Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K13: 2D Materials (General) -- Topology and Exotic PhenomenaFocus
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Sponsoring Units: DMP DCOMP Chair: Adina Luican-Mayer, Univ of Ottawa Room: BCEC 153B |
Wednesday, March 6, 2019 8:00AM - 8:12AM |
K13.00001: Growth and Transport Characterization of Epitaxial Graphene on Non-polar SiC Facets Yiran Hu, Vladimir Prudkovskiy, Antonio Tejeda, Amina Taleb Ibrahimi, Yue Hu, Clemens B. Winkelmann, Lei Ma, Claire Berger, Walt A. de Heer The ballistic transport and quantized conductance of e2/h observed in epitaxial graphene side wall nanoribbons at room temperature still lacks full explanation [1]. In an effort to solve this question, we produced graphene samples on specific SiC facets that host the side wall nanoribbons and fabricated standard transport measurement devices. Pre-growth patterning has been demonstrated to direct the graphene growth and improve growth uniformity. We did a variety of measurements, including AFM, Raman spectroscopy, ARPES, STS and transport measurements. These results indicate a charge-neutral graphene material, showing properties consistent with the observation of ballistic transport in side wall nanoribbons. |
Wednesday, March 6, 2019 8:12AM - 8:24AM |
K13.00002: Strain fields in graphene induced by nanopillars Slavisa Milovanovic, Lucian Covaci, Francois M Peeters The mechanical and electronic properties of a graphene membrane placed on top of a superlattice of nanopillars are investigated. We use molecular dynamics simulations to access the deformation fields and the tight-binding approaches to calculate the electronic properties. The system of interest consists of a triangular lattice of nanopillars with a period of a=750 nm over which the graphene layer is deposited. Ripples form in the graphene layer that span across the unit cell, connecting neighboring pillars, in agreement with recent experiments. We investigate the dependence of the pseudo-magnetic field (PMF) on unit cell parameters and the van der Waals interaction between graphene and the substrate. We find direct correspondence with typical experiments on pillars, showing intrinsic "slack" in the graphene membrane. PMF values are confirmed by the LDOS calculations at different positions of the unit cell showing pseudo-Landau levels at varying spacings. Our findings can be applied to other 2D materials (hBN, TMDs). Such systems are of interest as single-photon emitters where charge carriers are confined by the strain potential. Our study can be used as a guide to optimize parameters of the system for the improvement of the efficiency of the emitter. |
(Author Not Attending)
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K13.00003: Charged topological solitons in zigzag graphene nanoribbons. Luis Brey, M.P. Lopez-Sancho Zigzag graphene nanoribbons (ZZGN) have a magnetic ground state (GS) characterized by |
Wednesday, March 6, 2019 8:36AM - 9:12AM |
K13.00004: Topological Effects in 1D and 2D Materials: Topological Band Engineering, Optical Selection Rules, and Excitonic Shift Currents Invited Speaker: Steven G. Louie In this talk, I present several fascinating manifestations of topological effects in the electronic and optical properties of atomically thin one-dimensional (1D) and two-dimensional (2D) materials. First, we find that symmetry-protected topological phases exist in graphene nanoribbons (GNRs) [1]. Semiconducting GNRs of different width, edge shape, and terminating unit cells can belong to different electronic topological classes, characterized by a Z2 invariant. Junctions between segments of topologically distinct GNRs are predicted to support robust in-gap topological junction states which can be used for band engineering. Experimental realizations of these predictions have been achieved [2]. Second, we show that the conventional optical selection rules for excitons must be replaced in 2D by a novel simpler formula, owing to a topological characteristic inherent to the photoexcitation of excitons in 2D [3]. The new selection rule is dictated by a winding number of the interband optical transition matrix elements (a heretofore unrecognized topological invariant). This appealingly simple and general new rule is applied to elucidate the optical spectra of gapped graphene systems. Third, I present some recent work on the effects of electron-hole interactions on shift currents in non-centrosymmetric 2D crystals (so-called bulk photovoltaic effect), in which we show excitonic effects lead to an enormous enhancement and, more interestingly, gives rise to DC conduction with sub-bandgap-frequency excitations.[4] |
Wednesday, March 6, 2019 9:12AM - 9:24AM |
K13.00005: Topological Vortices vs. discommensuration dislocations in charge-density-wave 2H-TaSe2 Seong Joon Lim, Choong-Jae Won, Sang-Wook Cheong Charge density wave (CDW) discommensuration (DC) refers to a boundary between two different CDW phases and is a key to understand the phase transition from commensurate to incommensurate CDW. The phase transition can be rendered as a process of creating or eliminating DCs that results in change of incommensurability, and the place of such process, where DCs merge to each other, has been known as CDW dislocation. The idea of CDW DC dislocation has been primarily studied in 2H-TaSe2, which shows a clear phase transition from incommensurate to commensurate CDW. Although there have been several experimental and theoretical results supporting the idea, it has nevertheless not been verified at the atomic scale using, for example, scanning tunneling microscopy (STM). We present an observation of such entangled points of multiple DCs in 2H-TaSe2 by STM. The observation showed no phase slip which is against the idea of CDW dislocation, but the result instead unveiled the formation of topological vortices. In this talk an atomic scale comparison among the CDW DC dislocation model, the experimental results as well as a complicated vortex-like domain topology of 2H-TaSe2 will be presented. |
Wednesday, March 6, 2019 9:24AM - 9:36AM |
K13.00006: Quantum paraelastic two-dimensional materials Tyler Bishop, Erin Farmer, Afsana Sharmin, Alejandro Pacheco-Sanjuan, Pierre Darancet, Salvador Barraza-Lopez We study the elastic energy landscape of two-dimensional tin oxide (SnO) monolayers and discover a transition temperature using ab-initio molecular dynamics (MD), that is close to the value of the elastic energy barrier J derived from T = 0 K density functional theory calculations. The power spectra of the MD evolution permits identifying soft phonon modes likely responsible for the observed structural transformation. The mean atomic displacements obtained from a Bose-Einstein occupation of the phonon modes suggest the existence of a quantum paraelastic phase, that could be tuned charge doping, implying that SnO monolayers could be two-dimensional quantum paraelastic material with a charge-tunable quantum phase transition. |
Wednesday, March 6, 2019 9:36AM - 9:48AM |
K13.00007: Dependence of the hBN Layer Thickness on the Band Structure and Exciton Properties of Encapsulated WSe2 Monolayers Iann Gerber, Xavier Marie The optical properties of two-dimensional transition metal dichalcogenide monolayers such as MoS2 or WSe2 are dominated by excitons, Coulomb bound electron-hole pairs. Screening effects due the presence of hexagonal-BN surrounding layers have been investigated by solving the Bethe Salpeter Equation on top of GW wave functions in density functional theory calculations. We have calculated the dependence of both the quasi-particle gap and the binding energy of the neutral exciton ground state Eb as a function of the hBN layer thickness. This study demonstrates that the effects of screening at this level of theory are more short-ranged that it is widely believed. The encapsulation of a WSe2 monolayer by three sheets of hBN (∼ 1 nm) already yields a 20 % decrease of Eb whereas the maximal reduction is 27% for thick hBN. We have performed similar calculations in the case of a WSe2 monolayer deposited on stacked hBN layers. These results are compared to the recently proposed Quantum Electrostatic Heterostructure approach. |
Wednesday, March 6, 2019 9:48AM - 10:00AM |
K13.00008: Revealing of Dark Exciton States by Twisted Light in Two Dimensional Transition Metal Dichalcogenides Kristan Bryan C Simbulan, Teng-De Huang, Feng Li, Junjie Qi, Ting-Hua Lu, YannWen Lan Brightening spin- and momentum-forbidden dark exciton in two-dimensional (2D) transition metal dichalcogenides (TMDs) is nontrivial to further advances in optoelectronics applications. In this study, we demonstrate the effects of the interaction between twisted light (light possessing orbital angular momentum) and atomically thin TMD material in the optical measurements. Our results show a reproducible blue shift in the photoluminescence (PL) spectra of both monolayer and bilayer TMD material as the topological charge of the illuminating twisted light is incremented along positive and negative values. This phenomenon is attributed to the transfer of orbital angular momentum from twisted light onto the center-of-mass momentum of excitons which consequently brightens momentum-forbidden dark exciton states. Further, based on the observation on power-dependence of PL spectra, we found a red shift with increasing laser power across different topological charge values of the incident light. It reveals that there is another factor such as heat or strain induced by twisted light, which can replace the dominant mechanism. This study uncovers a new selection rule in 2D TMD materials that can potentially be a useful control mechanism for future optoelectronic device applications. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K13.00009: The influence of disorder in the external environment of 2D semiconductors on their electronic and optical properties Archana Raja, Lutz Waldecker, Jonas Zipfel, Yeongsu Cho, Samuel Brem, Jonas Ziegler, Takashi Taniguchi, Kenji Watanabe, Ermin Malic, Timothy Berkelbach, Tony F Heinz, Alexey Chernikov Disorder in solids typically stems from local fluctuations of material structure itself like composition, strain, and size. Here, we highlight a new source of disorder in atomically thin, two-dimensional (2D) materials: The variation in the effective strength of Coulomb interactions in the 2D material resulting from fluctuations in the dielectric screening of the adjoining environment. We experimentally monitor the influence of dielectric disorder for monolayer WS2 and WSe2 on SiO2, PDMS and h-BN by probing correlations between ground and excited state exciton resonances, which exhibit different sensitivities to the external dielectric environment. Our observations are described in a theoretical framework that considers variation in external dielectric screening and intrinsic phonon scattering channels as contributing to the ground and excited-state exciton linewidths. Even moderate fluctuations in the external dielectric permittivity are shown to induce inhomogeneous variations of the bandgap and exciton binding energies on the order of 100's of meV, constituting the major source of disorder in the studied samples. We identify elimination of dielectric disorder as key to achieving high material quality through encapsulation of 2D semiconductors in other van der Waals materials. |
Wednesday, March 6, 2019 10:12AM - 10:24AM |
K13.00010: Theory-assisted detection of nano-rippling and impurities in STEM images of angle-mismatched bilayer graphene Andrew O'Hara, Oleg S Ovchinnikov, Jordan A. Hachtel, Stephen Jesse, Sergei Kalinin, Albina Y Borisevich, Sokrates T Pantelides Two-dimensional (2D) materials commonly contain ripples and impurity atoms that limit carrier mobilities, create pseudo-magnetic fields, and affect other electronic and magnetic properties. While scanning transmission electron microscopy (STEM) provides high-accuracy determination of the atomic positions and columns in the image plane, it is difficult to obtain precise atomic positions in the perpendicular direction. Detection of impurities with similar atomic numbers can also be difficult in Z-contrast imaging. In the case of multilayer 2D materials such as bilayer graphene, misalignment of the layers results in a moiré pattern that further compounds the problem of atomic identification. In this work, we introduce a combined approach utilizing STEM imaging and density-functional-theory calculations to recover this information from the experimentally accessible xy-coordinates in twisted bilayer graphene. We find that the strain-induced rippling obeys the continuum model of elasticity and that the moiré-pattern-induced undulations are approximately an order of magnitude smaller. Additionally, using the presented methodology, we are able to establish the presence of a substitutional nitrogen impurity. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K13.00011: Strain enhancement of the Kondo effect in graphene Kevin Ingersent, Dawei Zhai, Sergio E Ulloa, Nancy Patricia Sandler The Kondo physics of screening of an impurity's magnetic moment by electrons in doped graphene has been predicted to exhibit peculiar features. However, conclusive experimental observation of the phenomenon remains elusive. One possible obstacle to its identification is a very small Kondo temperature TK in situations where the chemical potential lies near the Dirac point. Here, we propose to use mechanical deformations in graphene to recognize the unique fingerprints that the Kondo regime exhibits [1]. Inhomogeneous deformations are known to produce specific alternating changes in the local density of states that indicate sublattice symmetry breaking effects. These patterns can be magnified to produce significant enhancement or depression of TK for magnetic impurities positioned at different lattice sites. The deformation-induced changes, particularly the strong increase of TK expected at certain impurity locations, may lift the Kondo scale into the experimentally relevant range and are suitable for detection using local probes such as scanning tunneling microscopy. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K13.00012: Nano-optical imaging of 2D TMD alloys Dmitri Voronine Two-dimensional transition metal dichalcogenides (2D TMDs) are the materials of recent interest due to many promising applications. Novel materials and devices are based on the heterostructures formed by 2D TMDs. Alloys formed at TMD heterojunctions may enhance or limit the applications. It is important to characterize their optoelectronic properties with nanoscale spatial resolution. Tip-enhanced photoluminescence (TEPL) and tip-enhanced Raman scattering (TERS) techniques were used to image various TMD (MoS2, WS2, MoSe2, WSe2) alloys and heterostructures revealing detailed nanoscale features. Conventional far-field photoluminescence (PL) and Raman imaging provides highly averaged information with spectral congestion. In contrast, the TEPL and TERS methods, not limited by diffraction, provide substantial information related to nanoscale optical properties of 2D materials with resolution down to a few nanometers. The variations in the nanoscale optical properties correlating with the structural variation can provide a better understanding of the 2D TMD materials for the future development of highly efficient, flexible, lightweight optoelectronic devices. |
Wednesday, March 6, 2019 10:48AM - 11:00AM |
K13.00013: Stacking-dependent interlayer phonons in 3R and 2H MoS2 Jeremiah Van Baren, Gaihua Ye, Zhipeng Ye, Pouyan Rezaie, Jia-An Yan, Yu Peng, Zheng Liu, Rui He, Chun Hung Lui Atomically thin MoS2, a prototype two-dimensional semiconductor, commonly exhibits the 2H stacking order. In even layer numbers, 2H MoS2 restores the inversion symmetry and hence loses many attractive properties, such as second harmonic generation, piezoelectricity and spin-valley coupling.But researchers have recently grown MoS2 crystals with 3R stacking order, which in all layer numbers breaks the inversion symmetry and retains the valleytronic, piezoelectric and nonlinear optical properties as in the monolayer. |
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