Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E35: Phase Transitions in 2D Materials |
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Sponsoring Units: DCMP Chair: Adam Friedman, Naval Research Lab Room: LACC 409B |
Tuesday, March 6, 2018 8:00AM - 8:12AM |
E35.00001: Evidence for Chemical Vapor Induced 2H to 1T Phase Transition in Transition Metal Dichalcogenide Films Adam Friedman, Aubrey Hanbicki, F Perkins, Glenn Jernigan, James Culbertson, Paul Campbell Transition metal dichalcogenides (TMDs) show remarkable potential for use in chemical vapor sensor devices. They are inexpensive, inherently flexible, low-power, can be grown in large areas, and have shown high sensitivity and selectivity to electron donor analyte molecules. We present both conductance and optical evidence that the phase transition can be induced in MoX2 films by a saturating dose of strong electron donor vapor. We find that the conductance response to strong electron donors in both monolayer MoS2 and MoSe2 FET devices ceases after moderate exposure, with final value of the conductance being on order of that expected for the 1T phase. We also examine chemically exposed TMD films intermittently interrogated with Raman and photoluminescence spectroscopy. We observe the appearance of weak characteristic 1T phase Raman features for MoS2 and we observed a quenching of the photoluminescence of both TMD films that is recoverable with annealing. We compare the electronic and optical data to what would be expected for doping mechanisms or oxidation alone, and we conclude that the data cannot by explained solely by doping mechanisms. Our results suggest a mechanism for a new type of optoelectronic chemical vapor sensor. |
Tuesday, March 6, 2018 8:12AM - 8:24AM |
E35.00002: In pursuit of barrierless transition metal dichalcogenides lateral heterojunctions Oguz Gulseren, Yierpan Aierken, Cem Sevik, François Peeters, Deniz Cakir Two-dimensional materials are expected to become key components for novel applications for not only electronic devices but also for energy storage applications including super capacitors and batteries because of their exotic properties. Fully understanding of most of material properties needs an atomistic description from quantum mechanics. In this respect, we investigated the impact of charge and substitutional atom doping by replacing Mo atoms with either Re or Ta on the electronic transport properties of the hybrid metallic-semiconducting lateral junctions, formed between metallic (1T and 1Td) and semiconducting (2H) phases of MoS2 by means of first-principles and non-equilibrium Green function formalism based calculations. Our results clearly revealed the strong influence of interface and crystallographic orientation of the metallic phase on the transport properties of these systems. The Schottky barrier height, which is the dominant mechanism for contact resistance, was found to be as large as 0.63 eV and 1.19 eV for holes and electrons, respectively. |
Tuesday, March 6, 2018 8:24AM - 8:36AM |
E35.00003: Interface-Phase-Shift-Induced Interference and Enhanced Absorption of MoS2 Monolayers Eunah Kim, Jin-Woo Cho, Bo Ra Kim, Trang Thi Thu Nguyen, Sun-Kyung Kim, Seokhyun Yoon, Dong-Wook Kim The extremely thin nm-thick MoS2 layers limit optical absorption in spite of their very large absorption coefficients in visible wavelength range. To enhance the light-matter interaction in MoS2 layers, researchers have tried several approaches, using plasmonic metal nanostructures, Fabry-Perot-type cavities, and photonic nanostructures. In this work, we investigated how reflection and transmission phase-shift at the highly absorbing MoS2 interface could affect the absorption spectra of the MoS2 monolayers on SiO2/Si substrates (SiO2 thickness: 40 ~ 130 nm). Such interface-phase-shift gave rise to interference in MoS2 layer, although the layer thickness was only 0.7 nm, much smaller than the wavelength. We compared measured and calculated optical reflection spectra, which showed that the interference enhanced optical absorption in MoS2 monolayers in whole visible wavelength range. Raman intensity of MoS2 monolayers largely varied depending on the SiO2 thickness, which could be well explained by the interference-enhanced absorption in MoS2 layers. This work showed that proper choice of the SiO2 thickness could provide us a simple and useful means to improve broadband optical absorption in MoS2 monolayers. |
Tuesday, March 6, 2018 8:36AM - 8:48AM |
E35.00004: Strain Controlled Charge Density Wave Transition in 1T-TaS2 Films Xinyuan Lai, Zhenyuan Zhang, Jinhai Mao, Junxi Duan, Guohong Li, Michael Altvater, Eva Andrei We study the influence of strain on the phase transition between the Near Commensurate and the Commensurate Charge Density Wave states in thin films of 1T-TaS2. CDWs emerge due to the enhanced coupling between the conducting electrons and the crystal lattice when they are almost commensurate with each other. As a result, the formation of CDWs is sensitive to changes in the structure induced by external perturbations such as strain. To introduce strain in a controllable fashion we employed two methods. The first consisted of depositing thin 1T-TaS2 films onto arrays of micron size Au pillars and utilized the capillary force to introduce the strain. The pillars were supported on a highly doped Si substrate capped by 300nm of SiO2. The amount of strain was controlled by varying the pillars spacing. In any given sample the pillar spacing was varied in order to achieve an in-situ comparison of the different strain magnitudes. In the second method we introduced strain in suspended thin 1T-TaS2 films over trenches etched into SiO2 by utilizing gate induced electrostatic force. Our results show that the CDW transition temperature increases with the amount of strain, and offer direct evidence that strain provides a mechanism to stabilize the CCDW phase. |
Tuesday, March 6, 2018 8:48AM - 9:00AM |
E35.00005: Controlling the localization of topological gapless states in bilayer graphene with a gate voltage Marta Pelc, Wlodzimierz Jaskolski, Leonor Chico, Andres Ayuela, Garnett Bryant Bilayer graphene with stacking domain walls is known for having topologically protected gapless states when gated. If only the transition between the domains is defectless, the two states exist on each valley [1, 2]. Their number may be related to topological properties of the two domains. The origin and exact localization of such states, however, require more detailed study. We investigate these gapless states using atomistic lattice models, which permit to study their origin by following the formation of carbon bonds between layers. More importantly, we analyze the layer localization and show that it depends on the ratio of the gate potential to the interlayer hopping [3]. Two different regimes are thus defined for small and large gate voltages that may open a route for the use of topologically protected states in practical devices. |
Tuesday, March 6, 2018 9:00AM - 9:12AM |
E35.00006: Exploring the Electonic Properties of Strain-induced Grain Boundaries in the 2D Quantum Spin Hall State in 1T’-WSe2 Zahra Pedramrazi, Charlotte Herbig, Madeleine Philips, Dillon Wong, Yi Chen, Hsin-Zon Tsai, Shujie Tang, Hyejin Ryu, Artem Pulkin, Zahid Hussain, Sung-Kwan Mo, Zhi-Xun Shen, Oleg Yazyev, Eugene Mele, Michael Crommie Monolayer group VI transition metal dichalcogenides (MX2 group, where M is W or Mo and X is S, Se or Te) host novel electronic phases including a 2D quantum spin Hall (QSH) state in the 1T’ structure. The 1T’ phase has three orientational variants. Using scanning tunneling microscopy, we induce strain on monolayer 1T’-WSe2 islands, which results in switching between these three orientations and creates grain boundaries between two topologically nontrivial 1T’ domains. The electronic structure of these grain boundaries is then explored via scanning tunneling spectroscopy and compared with calculations of confined electronic modes on the grain boundary. |
Tuesday, March 6, 2018 9:12AM - 9:24AM |
E35.00007: Spin-Valley Coherent Phases in Bilayer Graphene at Charge Neutrality Ganpathy Murthy, Efrat Shimshoni, Herbert Fertig Bernal-stacked Bilayer Graphene in the quantum Hall regime can be tuned by a Zeeman field and an electric field perpendicular to the layers. The manifold of Landau levels near charge neutrality is 8-fold near-degenerate. We theoretically analyse this system at $\nu=0$ by using the Hartree-Fock approximation on a Hamiltonian that contains, in addition to the long-range Coulomb interaction, short-range interactions with valley anisotropy. The Hamiltonian has U(1)$_{valley}\times$U(1)$_{spin}$ symmetry. A crucial ingredient in our analysis is the nonperturbative treatment of the trigonal warping parameter $t_3$. We find a new phase that breaks both the U(1) symmetries, and occurs for small values of $B_{\perp}$ and intermediate values of the perpendicular electric field. This phase, of realized in experimental samples, would support multiple flavors of merons and skyrmions. |
Tuesday, March 6, 2018 9:24AM - 9:36AM |
E35.00008: Phase Transitions in Tetralayer Graphene at Charge Neutrality Shi Che, Yanmeng Shi, Ruoyu Chen, Jiawei Yang, Takashi Taniguchi, Kenji Watanabe, Dmitry Smirnov, Yafis Barlas, Chun Ning Lau As Bernal-stacked tetralayer graphene (4LG) hosts two intersecting massive Dirac bands, it supports an excellent platform for investigating multiband transport. Here we report studies on 4LG at the lowest Landau level half-filling in very high magnetic field, where the effect of Coulomb interactions is considerable. By exploring the dependence on magnetic and displacement fields of ν=0 quantum Hall state, the experimental result presents intriguing phase diagram of ground states. In particular, the tilted field magnetotransport reveals the interplay between crystal symmetry and Coulomb interactions in multiple Dirac band systems. |
Tuesday, March 6, 2018 9:36AM - 9:48AM |
E35.00009: Understanding Phase Transition of Epitaxial Two Layer Graphene during AFM Nanoindentation Tengfei Cao Phase transition of epitaxial multilayer graphene is systematically studied by combining atomic force microscopy (AFM) detection and density functional theory (DFT) calculation. Experimental results reveal that mechanical response of graphene to AFM indentation is dependent on layer numbers. The stiffness of bilayer graphene is extremely large and is comparable to diamond, and that of multilayer graphene (layer number larger than 3) is much smaller than SiC substrate. DFT simulations demonstrate that the ultrahardness two layer graphene comes from graphene/diamond film phase transition under nanoindentations. Similar evolution process of multilayer graphene is impeded by stacking patterns and high energy barriers. Graphene/SiC substrate interface greatly impacts phase transition process by inducing certain layer deformation of graphene, and by increasing carbon layer interactions. Surface dangling bonds saturation is prerequisite for diamond film formation. It not only determines phase transition barriers, but also stabilizes final diamond film. |
Tuesday, March 6, 2018 9:48AM - 10:00AM |
E35.00010: Ferroelectric quantum Hall phase revealed by visualizing interference of Landau level wavefunctions Mallika Randeria, Benjamin Feldman, Fengcheng Wu, Hao Ding, Andras Gyenis, Huiwen Ji, Robert Cava, Allan MacDonald, Ali Yazdani Novel broken symmetry states can spontaneously form due to Coulomb interactions in electronic systems with multiple internal degrees of freedom. A powerful approach to distinguish among such states in two-dimensional systems is to visualize their wavefunctions with a scanning tunneling microscope (STM)1. Here we investigate bismuth surface states, where strong spin-orbit coupling leads to six degenerate teardrop-shaped hole pockets. Our spectroscopic measurements reveal that this valley degeneracy is fully lifted at high magnetic field as a result of exchange interactions. We image the corresponding Landau level (LL) wavefunctions with a STM to address the nature of valley ordering for the case of a singly degenerate LL. The spatial patterns we observe around isolated defects contain unique signatures of interference between spin-textured valleys, which identifies the electronic ground state as a quantum Hall ferroelectric. Our observations confirm the recent prediction2 that interactions in strongly anisotropic valley systems favor the occupation of a single valley, which gives rise to ferroelectricity in bismuth. |
Tuesday, March 6, 2018 10:00AM - 10:12AM |
E35.00011: Nonlocal Optical Response in Topological Phase Transitions in the Graphene Family Lilia Woods, Pablo Rodriguez Lopez, Wilton De Melo Kort-Kamp, Diego Alejandro Dalvit We investigate the electromagnetic response of staggered two-dimensional materials of the graphene family, including graphene, silicene, germanene, and stanene, as they are driven through various topological phase transitions using external fields. Utilizing Kubo formalism, we compute their optical conductivity tensor taking into account the frequency and wave vector of the electromagnetic excitations, and study its behavior over the full electronic phase diagram of the materials. We compute the Plasmon dispersion relation for different phases. |
Tuesday, March 6, 2018 10:12AM - 10:24AM |
E35.00012: Observing a Scale Anomaly and A Universal Quantum Phase Transition in Graphene Omrie Ovdat, Jinhai Mao, Yuhang Jiang, Eva Andrei, Eric Akkermans One of the most interesting predictions resulting from quantum physics, is the violation of classical symmetries, collectively referred to as anomalies. A remarkable class of anomalies occurs when the continuous scale symmetry of a scale free quantum system is broken into |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E35.00013: Existence of non-trivial topological insulating phases in van der Waals heterostructure Sushant Behera, PRITAM DEB van der Waals heterostructures, formed of two dimensional materials with fascinating structural, electronic and magnetic properties, are now |
Tuesday, March 6, 2018 10:36AM - 10:48AM |
E35.00014: On the nature of the magnetism in Fe3GeTe2 Tom Berlijn, Giang Nguyen, Stuart Calder, Qiang Zou, Huibo Cao, Andrew May, Zheng Gai, An-Ping Li Magnetic ordering in layered van der Waals materials is not only of fundamental scientific interest, but also offers a pathway to tailor magnetic properties via confinement and heterostucture engineering. Recent studies have reported both ferromagnetic and antiferromagnetic order in the quasi 2D Fe3GeTe2.[1,2] In this talk we will address these seemingly contradicting results based on first principles calculations, scanning tunneling microscopy and inelastic neutron scattering. [1] V. Y. Verchenko et al, Inorg. Chem. 54, 8598 (2015) [2] J. Yi et al, 2D Mater 4, 011005 (2017) |
Tuesday, March 6, 2018 10:48AM - 11:00AM |
E35.00015: First-principles Calculations for the Change of Carrier-type in 2D MoTe2 by Laser Ilumination Eun-Ae Choi, Kyung Song, Si-Young Choi Nowadays, 2D TMD materials, MoTe2, have emerged as favorable candidates for replacing current Si devices at several nanometer levels with excellent properties for future electronic devices. It is very important to control the carrier-type to make complementary logic devices. There have been many attempts to control the polarity of carriers in 2D MoTe2, but there have been few reports on the simultaneous implementation of n-type and p-type in a single MoTe2 nanosheet. Recently, a study has been reported that made a p-n junction diode by selectively converting intrinsic n-type to p-type using laser illumination at 2D MoTe2 nanosheet with a thinkness of 20-30nm. Raman spectroscopic results showed that Te interstitial and MoO2 were generated after illumination. However, the exact cause of the carrier-type change by illumination was not clarified. In this study, we have found the stable structures of Te interstitial defects through the first-principles calculations and characterize them in each of the van der Waals gap and surface of MoTe2. Based on the simulation results, we explain the mechanism that the Te interstitial defect changes the carrier-type in 2D MoTe2. |
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