arXiv subjects

J. G. Hou

Publications and source records attributed to J. G. Hou.

At least 19 recordsLinked to original sources

arXiv2019

Visually Constructing the Chemical Structure of a Single Molecule by Scanning Raman Picoscopy

The strong spatial confinement of a nanocavity plasmonic field has made it possible to visualize the inner structure of a single molecule and even to distinguish its vibrational modes in real space. With such ever-improved spatial resolution, it is anticipated that full vibrational imaging of a molecule could be achieved to reveal molecular structural details. Here we demonstrate full Raman images of individual vibrational modes on the Ångström level for a single Mg-porphine molecule, revealing distinct characteristics of each vibrational mode in real space. Furthermore, by exploiting the underlying interference effect and Raman fingerprint database, we propose a new methodology for structural determination, coined as scanning Raman picoscopy, to show how such ultrahigh-resolution spectromicroscopic vibrational images can be used to visually assemble the chemical structure of a single molecule through a simple Lego-like building process.

physics.chem-ph↗

arXiv2012

Bipolar Magnetic Semiconductors: A New Class of Spintronics Materials

Electrical control of spin polarization is very desirable in spintronics, since electric field can be easily applied locally in contrast with magnetic field. Here, we propose a new concept of bipolar magnetic semiconductor (BMS) in which completely spin-polarized currents with reversible spin polarization can be created and controlled simply by applying a gate voltage. This is a result of the unique electronic structure of BMS, where the valence and conduction bands possess opposite spin polarization when approaching the Fermi level. Our band structure and spin-polarized electronic transport calculations on semi-hydrogenated single-walled carbon nanotubes confirm the existence of BMS materials and demonstrate the electrical control of spin-polarization in them.

cond-mat.mtrl-sci↗

arXiv2011

Evidence of Photocatalytic Dissociation of Water on TiO2 with Atomic Resolution

Photocatalytic water splitting reaction on TiO2 surface is one of the fundamental issues that bears significant implication in hydrogen energy technology and has been extensively studied. However, the existence of the very first reaction step, the direct photo-dissociation of water, has been disregarded. Here, we provide unambiguously experimental evidence to demonstrate that adsorbed water molecules on reduced rutile TiO2(110)-1\times1 surface can be dissociated under UV irradiation using low temperature scanning tunneling microscopy. It is identified that a water molecule at fivefold coordinated Ti (Ti5c) site can be photocatalytically dissociated, resulting in a hydroxyl at Ti5c and another hydroxyl at bridge oxygen row. Our findings reveal a missing link in the photocatalytic water splitting reaction chain, which greatly contribute to the detailed understanding of underlying mechanism.

physics.chem-ph↗

arXiv2011

Vacancy Induced Splitting of Dirac Nodal Point in Graphene

We investigate the vacancy effects on quasiparticle band structure of graphene near the Dirac point. It is found that each Dirac nodal point splits into two new nodal points due to the coherent scattering among vacancies. The splitting energy between the two nodal points is proportional to the square root of vacancy concentration. In addition, an extra dispersionless impurity band of zero energy due to particle-hole symmetry is found. Our theory offers an excellent explanation to the recent experiments.

cond-mat.quant-gas↗

arXiv2011

Atomic structure, energetics, and dynamics of topological solitons in Indium chains on Si(111) surfaces

Based on scanning tunneling microscopy and first-principles theoretical studies, we characterize the precise atomic structure of a topological soliton in In chains grown on Si(111) surfaces. Variable-temperature measurements of the soliton population allow us to determine the soliton formation energy to be ~60 meV, smaller than one half of the band gap of ~200 meV. Once created, these solitons have very low mobility, even though the activation energy is only about 20 meV; the sluggish nature is attributed to the exceptionally low attempt frequency for soliton migration. We further demonstrate local electric field-enhanced soliton dynamics.

cond-mat.mes-hall↗

arXiv2011

Chemical Vapor Deposition Growth of Graphene using Other Hydrocarbon Sources

Graphene has attracted a lot of research interests due to its exotic properties and a wide spectrum of potential applications. Chemical vapor deposition (CVD) from gaseous hydrocarbon sources has shown great promises for large-scale graphene growth. However, high growth temperature, typically 1000°C, is required for such growth. Here we demonstrate a revised CVD route to grow graphene on Cu foils at low temperature, adopting solid and liquid hydrocarbon feedstocks. For solid PMMA and polystyrene precursors, centimeter-scale monolayer graphene films are synthesized at a growth temperature down to 400°C. When benzene is used as the hydrocarbon source, monolayer graphene flakes with excellent quality are achieved at a growth temperature as low as 300°C. The successful low-temperature growth can be qualitatively understood from the first principles calculations. Our work might pave a way to undemanding route for economical and convenient graphene growth.

cond-mat.mtrl-sci↗

arXiv2010

Nearly Free Electron States in Graphene Nanoribbon Superlattices

Nearly free electron (NFE) state is an important kind of unoccupied state in low dimensional systems. Although it is intensively studied, a clear picture on its physical origin and its response behavior to external perturbations is still not available. Our systematic first-principles study based on graphene nanoribbon superlattices suggests that there are actually two kinds of NFE states, which can be understood by a simple Kronig-Penney potential model. An atom-scattering-free NFE transport channel can be obtained via electron doping, which may be used as a conceptually new field effect transistor.

cond-mat.mtrl-sci↗

arXiv2010

Topological Transition of Graphene from Quantum Hall Metal to Quantum Hall Insulator at $ν=0$

The puzzle of recently observed insulating phase of graphene at filling factor $ν=0$ in high magnetic field quantum Hall (QH) experiments is investigated. We show that the magnetic field driven Peierls-type lattice distortion (due to the Landau level degeneracy) and random bond fluctuations compete with each other, resulting in a transition from a QH-metal state at relative low field to a QH-insulator state at high enough field at $ν=0$. The critical field that separates QH-metal from QH-insulator depends on the bond fluctuation. The picture explains well why the field required for observing the insulating phase is lower for a cleaner sample.

cond-mat.dis-nn↗

arXiv2010

Accurate evaluation of the Green's function of disordered graphenes

An accurate simulation of Green's function and self-energy function of non-interacting electrons in disordered graphenes are performed. Fundamental physical quantities such as the elastic relaxation time τe, the phase velocity vp, and the group velocity vg are evaluated. New features around the Dirac point are revealed, showing hints that multi-scattering induced hybridization of Bloch states plays an important role in the vicinity of the Dirac point.

cond-mat.mes-hall↗

arXiv2009

Universal $\sqrt{2}\times\sqrt{2}$ structure and short-range charge order at the surfaces of BaFe$_{2-x}$Co$_{x}$As$_{2}$ compounds with various Co doping levels

The structure and electronic order at the cleaved (001) surfaces of the newly-discovered pnictide superconductors BaFe$_{2-x}$Co$_{x}$As$_{2}$ with x ranging from 0 to 0.32 are systematically investigated by scanning tunneling microscopy. A $\sqrt{2}\times\sqrt{2}$ surface structure is revealed for all the compounds, and is identified to be Ba layer with half Ba atoms lifted-off by combination with theoretical simulation. A universal short-range charge order is observed at this $\sqrt{2}\times\sqrt{2}$ surface associated with an energy gap of about 30 meV for all the compounds.

cond-mat.supr-con↗

arXiv2008

The shape of disorder broadened Landau subbands in graphene

Density of states (DOS) of graphene under a high uniform magnetic field and white-noise random potential is numerically calculated. The disorder broadened zero-energy Landau band has a Gaussian shape whose width is proportional to the random potential variance and the square root of magnetic field. Wegner-type calculation is used to justify the results.

cond-mat.mes-hall↗

arXiv2008

A first principles study on the electronic and magnetic properties of Ba$_{1-x}$K$_x$Fe$_2$As$_2$

We report a systematic first-principles study on the recent discovered superconducting Ba$_{1-x}$K$_x$Fe$_2$As$_2$ systems ($x$ = 0.00, 0.25, 0.50, 0.75, and 1.00). Previous theoretical studies strongly overestimated the magnetic moment on Fe of the parent compound BaFe$_2$As$_2$. Using a negative on-site energy $U$, we obtain a magnetic moment 0.83 $μ_B$ per Fe, which agrees well with the experimental value (0.87 $μ_B$). K doping tends to increase the density of states at fermi level. The magnetic instability is enhanced with light doping, and is then weaken by increasing the doping level. The energetics for the different K doping sites are also discussed.

cond-mat.supr-con↗

arXiv2008

Electronic structures of organic molecule encapsulated BN nanotubes under transverse electric field

The electronic structures of boron nitride nanotubes (BNNTs) doped by different organic molecules under a transverse electric field were investigated via first-principles calculations. The external field reduces the energy gap of BNNT, thus makes the molecular bands closer to the BNNT band edges and enhances the charge transfers between BNNT and molecules. The effects of the electric field direction on the band structure are negligible. The electric field shielding effect of BNNT to the inside organic molecules is discussed. Organic molecule doping strongly modifies the optical property of BNNT, and the absorption edge is red-shifted under static transverse electric field.

cond-mat.mtrl-sci↗

arXiv2008

Electronic Structures of SiC Nanoribbons

Electronic structures of SiC nanoribbons have been studied by spin-polarized density functional calculations. The armchair nanoribbons are nonmagnetic semiconductor, while the zigzag nanoribbons are magnetic metal. The spin polarization in zigzag SiC nanoribbons is originated from the unpaired electrons localized on the ribbon edges. Interestingly, the zigzag nanoribbons narrower than $\sim$4 nm present half-metallic behavior. Without the aid of external field or chemical modification, the metal-free half-metallicity predicted for narrow SiC zigzag nanoribbons opens a facile way for nanomaterial spintronics applications.

cond-mat.mtrl-sci↗

arXiv2008

A first principles study on organic molecules encapsulated BN nanotubes

The electronic structures of boron nitride nanotubes (BNNTs) doped by organic molecules are investigated with density functional theory. Electrophilic molecule introduces acceptor states in the wide gap of BNNT close to the valence band edge, which makes the doped system a $p$-type semiconductor. However, with typical nucleophilic organic molecules encapsulation, only deep occupied molecular states but no shallow donor states are observed. There is a significant electron transfer from BNNT to electrophilic molecule, while the charge transfer between nucleophilic molecule and BNNT is neglectable. When both electrophilic and nucleophilic molecules are encapsulated in the same BNNT, large charge transfer between the two kinds of molecules occurs. The resulted small energy gap can strongly modify the transport and optical properties of the system.

cond-mat.mtrl-sci↗

arXiv2008

Electronic Structure in Gapped Graphene with Coulomb Potential

In this paper, we numerically study the bound electron states induced by long range Coulomb impurity in gapped graphene and the quasi-bound states in supercritical region based on the lattice model. We present a detailed comparison between our numerical simulations and the prediction of the continuum model which is described by the Dirac equation in (2+1)-dimensional Quantum Electrodynamics (QED). We also use the Fano's formalism to investigate the quasi-bound state development and design an accessible experiments to test the decay of the supercritical vacuum in the gapped graphene.

cond-mat.mes-hall↗

arXiv2008

Half Metallicity in Hybrid BCN Nanoribbons

We report a first-principles electronic-structure calculation on C and BN hybrid zigzag nanoribbons. We find that half-metallicity can arise in the hybrid nanoribbons even though stand-alone C or BN nanoribbon possesses a finite band gap. This unexpected half-metallicity in the hybrid nanos-tructures stems from a competition between the charge and spin polarizations, as well as from the pi orbital hybridization between C and BN. Our results point out a possibility of making spintronic devices solely based on nanoribbons and a new way of designing metal-free half metals.

cond-mat.mtrl-sci↗

arXiv2007

Will Zigzag Graphene Nanoribbon Turn to Half Metal under Electric Field?

At B3LYP level of theory, we predict that the half-metallicity in zigzag edge graphene nanoribbon (ZGNR) can be realized when an external electric field is applied across the ribbon. The critical electric field to induce the half-metallicity decreases with the increase of the ribbon width. Both the spin polarization and half-metallicity are removed when the edge state electrons fully transferred from one side to the other under very strong electric field. The electric field range under which ZGNR remain half-metallic increases with the ribbon width. Our study demonstrates a rich field-induced spin polarization behavior, which may leads to some important applications in spinstronics.

cond-mat.mtrl-sci↗