Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H36: 2D Materials - van der Waals Heterostructures II |
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Sponsoring Units: DCMP Chair: Scott Schmucker, NIST Room: LACC 410 |
Tuesday, March 6, 2018 2:30PM - 2:42PM |
H36.00001: Ab-initio CrI3 Electronic Structure and Raman Spectra Daniel Larson, Efthimios Kaxiras First principles calculations with density functional theory are used to predict the phonon spectrum and Raman activity for chromium-triiodide (CrI3) in the high- and low-temperature crystal structures (C2/m and R-3, respectively). Comparison is made between bulk and monolayer samples. The results are used to study the effects of strain, polarization, and incident angle on Raman activity. |
Tuesday, March 6, 2018 2:42PM - 2:54PM |
H36.00002: Investigation of thermal conductivity in layered honeycomb magnet CrCl3 Christopher Pocs, Ian Leahy, Seung-Hwan Do, Kwang-Yong Choi, Minhyea Lee We investigate the in-plane thermal conductivity (κ) of quasi two-dimensional honeycomb magnet CrCl3, as a function of temperature (T) and applied field (H). In contrast to isomorphic cousin RuCl3, spin-orbit coupling in CrCl3, is expected to be weak, providing a window into phonon-related contributions to thermal transport. We find that in-plane κ is insensitive to the applied field direction, either in the ab-plane or along the c axis, supporting the assertion that spin-orbit coupling plays a minimal role. However, a large, non-saturating enhancement of κ(H) up to H = 18 T for a range of T as high as 60 K, suggests a strong suppression of spin-phonon scattering under applied field. We will discuss estimations of the phonon contribution to κ, and compare to the case of RuCl3. |
Tuesday, March 6, 2018 2:54PM - 3:06PM |
H36.00003: Toward Ultra-Efficient Solar Energy Regulation in WS2 - MoSe2 Heterojunction Photocells Jedediah Kistner-Morris, Fatemeh Barati, Dennis Pleskot, Jacky Wan, Nathaniel Gabor Biologically inspired technologies may prove to be the key to increased efficiency in |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H36.00004: Spatial Control of Charge Doping in van der Waals Heterostructures Salman Kahn, Wu Shi, Sheng-Yu Wang, Hsin-Zon Tsai, Dillon Wong, Takashi Taniguchi, Kenji Watanabe, Alex Zettl, Michael Crommie Advancements in designing innovative electronic devices and exploring novel physics are often limited by the spatial control of charge doping in 2D materials. A variety of conventional techniques, such as electrostatic gating, are used to achieve charge doping control but suffer from complicated fabrication processes that introduce impurities and lack flexibility. To overcome these challenges, we have introduced new methods of patterning doping profiles in van der Waals heterostructures by controllably exciting defect states in our device with light illumination and local field excitation. In this study we create rewritable nanoscale doping patterns with improved spatial control. These spatially dependent doping patterns are then characterized through local probe and transport methods. This local patterning technique will enable novel device designs and allow for the exploration of patterned superlattice structures in 2D materials. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H36.00005: Bottom-up graphene nanoribbon(GNR) device based on van der Waals heterostructure Kyunghoon Lee, Shuang Wu, Gabriela Borin Barin, Juan Llinas, Young-Jae Shin, Philip Kim, Roman Fasel, Felix Fisher, Michael Crommie, Jeffrey Bokor, Alex Zettl Driven by recent experimental breakthroughs via a bottom-up approach, a new exciting material candidate in the form of just few atoms wide graphene nanoribbons (GNRs) are emerging as strong candidates for realizing molecular electronics. GNRs are predicted to exhibit a rich variety of electronic and magnetic behaviors. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H36.00006: Origins of Electronic Localization and Superlattices in Misaligned 2D Heterostructures Stephen Carr, Daniel Massatt, Mitchell Luskin, Efthimios Kaxiras One can create a misaligned bilayer of 2D materials in many ways: from twist, strain, or lattice mismatch between layers. Experimental measurements of these materials (e.g. in bilayers of graphene or transition metal di-chalcogenides) have shown that the electrons' local density of states (LDoS) can be strongly influenced by the interlayer interaction. In some cases, the electronic wavefunction appears localized like an array of quantum dots. In others, a pattern of high-density regions connect like a network of Luttinger liquids. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H36.00007: On-chip broadband THz time domain spectroscopy of van der Waals heterostructures Joshua Island, Peter Kissin, Richard Averitt, Andrea Young Free space THz spectroscopy is generally limited to samples with sizes greater than the diffraction limited resolution of the experimental system. This restricts sample sizes to several hundreds of microns excluding investigations of smaller, exfoliable van der Waals materials such as graphene and transition metal dichalcogenides which have raised great interest. One method to beat the THz diffraction limit is to couple the radiation to an on-chip waveguide. Here, we present our progress towards development of a completely on-chip THz spectroscopy technique to investigate van der Waals heterostructures with sizes well below the diffraction limit. We employ photoconductive switches and coplanar waveguides to generate and control THz transients and detect the waveforms with picosecond time resolution. The versatility of our waveguide geometry allows for transmission, reflection, and emission spectroscopy all on a single chip. I will describe our efforts to investigate equilibrium and non-equilibrium phases of van der Waals heterostructures with these THz transients. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H36.00008: Exciton-Exciton Interactions Revealed through Interlayer Photoresponse of a Graphene-MoTe2-Graphene Heterostructure Photocell Dennis Pleskot, Trevor Arp, Vivek Aji, Nathaniel Gabor Heterostructures composed of atomic layer materials bonded through van-der-Waals interactions have demonstrated great potential for use in next generation optoelectronic devices. However, the role of strong exciton-exciton interactions in the interlayer photocurrent in such devices is not fully understood. Utilizing a newly developed dynamic photoresponse microscopy technique, we gain a comprehensive understanding of the MoTe2 heterostructure photoresponse. The power dependence of the interlayer photocurrent is well described by a single power law, where the power law exponent parameterizes the non-linearity of the photoresponse. We develop a detailed model that accounts for careful time integration of the dynamics, resulting in an analytic solution that reproduces the non-linear power dependence. We attribute the strong sub-linear power dependence to electron-hole generation and exciton-exciton annihilation. Additionally, we spatially resolve the deviations from this conventional power law behavior, which suggest a breakdown of the exciton-exciton annihilation model and the emergence of a novel electronic phase. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H36.00009: Observation of Fractional Chern Insulators in a van der Waals Heterostructure Eric Spanton, Alexander Zibrov, Haoxin Zhou, Takashi Taniguchi, Kenji Watanabe, Michael Zaletel, Andrea Young Chern bands are 2D bands which exhibit a quantized Hall conductivity when fully filled. Landau levels are a special case of Chern bands with a Chern number, C = 1. The Hofstadter butterfly, a fractal energy spectrum which forms in electronic systems with a lattice and a strong magnetic field, is also tunable Chern band structure where C can take on any integer value depending on magnetic field and electron density. An outstanding question is whether topological order driven by electron interactions (e.g. the paradigmatic fractional quantum Hall insulator) can exist at fractional filling of a non-Landau level Chern band, i.e. a fractional Chern insulator. We measured the magnetocapacitance of a graphite/hexagonal BN encapsulated bilayer graphene device with a moiré potential between the bilayer and hBN dielectric. We find a Hofstadter butterfly made up of many single-particle Chern bands which are tuned by electron density, magnetic, and electric fields. At fractional filling of some C =-1 and +2 bands, we observe fractional Chern insulators consistent with Laughlin-like states. Our observations open up the experimental study of new topological quantum phase transitions and the realization of lattice-defect based states which are inaccessible in traditional quantum Hall systems. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H36.00010: Coherent THz Radiation from Charge Transfer between Monolayers of Transition Metal Dichalcogenides Yue (Eric) Ma, Burak Guzelturk, Guoqing Li, Linyou Cao, Zhi-Xun Shen, Aaron Lindenberg, Tony Heinz We report the observation of coherent electromagnetic radiation from ultrafast charge transfer in MoS2/WS2 vertical heterostructures excited by an above-band-gap femtosecond laser. The emitted electric field profile is a near-half-cycle pulse in the THz frequency regime. The field polarities are consistent with the charge transfer direction expected for the type-II band alignment, and are opposite for heterostructures with opposite stacking order. The field strength scales linearly with excitation power and absorbance of the heterostructure. The observations are reproduced in samples made with different growth and fabrication methods. Using a high-bandwidth setup we obtain an upper limit of net charge transfer time constant of ~100 fs. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H36.00011: Photoresponse Imaging of Hot Carrier Dynamics in Graphene-Boron Nitride-Graphene Heterostructures Jacky Wan, Trevor Arp, Nathaniel Gabor The space and time evolution of hot charge carriers in Dirac electronic systems, such as graphene, may highlight the intriguing electron dynamics of Dirac fluids. Utilizing a near-infrared scanning pulsed laser, we spatially and temporally resolved the interlayer photocurrent between two graphene layers separated by an ultrathin tunneling barrier. We found the interlayer photocurrent I increases super-linearly with excitation power P, exhibiting clear power law growth of I ~ P2. This behavior is uniform across the heterostructure area and depends on temperature and applied bias voltage. Further, we utilized a two-pulse photoresponse measurement to access the interlayer charge carrier dynamics and extracted two timescales of the photoresponse, t1 on the order of 200 fs, and t2 about 2 ps, which we attributed to fast electron-electron scattering and slow electron-phonon interactions within a single graphene layer. These time scales are accessible due to the unique interlayer transport processes in the graphene-boron nitride-graphene heterostructure, which may allow detailed investigation of non-equilibrium correlated electron phases in this Dirac electronic system. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H36.00012: Assessment of the Energy Gaps in Graphene-h-BN Superlattices with a Hexagonal Boron Nitride Tunnel Barrier Suyong Jung, Hakseong Kim, Nicolas Leconte, Jeil Jung Engineering and detecting an energy gap in graphene has been an active research field ever since the crystalline carbon layer was isolated from its bulk form, hoping for next-generation device applications. Graphene/hexagonal boron nitride (h-BN) heterostructure can serve as an ideal experimental platform for generating energy gaps, originated from either atomic- or nano-scale altercations within hexagonal graphene-h-BN superlattices. Here, we report direct assessment of the energy gaps formed not only at the charge neutrality (main Dirac) point but also at the mini-zone boundary (second Dirac point, SDP) defined by the close packing of graphene and h-BN lattices. With a planar tunneling scheme using thin h-BN as a tunnel barrier, we investigate the progression of the energy gaps at varying the length of graphene-h-BN superlattices and an external magnetic field, revealing that the energy gap at the SDP responds differently to external variants in a microscopic scale when comparing with the gap at the main Dirac point. In addition, many-body interactions play a major role in deciding energy-gap sizes in tightly aligned graphene-h-BN superlattices as the effect from the interactions diminish as the twist angle increases. |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H36.00013: Unusual Coulomb excitations in tri-layer ABC-stacked graphene Chiun Yan Lin, Ming-Fa Lin The layer-based random-phase approximation is further developed to investigate electronic excitations in tri-layer ABC-stacked graphene. All the layer-dependent atomic interactions and Coulomb interactions are included in the dynamic charge screening. There exist rich and unique (momentum, frequency)-excitation phase diagrams, in which the complex single-particle excitations and five kinds of plasmon modes, are dominated by the unusual energy bands and doping carrier densities. The latter frequently experience the significant Landau damping due to the former, leading to the coexistence/destruction in the energy loss spectra. Specifically, the only acoustic plasmon in pristine case is dramatically changed into an optical mode even at very low doping. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H36.00014: Electronic Thermal Conductance Measurement of Ultraclean Bilayer Graphene using Johnson Noise Thermometry Artem Talanov, Jesse Crossno, Kemen Linsuain, Jonah Waissman, Marine Arino, Hugo Bartolomei, Takashi Taniguchi, Kenji Watanabe, Kin Chung Fong, Philip Kim Strongly interacting electron systems can exhibit exotic new physics, such as the hydrodynamic regime, where particles' behavior is best described as a viscous fluid rather than as individual particles. Several such hydrodynamic systems have recently been both predicted and shown experimentally to violate the Wiedemann Franz (WF) law, and the link between hydrodynamic transport and the violation of WF has been explored. In this study, we investigate thermal conductance in ultraclean bilayer graphene samples as a function of temperature, carrier density, and displacement field. We measure thermal conductance by Joule-heating the graphene and measuring its resulting thermal response via Johnson noise thermometry. We collect Johnson noise over a several hundred MHz bandwidth, improving upon our previously-developed noise measurement technique with cryogenic low-noise amplifiers and symmetric LC matching circuits. Measuring the zero-bias Johnson noise of the device at several bath temperatures allows us to calibrate the effective gain and system noise for several orders of magnitude of device resistance. Our data shows an enhancement of the thermal conductance above the value predicted by the WF law near the charge neutrality point. |
Tuesday, March 6, 2018 5:18PM - 5:30PM |
H36.00015: In Situ TEM Observation of Thermoelectric Cooling in a Bismuth Telluride and Bismuth-Antimony Telluride Nanoscale Heterojunction Gurleen Bal, Matthew Mecklenburg, William Hubbard, Brian Zutter, Roshni Patil, WILLIAM KESSEL, Bozo Vareskic, Graydon Flatt, Shaul Aloni, B. Regan Thermoelectrics have a wide variety of applications, but their efficiency must be improved before they become economical for non-niche applications. We demonstrate cooling in thermoelectric heterojunctions constructed from 2D flakes of exfoliated bismuth telluride and bismuth-antimony telluride using a new transmission electron microscopy (TEM)-based technique: plasmon energy expansion thermometry (PEET). PEET measures temperature by measuring the sample material’s bulk plasmon energy. The plasmon energy is related to the electron density, which in turn is related to temperature via the material’s coefficient of thermal expansion. Because the plasmon peaks in bismuth telluride and bismuth-antimony telluride are too broad to be precisely located, we evaporate indium nanoparticles near the heterojunction to serve as local nano-thermometers. Measuring the nanoparticles’ temperature as a function of the junction current, we have observed heterojunction cooling by 300C. With better ohmic contacts and improved device geometry, we hope to increase the thermoelectric efficiency of these 2D devices relative to their bulk 3D counterparts. |
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