - 3:06PM |
H12.00001: Theory of 2D Materials: Excitons, Valley-Spin Physics, and Magnetism. Invited Speaker: Ting Cao Recent advances in the experimental and theoretical studies of atomically thin two-dimensional (2D) materials have opened up opportunities in exploring new phenomena and properties as well as related applications absent in conventional bulk materials. In the first part of my talk, we will present theoretical studies on the optical responses of monolayer transition metal dichalcogenides. By ab initio GW-BSE calculations, we demonstrate a previously unrecognized valley-spin character of bright excitons, which leads to interesting ultrafast phenomena in monolayer transition metal dichalcogenides [1]. We then discuss the theory of dark excitons and trions, and how they could be brightened by applying an external magnetic field [2] or using other approaches. In the second part of my talk, we will discuss theoretical studies of magnetism in 2D materials. We discuss the physical origins and control of ferromagnetism in materials with different chemical species and structural configurations. We further connect our theoretical discoveries to experimental results and explore their potential applications. |
Tuesday, March 5, 2019 3:06PM - 3:18PM |
H12.00002: A theoretical proposal of gate-induced quantum anomalous hall effect in 2D ferromagnetic multilayers Yuan Gao, Di Xiao, Wenguang Zhu The quantum anomalous hall effect (QAHE) is the manifestation of topological electronic structure characterized by a finite Chern number and helical edge electron states and may have potential applications in future electronic devices with low energy consumption. However, the QAHE so far can only be achieved at very low temperatures. Thus, the search for new quantum anomalous hall systems with elevated temperature is in demand. Here we propose a new design based on recently discovered 2D ferromagnetic semiconductors. Using first-principles calculations, we demonstrate that by applying a moderate electric field a 2D ferromagnetic multilayer can be converted into the quantum anomalous hall state. The topological nature and band gap as a function as the applied electric field and film thickness are systematically investigated in details. |
Tuesday, March 5, 2019 3:18PM - 3:30PM |
H12.00003: Simulating the nanomechanical response of cyclooctatetraene molecules on a graphene device Sehoon Oh, Michael F Crommie, Marvin L Cohen We have investigated the atomic and electronic structures of cyclooctatetraene molecules on graphene and analyzed their dependence on external gate voltage using first-principles calculations. The external gate voltage is simulated by adding or removing electrons using density functional theory calculations. This allowed us to investigate how changes in carrier density modify the molecular shape, orientation, adsorption site, diffusion barrier, and diffusion path. The results of the calculation imply that the shape and mobility of the adsorbed molecules can be controlled by externally gating graphene devices. |
Tuesday, March 5, 2019 3:30PM - 3:42PM |
H12.00004: Universal scaling laws of electron emission phenomena in two-dimensional material electrical contacts Yee Sin Ang, Hui Ying Yang, Lay Kee Ang Electrically contacting two-dimensional (2D) material with another material often leads to the formation of an interfacial Schottky barrier. Charge injection across such barrier is commonly analysed using the classic Richardson-Dushman (thermionic emission) and Fowler-Nordheim (field emission) models despite the fact that the assumptions underlying such models are often inconsistent with the physical properties of most 2D materials. Here we formulate generalized models of electron injection across 2D material interfaces1-3. We show that the thermionic transport across a 2D material Schottky contact is governed by universal scaling laws broadly applicable for large classes of 2D systems. We further uncover a new universal scaling behavior in the vertical electron tunneling across 2D material van der waals heterostructures4. Our models signal the breakdown of century-old classic electron emission models in 2D materials, and paves the way towards the better understanding and design of 2D material electronic devices. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H12.00005: Design of Atomically Precise GNR-Based Negative Differential Resistance Device Zhongcan Xiao, Chuanxu Ma, Jingsong Huang, Liangbo Liang, Wenchang Lu, Kunlun Hong, Bobby G Sumpter, An-Ping Li, Jerry Bernholc Down-scaling device dimensions to the nanometer range raises significant challenges to traditional device design, due to potential current leakage across nanoscale dimensions and the need to maintain reproducibility while dealing with atomic-scale components. Here we investigate negative differential resistance (NDR) devices based on atomically precise graphene nanoribbons. Our computational evaluation of the traditional double-barrier resonant tunneling diode NDR structure uncovers important issues at the atomic scale, concerning the need to minimize the tunneling current between the leads while achieving high peak current. We propose a new device structure consisting of multiple short segments that enables high current by the alignment of electronic levels across the segments while enlarging the tunneling distance between the leads. The proposed structure can be built with atomic precision using a scanning tunneling microscope (STM) tip during an intermediate stage in the synthesis of an armchair nanoribbon. An experimental evaluation of the band alignment at the interfaces and an STM image of the fabricated active part of the device are also presented. This combined theoretical-experimental approach opens a new avenue for the design of nanoscale devices with atomic precision. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H12.00006: Deep Learning Assisted Optical Identification of Exfoliated Two-Dimensional Crystals Bingnan Han, Yuxuan Lin, Wenyue Li, Nannan Mao, Yafang Yang, Haozhe Wang, Wei Sun Leong, Pablo Jarillo-Herrero, Tomas Palacios, Jihao Yin, Jing Kong Up to now, hundreds of two-dimensional materials are being studied in the fields of condensed matter physics, material sciences and electrical engineering. The overwhelming approach to obtain 2D crystals in laboratory is a combination of the mechanical exfoliation and the exhaustive search under an optical microscope by a well-trained researcher. Here we report a generic flake-hunting approach assisted by deep learning that can achieve the automatic, real-time, accurate, and robust optical identification of the type and the thickness of various 2D crystals. A semantic segmentation method using the encoder-decoder convolutional neural networks (SegNet) was developed and trained to identify the type and the thickness of the mechanically exfoliated 2D crystals on a SiO2/Si wafer. Besides the commonly used parameters such as the optical contrasts of the 2D crystals, deep graphical features can also be extracted and harnessed by the SegNet for accurate and robust identification. Our proposed method can be used for a wide range of research topics where initial screening and identification of nanomaterials are necessary. |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H12.00007: Ab Initio Study of the Mechanisms of Nitrogen Functionalization of Graphene Olivier Malenfant-Thuot, Michel Cote Our research aims to better understand the process of nitrogen functionalization of graphene, at the atomic level. We conducted electronic structure calculations, in the Density Functional Theory framework, to study the dynamics of different incorporation mechanisms of nitrogen atoms. We used the Nudged Elastic Band module, available in the BigDFT ab initio code, to calculate reaction pathways and energy barriers for a diverse set of migration and incorporation reactions, with or without the presence of native defects in the graphene sheet. By analyzing these results, we can predict which processes and functionalization configurations are more likely to be obtained in particular conditions. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H12.00008: Model for the metal-insulator transition in graphene superlattices and beyond Noah Yuan, Liang Fu We propose a two-orbital Hubbard model on an emergent honeycomb lattice to describe the low-energy physics of twisted bilayer graphene. Our model provides a theoretical basis for studying metal-insulator transition, Landau level degeneracy lifting, and unconventional superconductivity that are recently observed. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H12.00009: Origin of Magic Angles in Twisted Bilayer Graphene Grigory Tarnopolsky, Alex J Kruchkov, Ashvin Vishwanath Twisted bilayer graphene (TBG) is known to feature isolated and relatively flat bands near charge neutrality, when tuned to special magic angles. However, different criteria for the magic angle such as the vanishing of Dirac speed, minimal bandwidth or maximal band gap to higher bands typically give different results. We study a modified continuum model for TBG which has an infinite sequence of magic angles θ at which, we simultaneously find that (i) the Dirac speed vanishes (ii) absolutely flat bands appear at neutrality and (iii) band gaps to the excited bands are maximized. When parameterized in terms of α ~ 1/θ, they recur with the simple periodicity of Δα ≈ 3/2, which, beyond the first magic angle, differs from earlier calculations. Further, in this model we prove that the vanishing of the Dirac velocity ensures the exact flatness of the band and show that the flat band wave functions are related to doubly-periodic functions composed of ratios of theta functions. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H12.00010: Data-driven machine-learning clustering analysis to automatically classify exfoliated graphene flakes from optical microscope images Satoru Masubuchi, Tomoki Machida Machine-learning techniques enable the recognition of a wide range of images to complement the human intelligence. Since the advent of exfoliated graphene on SiO2/Si substrates, the identification process of graphene has relied on the optical microscopy imaging. Here, we develop the data-driven clustering analysis method to automatically identify the positions, shapes, and thickness of graphene flakes from the large amount of the optical microscope images of exfoliated graphene on SiO2/Si. Application of the feature extraction algorithm to the images yielded the optical and morphology feature values of the regions surrounded by the edges. The feature values formed the discrete clusters in the optical feature space, which were derived from 1-, 2-, 3-, and 4-layer graphene. The cluster centers are detected by the unsupervised machine-learning algorithm, enabling highly accurate classification of monolayer, bilayer, and trilayer graphene. The analysis can be applied to substrates with differing SiO2thickness, which demonstrates the generality of the approach. |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H12.00011: Theoretical investigation of 30 degree twisted bilayer graphene Kejie Bao, Yiou Zhang, Junyi Zhu, Shuyun Zhou To understand the unique stability and coupling mechanism in the interesting incommensurate bilayer graphene on top of Pt substrate with 30 degree twisting angles, we performed theoretical derivations and discovered a general interlay scattering mechanism: for any two vectors of one layer, if their difference equals the difference between the reciprocal vectors of the two layers, they can be coupled via a scattering of another layer. This scattering condition is general for any bilayer structures. Based on our first principles calculations, we found that such a coupling mechanism allows a coupling of K and K’ wave vectors of the top layer to couple and form a gap at the M point of the lower layer. This mechanism also enhances a p-d orbital coupling with substrate and explains why the incommensurate bilayer is even more stable than an AB stacking bilayer, where no significant p-d coupling exist. This discovery largely widens the scope of periodic structures and the general coupling condition provides important insights in the design and manufacture of two dimensional heterostructures that may lead to important electronic and spintronic devices. |
Tuesday, March 5, 2019 5:06PM - 5:18PM |
H12.00012: Unconventional superconductivity and density waves in twisted bilayer graphene Hiroki Isobe, Noah Yuan, Liang Fu We study electronic ordering instabilities of twisted bilayer graphene with n=2 electrons per supercell, where correlated insulator state and superconductivity are recently observed. Motivated by the Fermi surface nesting and the proximity to Van Hove singularity, we introduce a hot-spot model to study the effect of various electron interactions systematically. Using the renormalization group method, we find d/p-wave superconductivity and charge/spin density wave emerge as the two types of leading instabilities driven by Coulomb repulsion. The density wave state has a gapped energy spectrum at n=2 and yields a single doubly-degenerate pocket upon doping to n>2. The intertwinement of density wave and superconductivity and the quasiparticle spectrum in the density wave state are consistent with experimental observations. |
Tuesday, March 5, 2019 5:18PM - 5:30PM |
H12.00013: Twisted bilayer graphene as a phononic metamaterial William Dorrell, Harris Pirie, Bowei Liu, Yu Liu, Nathan Drucker, Alex J Kruchkov, Jenny Hoffman The twist angle within stacked van der Waals materials represents a novel degree of freedom to tune electronic properties. In bilayer graphene, varying the twist angle hybridizes the Dirac cones from each layer, resulting in flat bands that localize charge and induce unconventional superconductivity. Recently, graphene-like Dirac cones were observed in the phononic band structure of a metamaterial consisting of a honeycomb lattice of steel pillars in air. However, varying the twist angle has not been explored in metamaterials, due to the difficulty in coupling two macroscopic layers. Here we develop a method to couple layered phononic metamaterials using intermediary membranes, and we numerically demonstrate a classical system with flat phononic bands analogous to the electronic structure at magic angle twisted bilayer graphene. Our results provide a more tangible route to comprehending the behavior of quantum materials and may yield applications in super-resolution imaging. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700