Events

Seminar


Seminar calendar : 02-Oct | 23-Oct | 20-Nov


Signature of Helical Transport in Quantum Spin Hall Insulator Atomically Thin Na3Bi

Who: Prof Michael Fuhrer, Monash University, Australia
When: Wed, 02-Oct-2019, 11am
Where: Physics Conference Room (S11-02-07)
Host: Assoc Prof Shaffique Adam

Abstract:
I will discuss our work on films of Na3Bi (a topological Dirac semimetal) which, when thinned to a few atomic layers, becomes a large bandgap (>300 meV) quantum spin Hall insulator, with an electric-field tuned topological-to-conventional transition, making it a promising platform for topological electronics[1-5]. We study thin films of Na3Bi grown in ultra-high vacuum by molecular beam epitaxy[1], characterized with electronic transport, scanning tunneling microscopy (STM), and angle-resolved photoemission spectroscopy. When thinned to a few atomic layers Na3Bi is a large gap (>300 meV) 2D topological insulator with topologically protected edge modes observable in STM[4]. Electric field applied perpendicular to the Na3Bi film, by potassium doping or by proximity of an STM tip, closes the bandgap completely and reopens it as a conventional insulator[4]. To enable electrical transport experiments, we prepare ultra-thin Na3Bi on insulating sapphire. A non-local transport measurement design allows us to observe edge conduction in millimeter-scale samples when the Fermi level is brought into the bulk gap by doping with acceptor molecule F4-TCNQ. In the edge conduction regime, helical conduction gives rise to a giant negative magnetoresistance due to suppression of spin-flip scattering which becomes inelastic in a magnetic field[5]. Comparison to a simple theoretical model indicates >98% of the resistance is due to spin-flip scattering, well beyond the limit of 2/3 for a generic non-helical metal with exchange-mediated scattering, providing an unambiguous signature of helical transport.

References:

  1. Nano Letters 16 (5), 3210-3214 (2016).
  2. Physical Review Materials 1, 054203 (2017).
  3. Science Advances 3, eaao6661 (2017).
  4. Nature 564, 390-394 (2018).
  5. arXiv:1906.01214

About the speaker:

Michael S. Fuhrer received his B.S. in Physics from the University of Texas at Austin in 1990, and Ph. D. in Physics from the University of California at Berkeley in 1998. After a postdoctoral appointment at Lawrence Berkeley National Laboratory, Fuhrer joined the faculty at the University of Maryland as an Assistant Professor in 2000, and from 2009-2012 was Professor, and Director of the Center for Nanophysics and Advanced Materials. In 2012 Fuhrer was awarded an ARC Laureate Fellowship, and moved to Monash University as Professor of Physics in 2013. Fuhrer founded the Monash Centre for Atomically Thin Materials, and directs the ARC Centre of Excellence in Future Low-Energy Electronics Technologies. Fuhrer's current research interests lie in understanding the electronic properties of atomically-thin materials (such as graphene and transition-metal chalcogenides), and topological materials. Fuhrer is a Fellow of the American Physical Society and the American Association for the Advancement of Science.



Natural Orbital Functional Theory for Multiplets

Who: Prof Mario Piris, University of the Basque Country/Donostia International Physics Centre, Spain
When: Wed, 23-Oct-2019, 11am
Where: Physics Conference Room (S11-02-07)
Host: Prof Antonio Helio Castro Neto

Abstract:
The energy of an electron system can be determined exactly from the knowledge of the one- and two-particle reduced density matrices (1- and 2-RDMs). In practical applications, we employ this exact energy functional but using an approximate 2-RDM that is built from the 1-RDM. Approximating the energy functional has important consequences: the theorems obtained for the exact functional of the 1-RDM [1] are no longer valid. As a consequence, the functional N-representability problem arises, that is, we have to comply the requirement that reconstructed 2-RDM must satisfy N-representability conditions to ensure a physical value of the approximate ground-state energy. In this presentation, the role of the N-representability in approximate one-particle functional theories [2] will be analyzed.

The 1-RDM functional is called Natural Orbital Functional (NOF) [3] when it is based upon the spectral expansion of the 1-RDM. Appropriate forms of the two-particle cumulant have led to different implementations [4], being the most recent an interacting-pair model called PNOF7 [5]. The latter is able to treat properly the whole static (non-dynamic) correlation and the intrapair dynamic correlation. The interpair electron correlation is recovered by the NOF-MP2 method [5]. In the second part of the presentation, a new NOF [6] that allows describing an electronic system with any value of the spin, that is, a new reconstruction of the 2-RDM for spin multiplets will be presented.

Calculation of ionization potentials of the first-row transition-metal atoms is presented as test case. The values obtained agree with those reported at the CCSD(T) and experimental data [6]. In addition, the correlation energies obtained by using PNOF7 for the two-dimensional square lattice Hubbard model of up to 144 electrons with different spin multiplicities are also presented. Our results are comparable to those of exact diagonalizations, density matrix renormalization group (DMRG) and auxiliary-field quantum Monte Carlo (AFQMC) calculations. Accurate results are obtained when particle-hole symmetry is broken away from half-filling. Finally, dissociation processes are studied in two-dimensional hydrogen networks in order to consider the missing interactions of the Hubbard model.

References:

  1. T. L. Gilbert, Phys. Rev. B 12, 2111 (1975); M. Levy, Proc. Natl. Acad. Sci. USA 76, 6062 (1979); S. M. Valone, J. Chem. Phys. 73, 1344 (1980).
  2. M. Piris, in Many-body approaches at different scales: a tribute to N. H. March on the occasion of his 90th birthday, Chap. 22, pp. 231-247. New York: Springer (2017).
  3. M. Piris, Adv. Chem. Phys. 134, 387-427 (2007).
  4. M. Piris, J. M. Ugalde, Int. J. Quantum Chem. 114, 1169-1175 (2014).
  5. M. Piris, Phys. Rev. Lett. 119, 063002 (2017); Phys. Rev. A 98, 022504 (2018).
  6. M. Piris, Phys. Rev. A 100 (2019). arXiv: 1908.05501 [physics.chem-ph]

About the speaker:
Mario Piris is research Professor. In 1988, he was awarded a master degree in Physics and Mathematics from the Lomonosov University of Moscow. He earned his PhD in 1997 from the University of Havana, and in 2000 got a position of Professor of Physics at the same university. He was awarded the Humboldt fellowship in 2001, and started working at the University of Erlangen, Germany. From 2005, he has been in the University of the Basque Country. In 2010, he got his current position of IKERBASQUE research Professor at the Donostia International Physics Center (DIPC). He has published around 100 scientific papers and presented his results in more than 50 international conferences. His work has always focused on the development of new methods far from mainstream. He is the lead author of the DoNOF program package.



Modifications of Transition Metal Dichalcogenides by Defect and Interface Engineering

Who: Prof Matthias Batzill, Department of Physics, University of South Florida, USA
When: Wed, 20-Nov-2019, 11am
Where: Physics Conference Room (S11-02-07)
Host: Prof Andrew Wee

Abstract:
Modifying and manipulating properties of 2D materials is central for achieving the potential of these materials. In our group we aim at synthesizing 2D materials by molecular beam epitaxy and study approaches for controlled modifications by alloying, doping, one-dimensional modifications (edges or grain boundaries) or interfacing with dissimilar materials. In the first part of the talk we present our studies on the formation of metallic mirror twin grain boundaries in MoSe21 or MoTe22 by incorporation of excess Mo into the lattice (see Figure). Very high density of MTB networks can be obtained in MoTe2 that effectively metallizes the material and thus may act as a metallic contact patch.3 Such line defects may also increase electrocatalytic properties for hydrogen evolution reactions.4 On a more fundamental level, we show that these 1D metallic grain boundaries host a one-dimensional electron gas and we present the first angle resolved photoemission (ARPES) studies of the electronic properties of such line defects. These studies show evidence for the presence of Tomonaga-Luttinger Liquid behavior.5 The second part of the talk focuses on the differences in charge density wave properties of metallic TMDCs in the monolayer compared to bulk materials. Here we focus on the layer dependent variations of the properties of VSe26 and VTe2.7 For both materials we observe a charge density wave (CDW) behavior in the monolayer limit compared to their bulk properties. From ARPES and DFT we can also show that CDW competes with ferromagnetic ordering in monolayer VSe2.6 In VTe2 we observe a structural transition from a distorted-1T’’ structure in the bulk to a simple hexagonal 1T structure in the monolayer. This structural transition is accompanied by a CDW instability for the monolayer that gives rise to a 4×4 CDW at low temperatures, in contrast to bulk VTe2 that does not exhibit a CDW instability.7

Figure: Line defect formation in 2H-MoTe2 with increasing Mo-deposition and the defect-induced band gap states measured by ARPES, showing linear (1D) Fermi-surface and dispersing bands.

References:

  1. Y Ma, S Kolekar, H Coy Diaz, J Aprojanz, I Miccoli, C Tegenkamp, M Batzill. Metallic Twin Grain Boundaries Embedded in MoSe2 Monolayers Grown by Molecular Beam Epitaxy. ACS Nano 11, 5130-5139 (2017)
  2. HC Diaz, Y Ma, R Chaghi, M Batzill. High density of (pseudo) periodic twin-grain boundaries in molecular beam epitaxy-grown van der Waals heterostructure: MoTe2/MoS2. Appl. Phys. Lett. 108, 191606 (2016)
  3. PM Coelho, HP Komsa, H Coy Diaz, Y Ma, AV Krasheninnikov, M Batzill. Post-Synthesis Modifications of Two-Dimensional MoSe2 or MoTe2 by Incorporation of Excess Metal Atoms into the Crystal Structure. ACS Nano 12, 3975-3984 (2018).
  4. T Kosmala, H Coy Diaz, HP Komsa, Y Ma, AV Krasheninnikov, M Batzill, S Agnoli. Metallic Twin Boundaries Boost the Hydrogen Evolution Reaction on the Basal Plane of Molybdenum Selenotellurides. Adv. Energy Mat. 1800031 (2018).
  5. Y Ma, et al. Angle resolved photoemission spectroscopy reveals spin charge separation in metallic MoSe2 grain boundary. Nature Commun. 8, 14231 (2017).
  6. P Coelho et al. Charge Density Wave State Suppresses Ferromagnetic Ordering in VSe2 Monolayers. J. Phys. Chem. C 123 14089-14096 (2019).
  7. P Coelho et al. Monolayer Modification of VTe2 and its Charge Density Wave. J. Phys. Chem. Lett. 10, 4987-4993 (2019).

About the speaker:
Dr. Batzill graduated from the University of Goettingen, Germany and obtained a PhD in Physics from the University of Newcastle upon Tyne, UK in 1999. He went on to do post-doc research at the Universities of Southern California in Los Angeles and Tulane University in New Orleans, before starting his independent research group at the University of South Florida (USF) in Tampa, USA in 2006, where he currently is a full Professor of Physics. He received a NSF-CAREER award and NSF creative extension award. He also has been a Hans-Fischer Fellow of the Technical University of Munich, Germany and won several outstanding research achievement and faculty awards at USF. He gave over 80 invited talks and published more than 120 publications with an H-index of 42 (google scholar). His research expertise is in the physics and chemistry of surfaces and interfaces with current focus on 2D materials.


 

Semestral Stargazing for Sem 1

Time / Location: 7pm - 10pm / NUS football Field

Dates: Friday 30 August 2019
Friday 13 September 2019
Friday 18 October 2019
Friday 1 November 2019

For more information and queries, please contact: Dr Abel Yang. Email: phyyja@nus.edu.sg, Tel: 6516 1246

 

Annular Solar Eclipse

Date / Location: Thursday 26 December 2019 / NUS football Field

Please visit website: SEAAN Meeting 2019 for more information.

 

Archive

  • Past seminars
    • "Charge Injection for 2D Materials Contact and Fractional Modeling Prof Ricky L. K. Ang, Singapore University of Technology and Design, Singapore, 11-Sep
    • Surprising Physics of Nanopore Transport by Prof Aleksei Aksementev, Department of Physics, The University of Illinois at Urbana-Champaign, USA, 6 Aug
    • "Spinning the Heat”: from Amorphous Graphene to Ballistic Spin Devices by Prof Stephan Roche, ICREA and Catalan Institute of Nanoscience and Nanotechnology, Spain, 04-Sep
    • Surprising Physics of Nanopore Transport by Prof Aleksei Aksementev, Department of Physics, The University of Illinois at Urbana-Champaign, USA, 6 Aug
    • 2D Atomic and Molecular Lattices: Rational Synthesis and New Properties by Prof. Thomas J. Kempa, Dept. of Chemistry, Dept. of Materials Science and Engineering, Johns Hopkins University, 24 Jul
    • Efficient and Stable Single-Layer Organic Light-Emitting Diodes by Prof Paul W.M. Blom, Max Planck Institute for Polymer Research, Mainz, Germany, 10 Jul
    • Non-Commutative Blahut-Arimoto Algorithms by Mr Navneeth Ramakrishnan, Imperial College London, 26 Jun
    • An Introduction to Ultrafast Nonlinear Refraction and Absorption by Prof David J. Hagan, University of Central Florida, 24 Jun
    • The Stability of Mechanosensing Force-Transmission Supramolecular Linkages by Prof Jie Yan (Department of Physics, Mechanobiology Institute at NUS), 24 Apr
    • The Molecule-2D Interface by Prof Andrew Wee (Department of Physics at NUS), 10 Apr
    • Synthesis, Properties and Applications of Novel 2D Materials by Jiadong Zhu (from the Liu Zheng group at NTU), 20 Mar
    • Signatures of Localization in Interacting Systems Coupled to a Nonequilibrium Bath by Xiansong Xu (from the Dario Poleti group at SUTD), 20 Mar
    • Discovering Novel 2D Materials for Advanced Heterostructures With Tailored Properties by Prof Kristian Sommer Thygesen (Department of Physics, Technical University of Denmark, Denmark), 20 Feb
    • Studying Quantum Interference with Superconducting Circuits by Dr Yvonne Yuan Gao (A*STAR quantum computing programme), 19 Feb
    • Towards controlled assemblies of ligand-stabilized noble metal nanoclusters by Prof. Hannu Häkkinen (University of Jyväskylä, Finland), 18 Feb
  • Germany Immersion Trip, 14 June - 5 July, Germany
  • 26th Annual IPS Astronomy Retreat 2019, 22 - 25 June 2019, 4D3N, Tiara Desearu Seaview Residence, Johor, Malaysia
  • Science Open House 2019, LT27 Foyer, 11 May
  • NUS Open Day @ Steven Riady Centre, UTown, 9 Mar