Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S57: Quantum and Ferroelectric Phenomena in Layered MaterialsFocus
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Sponsoring Units: DMP Chair: Peter Bøggild Room: Mile High Ballroom 3A |
Thursday, March 5, 2020 11:15AM - 11:27AM |
S57.00001: Visualizing and Manipulating Bilayer Graphene Quantum Dot States Zhehao Ge, Frederic Joucken, Eberth A Quezada, Diego Rabelo da Costa, John Davenport, Biran Giraldo, Nobuhiko Kobayashi, Takashi Taniguchi, Kenji Watanabe, Tony Low, Jairo Velasco Jr. Quantum confinement enables the control of a material's electronic, spin, and optical properties. A common platform to realize quantum confinement is a quantum dot (QD), which can be achieved through various methods such as electrostatic, self-assembly, and etching. Electrostatically defined monolayer and bilayer graphene QDs are unique compared to lateral semiconductor QDs because the quasi particles in the former possess chirality. As a result, upon normal incidence on a potential barrier monolayer graphene (MLG) features a 100% transmission (Klein tunneling) and bilayer graphene (BLG) features a 100% reflection (anti-Klein tunneling). Recently, several scanning tunneling spectroscopy (STS) studies have been performed on exposed electrostatically defined MLG QDs and have revealed chiral bound states with a tunable Berry phase. However, to date, no STS studies have directly probed the quasi-bound states in BLG QDs. Here we present our latest experimental progress on directly probing and manipulating the quasi-bound states in exposed BLG QDs with a scanning tunneling microscope (STM). |
Thursday, March 5, 2020 11:27AM - 11:39AM |
S57.00002: Graphene quantum dot bolometers for high frequency EPR of single molecule magnets Luke St. Marie, A El Fatimy, Jakub Hruby, Ivan Nemec, James Hunt, Rachael Myers-Ward, David Kurt Gaskill, Mattias Kruskopf, Yanfei Yang, Randolph Elmquist, Raphael Marx, Joris van Slageren, Petr Neugebauer, Paola Barbara Graphene’s properties, including broadband absorption and a small heat capacity, make it an ideal material for hot-electron bolometers. By nanostructuring graphene to open a quantum confinement gap, we have fabricated bolometers with extremely high responsivity. We use these graphene quantum bolometers to perform EPR spectroscopy of single molecule magnets (SMMs). We take advantage of the bolometers’ extreme sensitivity to conduct these measurements on thin layers of SMMs deposited by sublimation, allowing for the observation of substrate effects not detectable when measuring bulk samples of SMMs using standard techniques. |
Thursday, March 5, 2020 11:39AM - 11:51AM |
S57.00003: Manipulation of quantum geometrical properties in Weyl semimetal Jun Xiao, Ying Wang, Hua Wang, Sri Chaitanya Das Pemmaraju, Siqi Wang, Philipp Karl Muscher, Edbert Jarvis Sie, Clara M Nyby, Thomas Devereaux, Xiaofeng Qian, Xiang Zhang, Aaron Lindenberg Quantum materials with novel phases of matter are the key building blocks of energy efficient quantum electronics and powerful quantum computation. Exploiting control of those materials is fascinating to achieve new functionalities and information algorithm in future quantum devices. Quantum nanomaterials like layered materials, has revealed many exotic properties such as extremely large magnetoresistance (MR)1, type-II Weyl electron transport and diverging Berry curvature2. On the other hand, the nature of layered materials leads to ultra large tunability of physical properties via external stimuli. |
Thursday, March 5, 2020 11:51AM - 12:03PM |
S57.00004: Quantum transport of a two-dimensional hole gas in a diamond/h-BN heterostructure Yosuke Sasama, Katsuyoshi Komatsu, Satoshi Moriyama, Masataka Imura, Tokuyuki Teraji, Shiori Sugiura, Taichi Terashima, Shinya Uji, Kenji Watanabe, Takashi Taniguchi, Takashi Uchihashi, Yamaguchi Takahide Diamond attracts increasing attention as a next-generation semiconducting material because of its fascinating properties such as a wide-band gap, high thermal conductivity, high breakdown electric field, and high mobility. In this study, we fabricated a p-type diamond field-effect transistor (FET) using a diamond/hexagonal boron nitride (h-BN) heterostructure. A single-crystalline h-BN was cleaved by the scotch tape method and laminated on a hydrogen-terminated diamond surface. The laminated h-BN flake was used as a gate dielectric. The excellent insulating properties of h-BN enabled the mobilities of holes accumulating at the diamond surface to be higher than 300 cm2V-1s-1 at room temperature for hole densities above 5×1012 cm-2. The high mobility allowed to observe Shubnikov–de Haas oscillations in both the longitudinal and Hall resistivities. The oscillations provided valuable information on the hole transport at the diamond surface, such as the cyclotron effective mass, quantum lifetime, and two dimensionality. The high-quality two-dimensional hole gas demonstrated in this study will lead to new studies of quantum transport in diamond and development of high-performance diamond electric devices. (Y. Sasama et al., APL mater. 6 111105 (2018), arXiv:1907.13500) |
Thursday, March 5, 2020 12:03PM - 12:15PM |
S57.00005: Electrostatically gated quantum dots in van der Waals materials Justin Boddison-Chouinard, Alexander M Bogan, Pawel Hawrylak, Sergei Studenikin, Louis Gaudreau, Andrew Stanislaw Sachrajda, Adina A Luican-Mayer Quantum confinement of electrons into quantum dots has been thoroughly explored in materials such as silicon or gallium arsenide showing interesting physical phenomena as well as promise for use in quantum technologies. With rapid advancement in the fabrication of van der Waals heterostructures and their devices, quantum confinement provides a route for harnessing the properties of 2D materials towards building novel quantum computing platforms. Working towards that goal, in this talk we present the design and fabrication of electrostatically gated quantum structures based on monolayer and few atomic layers of molybdenum disulfide (MoS2) and bilayer graphene. Furthermore, we show and discuss our preliminary electron transport results aimed at probing the confined electron states in these structures. |
Thursday, March 5, 2020 12:15PM - 12:27PM |
S57.00006: Transport Properties in Interacting Graphene Quantum Dots Filipe Matusalem, Alexandre R Rocha Chemically derived graphene quantum dots (QD) hold great promise for applications in electronics, optoelectronics, and bioelectronics. Using a gate electrode, it is possible to control the number of electrons contained in the dot as well as the current flow when electrodes are attached. Experimentally, the fabrication of atomically precise graphene QDs consisting of low-bandgap armchair graphene nanoribbon (AGNR) segments were recently reported [1]. In this work, we apply a combination of dynamic mean field theory [2] and a molecular-based non-equilibrium Green's function technique coupled to DFT [3,4] to tackle the problem of interacting electrons in graphene quantum dots in AGNR including a gate electrode. We observe conductance peaks associated with the localized states related to the graphene QDs presented in the AGNR in a realistic parameters range giving us precise gap tunability. Such device can be used to design a graphene-based single electron transistor. |
Thursday, March 5, 2020 12:27PM - 1:03PM |
S57.00007: Strain-based room-temperature non-volatile MoTe2 ferroelectric phase change transistor Invited Speaker: Stephen M Wu As transistors continue to scale down in size, physical limitations from nanoscale field-effect operation begin to cause undesirable effects that are detrimental to the further advancement of computing. We explore an alternative to conventional field-effect transistor operation by using dynamic strain engineering on 2D van der Waals materials to induce electronic/structural phase transitions. Strain has been widely popularized in industry in its static form to enhance silicon mobility, but using strain to dynamically control materials properties has been more challenging for 3D bonded systems due to substrate limitations and defect formation. Systems involving 2D materials are freed from substrate constraints and have high elastic limits, but have not been heavily explored for dynamic strain engineering due to the difficulty in transferring strain into a material that is weakly bonded out-of-plane. In this talk, we focus on challenges in achieving dynamically controllable strain in 2D-bonded materials and how these challenges can be overcome in a scalable on-chip device. We introduce one implementation of such a device using both static thin film stress capping layers and ferroelectric oxide gate-dielectrics. Here, MoTe2 can be reversibly switched with electric-field induced strain between the 1T’-MoTe2 (semimetallic) phase and a semiconducting MoTe2 phase in a three-terminal field effect transistor geometry. Using strain, we achieve large non-volatile changes in channel conductivity (Gon/Goff~107 vs. Gon/Goff~0.04 in control devices) at room temperature. Using this implementation as a starting point, other phase transitions in 2D materials may be explored using this ‘straintronic’ device concept, which may enable low-power, high-speed, non-volatile, gate-controllability over a wide variety of exotic states of matter. |
Thursday, March 5, 2020 1:03PM - 1:15PM |
S57.00008: Imaging Strain-Induced Quantum Emitters Ashley Cavanagh, Dylan Renaud, Marko Loncar, Robert Moore Westervelt
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Thursday, March 5, 2020 1:15PM - 1:27PM |
S57.00009: Spin-orbit torques in heterostructures of 2D van der Waals magnets Vishakha Gupta, Gregory M Stiehl, Thow Min Cham, Joseph Mittelstaedt, ARNAB BOSE, Kaifei Kang, Shengwei Jiang, Kin Fai Mak, Jie Shan, Robert Buhrman, Daniel Ralph The discovery of intrinsic magnetism in 2D materials has opened an exciting new platform for spintronics, allowing fundamental studies of efficient mechanisms for electrically controlling magnetic materials. Here we explore spin-orbit torques in heterostructures of ultrathin single-crystal magnetic insulators and large spin-orbit coupling (SOC) materials: heavy metals and topological insulators. We report current induced switching in insulating ferromagnet Cr2Ge2Te6 at current densities as low as 106 Acm-2 from spin-orbit torques generated in heterostructures with Pt and Ta. We will discuss methods for quantifying the efficiency of these torques using both optical and electrical-transport techniques. |
Thursday, March 5, 2020 1:27PM - 1:39PM |
S57.00010: Fabrication of Hall Micromagnetometers for Probing Two-Dimensional Magnets Sean Nelson, Marc Bockrath, Joshua Goldberger, Daniel Weber Two-dimensional (2D) magnetic materials1 have emerged as a promising area for both new physics and potential spintronics applications. Recent work has shown the ballistic Hall micromagnetometry using graphene Hall bars can be used to detect the magnetization of atomically thin 2D magnetic materials.2 We will discuss our approach to the fabrication of such devices on top of 2D magnetic layers. The magnetic layers are first exfoliated onto a clean Si/SiO2 chip. A Hall bar is fabricated using a standard dry transfer technique, layer stacking, and electron beam lithography. The Hall bar is placed so that it half covers the magnetic layer. As in reference 1, the region covering the magnetic material is actively compared to the region not covering the magnetic material, which acts as a control. We perform low temperature magnetoconductance measurements, and the latest results will be discussed. |
Thursday, March 5, 2020 1:39PM - 1:51PM |
S57.00011: Effect of Potential Fluctuation on Ferroelectric-Gated Bilayer Graphene Hanying Chen, Zhiyong Xiao, Yifei Hao, Kenji Watanabe, Takashi Taniguchi, Xia Hong We investigated the effect of a ferroelectric gate on the transport properties of bilayer graphene (BLG). BLG flakes were mechanically exfoliated on epitaxial 300 nm Ba0.6Sr0.4TiO3 (BSTO) thin films grown on Nb-doped SrTiO3 substrates. Selected flakes were fabricated into field-effect transistor devices. At 2 K, the BLG devices exhibit field-effect mobility up to µFE ~ 3,000 cm2/Vs and quantum Hall effect, from which we deduced a dielectric constant of 113 for BSTO. The devices exhibit resistance hysteresis induced by nonvolatile polarization switching at high gate voltage. The temperature dependence of BLG at the charge-neutral point can be well described by thermally activated behavior at high temperature, which evolves to nearest-neighbor hopping below 80 K. We extracted activation energy of ~20 meV and hopping energy of ~0.3 meV. The result indicates that the epitaxial BSTO films yield less potential fluctuation compared with conventional SiO2 substrate. We will also discuss the effects of remote interfacial optical phonon scattering and capping with a top h-BN layer on these ferroelectric-gated BLG devices. |
Thursday, March 5, 2020 1:51PM - 2:03PM |
S57.00012: Improved Adhesion of Two-Dimensional Materials to Ferroelectrics from Poling Carla Watson, Tasneem Khan, Tara Pena, Stephen M Wu Ferroelectric materials can control mechanical strain in coupled 2D materials to induce phase transitions with applied electric field [1]. 2D flake-to-ferroelectric adhesion is crucial to device performance since it affects the efficiency of strain transfer. We discuss a framework to enhance 2D-ferroelectric adhesion through repeated ferroelectric poling. We explore this using exfoliated MoTe2 thin flakes on structurally mixed-phase rhombohedral (R)/tetragonal (T) BiFeO3 (BFO) ferroelectric thin films grown with a metallic La0.7Sr0.3MnO3 (LSMO) bottom counterelectrode for switching. Flakes with less adhesion are less conformal to the underlying R/T-BFO stripe domains due to a competition between strain energy and adhesive force. We quantify adhesion by measuring substrate conformality before and after local out-of-plane electric-field poling with a conductive AFM probe. Local poling causes both ferroelectric switching and structural transformation between R and T phases of BFO, which contribute to enhanced substrate conformality as well as a route to on-demand nanoscale strain engineering. |
Thursday, March 5, 2020 2:03PM - 2:15PM |
S57.00013: Control of transport properties of hybrid molybdenum disulfide (MoS2) – ferroelectric devices via domain engineering Alexey Lipatov, Tao Li, Nataliia S. Vorobeva, Alexander Sinitskii, Alexei Gruverman We demonstrate a concept of programmable ferroelectric devices comprised of two-dimensional (2D) and ferroelectric (FE) materials, which enables precise modulation of the in-plane conductivity of a 2D channel material through nanoengineering of FE domains with out-of-plane polarization. The functionality of these new devices has been demonstrated using field-effect transistors (FETs) fabricated with monolayer molybdenum disulfide (MoS2) channels on the Pb(Zr,Ti)O3 substrates. Using piezoresponse force microscopy, we show that local switching of FE polarization by a conductive probe can be used to tune the conductivity of the MoS2 channel. Specifically, patterning of the nanoscale domains with downward polarization creates conductive paths in a resistive MoS2 channel so that the conductivity of an FET is determined by the number and length of the paths connecting source and drain electrodes. In addition to the device programmability, we demonstrate the device ON/OFF cyclic endurance by successive writing and erasing of conductive paths in a MoS2 channel. These findings may inspire the development of advanced energy-efficient programmable synaptic devices based on combination of 2D and FE materials. |
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