Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S16: Transport in Nanostructures -- 2D Materials and Their HeterostructuresFocus
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Sponsoring Units: DMP Chair: Shixiong Zhang, Indiana University Bloomington Room: BCEC 155 |
Thursday, March 7, 2019 11:15AM - 11:51AM |
S16.00001: Electron transport in III-VI and IV-VI mono-chalcogenide nanostructures Invited Speaker: Xuan Gao One focal point in recent studies of 2D semiconductors beyond transition metal dichalcogenides (e.g. MoS2) is non-transition metal based III-VI and IV-VI monochalcogenides. In this talk, I will highlight our transport studies of monochalcogenide InSe, SnS and SnSe for future 2D semiconductor applications. First, few and multilayer InSe nanoflakes are demonstrated to be promising 2D semiconductor nanostructures with high electron mobility and gate tunable Rashba spin-orbit coupling for high-performance n-type transistor and spintronic devices. Then I will discuss the gate and doping control of nanostructured IV-VI monochalcogenide SnS and SnSe's electrical and thermoelectric transport properties. In particular, the impact of SnS and SnSe's intrinsic p-type nature in the device behavior will be addressed. References: Nano Letters 15 , 3815 (2015); Nano Letters 18, 4403 (2018); Nanoscale 8, 19050 (2016); Jour of Appl Phys, 123, 115109 (2018). |
Thursday, March 7, 2019 11:51AM - 12:03PM |
S16.00002: Inducing transport-anisotropy in graphene with 1D superlattices Yutao Li, Scott A Dietrich, Carlos Forsythe, Shaowen Chen, Takashi Taniguchi, Kenji Watanabe, James Hone, Cory Dean Graphene devices subjected to one-dimensional periodic superlattice potential have been studied extensively over the last decade. Previous experimental studies on such systems interpreted their data in fundamentally different ways: either based on commensurate effects such as Fabry-Perot interference, or a band structure based picture. Here we exploit our recently developed technique of dielectric superlattices to pattern graphene with a 1D superlattice, with periodicity as low as as low as 55nm. We observe strong anisotropy in electrical transport features along directions parallel versus perpendicular to the SL basis vector. For transport parallel to the SL basis vector, we see features consistent with Fabry-Perot interference at zero magnetic field, and Hoftstader butterfly-like features at high field that suggest band structure modifications. Possible interplay between commensurability effects and band structure modifications in such systems are discussed. |
Thursday, March 7, 2019 12:03PM - 12:15PM |
S16.00003: Commensurate Lattice Vector Dependent Thermal Conductivity of Twisted Bilayer Graphene Chenyang Li, Bishwajit Debnath, Roger Lake The in-plane thermal conductivity, as well as phonon dispersion of twisted bilayer graphene (TBG), are investigated as a function of temperature and rotation angle using both nonequilibrium molecular dynamics (NEMD) and density functional theory (DFT) combined with the phonon Boltzmann transport equation. The central result from the NEMD calculations is that the thermal conductivity decreases approximately linearly with the increasing lattice constant of the commensurate TBG unit cell. Comparisons of the phonon dispersions from both the DFT and NEMD calculations show that misorientation has the negligible effect on the low-energy phonon frequencies and velocities. However, the larger periodicity of TBG reduces the Brillouin zone size to the extent that the zone edge acoustic phonons are thermally populated allowing Umklapp scattering to reduce their lifetimes. This explanation is supported by DFT calculations of phonon-phonon lifetimes. |
Thursday, March 7, 2019 12:15PM - 12:27PM |
S16.00004: Anomalous Hall effect in strain-engineered graphene systems Sheng-Chin Ho, Ching-hao Chang, Yi-Chiang Heish, Shun-Tsung Lo, Thi-Hai-Yen Vu, Tse-Ming Chen The pseudo-magnetic field in graphene is of significant interest because it provides a means to modify the band structures and consequently the electronic properties through mechanical deformations or, more precisely, by engineering the strain1,2. Here, we report the observation of a non-vanishing Hall conductivity in a strain-engineered graphene system in the absence of an external magnetic field. |
Thursday, March 7, 2019 12:27PM - 12:39PM |
S16.00005: Probing spin fluctuations in graphene nanoribbons with STM tunnelling spectroscopy Ricardo Ortiz Cano, Joaquin Fernandez-Rossier We address the question of how scanning electron tunnelling spectroscopy can be used to probe spin excitations that emerge in localized states in atomically precise graphene ribbons. We consider both sequential and cotunnelling regimes, and discuss how the dI/dV curves provide information about the fluctuating local moments that are expected to occur in zigzag edges and other zero modes that occur in graphene ribbons. We model the graphene nanoribbons with a Hubbard model and we obtain the relevant electronic states by numerical diagonalization in a reduced active space. We conclude that inelastic electron tunnelling spectroscopy provides an image, with atomic scale resolution, of the spectral function of the low energy spin excitations in nanographene, and should provide unambiguous evidence of the emergence of local moments in this class of systems. |
Thursday, March 7, 2019 12:39PM - 12:51PM |
S16.00006: High efficiency thermoelectricity with indirect excitons in a transition-metal dichalcogenide nanostructure Chunjing Jia, Kai Wu, Brian Moritz, Thomas Devereaux High thermoelectric efficiency requires a large Seebeck coefficient and electric conductivity while maintaining low thermal conductivity. Recent development of modern synthesis techniques in nanomaterials provides new approaches to conquer such limits and usher the study of the thermoelectric effect into a new era. We propose to use indirect excitons (IEs) in two-dimensional TMDC nanostructures as a highly efficient thermoelectric device. We develop the exciton transport theory and numerically simulate the thermoelectric transport coefficients based on materials-specific parameters obtained from ab initio density functional theory calculations and experiments. Our numerical simulation shows that the excitons in bilayer TMDCs can dramatically enhance the figure of merit and the power factor an order of magnitude compared with those of separated TMDC monolayers. These enhancements are general consequences of increasing the Seebeck coefficient and electric conductivity of IEs simultaneously, thus demonstrating robustness for enhancing the thermoelectric effect. |
Thursday, March 7, 2019 12:51PM - 1:03PM |
S16.00007: Spin Hall effect for polaritons in a TMDC monolayer embedded in a microcavity Oleg Berman, Roman Kezerashvili, Yurii Lozovik The spin Hall effect (SHE) for polaritons in a transition metal dichalcogenides (TMDC) monolayer embedded in a microcavity is predicted. We demonstrate that two counterpropagating laser beams incident on a TMDC monolayer can deflect a polariton flow due to generation the effective gauge vector and scalar potentials. The components of polariton conductivity tensor for a weakly-interacting Bose gas of polaritons in the presence of Bose-Einstein condensation (BEC) and superfluidity and for non-interacting polaritons without BEC are obtained. We propose to study the superfluidity of microcavity polaritons by experimental measurement of components of a total conductivity tensor as functions of the effective gauge magnetic field at different temperatures. It is shown that the concentrations of the normal and superfluid components and the Kosterlitz-Thouless temperature of occurrence of superfluidity can be determined by experimental measurement the components of the total conductivity tensor. The possible experimental observation of the SHE for microcavity polaritons is proposed, which provides the signature of the superfluidity of microcavity polaritons. |
Thursday, March 7, 2019 1:03PM - 1:15PM |
S16.00008: Quantum scattering in two-dimensional nanostructures using a novel method of sources and absorbers Sathwik Bharadwaj, L Ramdas Ram-Mohan There have been several attempts in the literature to provide a variational approach to solve quantum scattering problems. However, the asymptotic boundary conditions (BCs) used in such methods do not take crucial evanescent modes into account, making them unsuitable for applications in meso- and nano-scale devices. Further, widely used atomistic models are computationally expensive for any device scale applications. We develop a method based on sources and absorbers to study quantum scattering in two-dimensional systems. The Cauchy BCs that are essential in the action integral formulation of scattering are reduced to simpler Dirichlet BCs by introducing totally absorbing “stealth regions.” Solutions decay within the stealth regions, thereby vanishing at the finite boundaries. A Green’s function source is constructed to provide incident plane waves in the active scattering regime. This method overcomes the limitations of the currently prevalent approaches to provide a complete non-asymptotic variational description for scattering in confined as well as open domain quantum systems. We also discuss the applications of our method in simulating nanoscale rectifiers and enhancement of the thermoelectric power in quantum waveguides with embedded defects. |
Thursday, March 7, 2019 1:15PM - 1:27PM |
S16.00009: Strong exciton-plasmon coupling in two-dimensional transition metal dichalcogenides Aaron Rose, Jeremy R Dunklin, Hanyu Zhang, Juan M. Merlo, Michael J Naughton, Jao van de Lagemaat Two-dimensional transition metal dichalcogenides (2-D TMDs) show promise as photocatalytic materials for water splitting due to their strong light-matter interactions and favorable band gaps and band alignments. Incorporation of plasmonics into these systems is expected to enhance performance due to a variety of effects including large exciton-plasmon coupling. |
Thursday, March 7, 2019 1:27PM - 1:39PM |
S16.00010: First principles study of electron transport through diarythylene-transition metal dichalcogenide molecular switch Ali Ramazani, Veera Sundararaghavan, Nicholas Fang Computational methods are fast becoming an integral part of nanoelectronics design process. With increasing computational power, electron transport simulation methods such as Non-equilibrium Green’s function (NEGF) methods now become of great interest in studying and designing new electronic materials. In this research, we study and design a single molecule switch based on a transition metal dichalcogenide (TMD) electrode (molybdenum disulfide MoS2 and a photo-chromic molecule. The chosen molecule, Diarylethene, is one of the only few thermally irreversible photochromes. The 1T phase of TMD monolayer has metallic properties and can therefore act as a conducting electrode for these molecular switches. In this study, we compare and contrast different chemistry and spacer groups with respect to their response as a molecular switch, focusing on the ON/OFF transmission ratio at the Fermi level. We identify promising chemistries for further experimentation. If experimentally realized, these switches are expected to become integral part of various applications including molecular memories, photon detectors and logic devices. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S16.00011: Frequency-Domain Magneto-Optical Kerr Effect for thermal property measurements of anisotropic 2-dimensional materials Simone Pisana, Mizanur Rahman, Mohammadreza Shahzadeh The rapidly increasing number of 2-dimensional materials that have been isolated or synthesized provides an enormous opportunity to realize new device functionalities. Whereas optical and electrical characterization has been more readily applicable to these new materials, quantitative thermal characterization is more challenging due to the difficulties with localizing heat flow. Optical pump-probe techniques that are well-established for the study of bulk materials or thin-films have limited sensitivity to lateral heat transport, and the characterization of the thermal anisotropy that is common in 2-dimensional materials is therefore challenging. Here we present a new method to quantify the thermal properties based on the magneto-optical Kerr effect that yields enhanced sensitivity to in-plane heat transport. Using a magnetic material as transducer for heat transport allows the use of semi-transparent layers that are very thin, increasing the in-plane thermal gradients. We apply this approach to measure the thermal properties of a range of 2-dimensional materials which are of interest for device applications, including single layer graphene and h-BN, multilayer h-BN and MoS2, and bulk MoSe2. |
Thursday, March 7, 2019 1:51PM - 2:03PM |
S16.00012: Quantum Dragon Solutions for the Tight Binding Model of 2D Ribbon Nanodevices. Godfred Inkoom, Mark Novotny, Tomas Novotny We present quantum dragon solutions for electron transport for the tight binding model with strong disorder in nanoribbons based on 2D hexagonal, rectangular, and square-octagonal graphs. When the nanodevice is connected between two thin semi-infinite leads, the Landauer formula gives the electrical conductance, $ G $. The electron transmission probability, ${\cal T}(E)$ from the solution of the time-independent Schr{\"o}dinger equation, yields $ G=\big(2e^2/h\big){\cal T}(E)$. In the presence of uncorrelated randomness, $ {\cal T}(E)\ll 1 $ for most electron energies, $ E $. Recently, the theoretical discovery of a large class of nanostructures called quantum dragons has been published [1]. Quantum dragon nanodevices have strong, correlated randomness but have ${\cal T}(E)=1$ for all $ E $ of electrons which propagate through the leads. Here, we show that both uniform leads and dimerized leads coupled to hexagonal, rectangular, and square-octagonal graphs with different boundary conditions can have the quantum dragon property [2]. We discuss how added disorder affects ${\cal T}(E)$ near a quantum dragon solution, and discuss experiments relevant to quantum dragon nanodevices. |
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