Yuxuan Jiang

EELS analysis of cation valence states and oxygen vacancies in magnetic oxides

Transition metal oxides are a class of materials that are vitally important for developing new materials with functionality and smartness. The unique properties of these materials are related to the presence of elements with mixed valences of transition elements. Electron energy-loss spectroscopy (EELS) in the transmission electron microscope is a powerful technique for measuring the valences of some transition metal elements of practical importance. This paper reports our current progress in applying EELS for quantitative determination of Mn and Co valences in magnetic oxides, including valence state transition, quantification of oxygen vacancies, refinement of crystal structures, and identification of the structure of nanoparticles.

Studies of Mn valence conversion and oxygen vacancies in La1−xCaxMnO3−y using electron energy-loss spectroscopy

Using the white line intensities, electron energy-loss spectroscopy in a transmission electron microscope has been employed to characterize the valence conversion and oxygen vacancies in La1−xCaxMnO3−y. For a nominal doping composition x=0.33, the ratio of Mn4+ to Mn3+ is determined to be more than 0.25 but less than 0.5, and the content of oxygen vacancy y is no more than 0.065 (equivalent to 2.2 at. % of the oxygen content). At ymax=0.065, 60% of the residual charge introduced by Ca doping is balanced by the conversion of Mn3+to Mn4+ and 40% by oxygen vacancy.

Synthesis and properties of green phosphor SrGa2S4:Eu2+ for field emission displays by an environmentally clean technique

An environmentally clean synthesis technique is reported for preparing the green phosphor SrGa2S4:Eu 21 . This method is based on solid state chemical reactions with the use of raw materials: sodium dimethyldithiocarbomate, tetramethylammonium chloride, gallium nitrate, europium nitrate, strontium sulfide and sulfur. The method is shown to produce powders with quasispherical shape, smooth surface morphologies, uniform particle size distribution and high luminous efficiencies (,20.5 lm/W at 2 kV and 30.8 lm/W at 5 kV). q 2000 Elsevier Science Ltd. All rights reserved.

Probing terahertz surface plasmon waves in graphene structures

Epitaxial graphene mesas and ribbons are investigated using terahertz (THz) near-field microscopy to probe surface plasmon excitation and THz transmission properties on the sub-wavelength scale. The THz near-field images show variation of graphene properties on a scale smaller than the wavelength, and excitation of THz surface waves occurring at graphene edges, similar to that observed at metallic edges. The Fresnel reflection at the substrate SiC/air interface is also found to be altered by the presence of graphene ribbon arrays, leading to either reduced or enhanced transmission of the THz wave depending on the wave polarization and the ribbon width.

Probing the semiconductor to semimetal transition in InAs/GaSb double quantum wells by magneto-infrared spectroscopy

We perform a magneto-infrared spectroscopy study of the semiconductor to semimetal transition of InAs/GaSb double quantum wells from the normal to the inverted state. We show that owing to the low carrier density of our samples (approaching the intrinsic limit), the magneto-absorption spectra evolve from a single cyclotron resonance peak in the normal state to multiple absorption peaks in the inverted state with distinct magnetic field dependence. Using an eight-band Pidgeon-Brown model, we explain all the major absorption peaks observed in our experiment. We demonstrate that the semiconductor to semimetal transition can be realized by manipulating the quantum confinement, the strain, and the magnetic field. Our work paves the way for band engineering of optimal InAs/GaSb structures for realizing novel topological states as well as for device applications in the terahertz regime.

Landau-level spectroscopy of massive Dirac fermions in single-crystalline ZrTe 5 thin flakes

Y. Jiang, Z. L. Dun, H. D. Zhou, Z. Lu, 4 K.-W. Chen, 4 S. Moon, 4 T. Besara, T. M. Siegrist, 5 R. E. Baumbach, D. Smirnov, and Z. Jiang ∗ School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332 Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996 National High Magnetic Field Laboratory, Tallahassee, Florida 32310 Department of Physics, Florida State University, Tallahassee, Florida 32306 Department of Chemical and Biomedical Engineering, Florida A&M University-Florida State University (FAMU-FSU) College of Engineering, Florida State University, Tallahassee, Florida 32310 (Dated: March 27, 2017)

Efficient generation of neutral and charged biexcitons in encapsulated WSe2 monolayers

Higher-order correlated excitonic states arise from the mutual interactions of excitons, which generally requires a significant exciton density and therefore high excitation levels. Here, we report the emergence of two biexcitons species, one neutral and one charged, in monolayer tungsten diselenide under moderate continuous-wave excitation. The efficient formation of biexcitons is facilitated by the long lifetime of the dark exciton state associated with a spin-forbidden transition, as well as improved sample quality from encapsulation between hexagonal boron nitride layers. From studies of the polarization and magnetic field dependence of the neutral biexciton, we conclude that this species is composed of a bright and a dark excitons residing in opposite valleys in momentum space. Our observations demonstrate that the distinctive features associated with biexciton states can be accessed at low light intensities and excitation densities.High-order correlated states in atomically thin transition metal dichalcogenides may be facilitated by long-lived optically dark excitons. Here, the authors report experimentally the emergence of neutral and charged biexciton species at low light intensities in encapsulated WSe2 monolayers.

Engineering Dirac materials: metamorphic InAs1-xSbx/InAs1-ySby superlattices with ultra-low bandgap

Quasiparticles with Dirac-type dispersion can be observed in nearly gapless bulk semiconductors alloys in which the bandgap is controlled through the material composition. We demonstrate that the Dirac dispersion can be realized in short-period InAs1-xSbx/InAs1-ySby metamorphic superlattices with the bandgap tuned to zero by adjusting the superlattice period and layer strain. The new material has anisotropic carrier dispersion: the carrier energy associated with the in-plane motion is proportional to the wave vector and characterized by the Fermi velocity vF, and the dispersion corresponding to the motion in the growth direction is quadratic. Experimental estimate of the Fermi velocity gives vF = 6.7 × 105 m/s. Remarkably, the Fermi velocity in this system can be controlled by varying the overlap between electron and hole states in the superlattice. Extreme design flexibility makes the short-period metamorphic InAs1-xSbx/InAs1-ySby superlattice a new prospective platform for studying the effects of charge-carrier chirality and topologically nontrivial states in structures with the inverted bandgaps.

Negative magnetoresistance in Ti-cleaned single-layer graphene

Symmetric graphene tunnelingfield-effect transistors (SymFETs) consisting of two independently gated graphene layers separated by a potential barrier have been proposed as a room-temperature resonant tunneling device. In a SymFET, the inelastic-coherent length (L) of the electrons is considered to be one of the important characteristic scattering lengths. Weak localization (WL) is a good tool to estimate L In this study, the surface of the chemical-vapor- deposited single-layer graphene was Ticleaned after performing the conventional fabrication processes for graphene transistors. We found that the charge-neutral point (VCNP) shifted to a lower back-gate voltage and the mobility increased owing to Ticleaning. Ti-cleaned Hall bars were investigated at 0.3 K under magnetic fields of up to 14 T. Negative magnetoresistance(MR) appears because of the WL effect, and the MR increases as the back-gate voltage (VG) approaches VCNP. From a fitting analysis using the theoretical formulation of WL, we found that L was greater than 100 nm and that L decreased as VG approached VCNP because of electron-hole puddles and electron-electron interaction.

THz near-field microscopy of graphene structures

Properties of graphene can be tuned electrically and chemically, providing a promising system for application in terahertz (THz) devices. Graphene response can be enhanced even further by means of coupling electromagnetic waves into plasmon modes, frequency of which is controlled by geometrical parameters. To probe excitation of confined plasmon modes and surface wave excitation, epitaxial graphene and its structures are investigated using THz near-field microscopy. Detected near-field images suggest excitation of THz surface waves occurring at graphene edges, similar to that observed at metallic edges, and excitation of confined plasmon modes. We will also discuss the impact of graphene inhomogeneity on local THz transmission properties on the sub-wavelength scale.