arXiv subjects

Feng Wang

Publications and source records attributed to Feng Wang.

At least 487 records · Page 27Linked to original sources

arXiv2014

A diffusive logistic problem with a free boundary in time-periodic environment: favorable habitat or unfavorable habitat

We study the diffusive logistic equation with a free boundary in timeperiodic environment. To understand the effect of the dispersal rate $d$, the original habitat radius $h_0$, the spreading capability $μ$, and the initial density $u_0$ on the dynamics of the problem, we divide the time-periodic habitat into two cases: favorable habitat and unfavorable habitat. By choosing $d$, $h_0$, $μ$ and $u_0$ as variable parameters, we obtain a spreading-vanishing dichotomy and sharp criteria for the spreading and vanishing in time-periodic environment. We introduce the principal eigenvalue $λ_1(d, α, γ, h(t), T)$ to determine the spreading and vanishing of the invasive species. We prove that if $λ_1(d, α, γ, h_0, T)\leq 0$, the spreading must happen; while if $λ_1(d, α, γ, h_0, T)> 0$, the spreading is also possible. Our results show that the species in the favorable habitat can establish itself if the dispersal rate is slow or the occupying habitat is large. In an unfavorable habitat, the species vanishes if the initial density of the species is small, while survive successfully if the initial value is big. Moreover, when spreading occurs, the asymptotic spreading speed of the free boundary is determined.

math.AP↗

arXiv2014

Characterization and manipulation of individual defects in insulating hexagonal boron nitride using scanning tunneling microscopy

Defects play a key role in determining the properties of most materials and, because they tend to be highly localized, characterizing them at the single-defect level is particularly important. Scanning tunneling microscopy (STM) has a history of imaging the electronic structure of individual point defects in conductors, semiconductors, and ultrathin films, but single-defect electronic characterization at the nanometer-scale remains an elusive goal for intrinsic bulk insulators. Here we report the characterization and manipulation of individual native defects in an intrinsic bulk hexagonal boron nitride (BN) insulator via STM. Normally, this would be impossible due to the lack of a conducting drain path for electrical current. We overcome this problem by employing a graphene/BN heterostructure, which exploits graphene's atomically thin nature to allow visualization of defect phenomena in the underlying bulk BN. We observe three different defect structures that we attribute to defects within the bulk insulating boron nitride. Using scanning tunneling spectroscopy (STS), we obtain charge and energy-level information for these BN defect structures. In addition to characterizing such defects, we find that it is also possible to manipulate them through voltage pulses applied to our STM tip.

cond-mat.mes-hall↗

arXiv2014

Brownian Motion of Arbitrarily Shaped Particles in Two-Dimensions

Here we implement microfabricated boomerang particles with unequal arm lengths as a model for non-symmetry particles and study their Brownian motion in a quasi-two dimensional geometry by using high precision single particle motion tracking. We show that due to the coupling between translation and rotation, the mean squared displacements of a single asymmetric boomerang particle exhibit a non-linear crossover from short time faster to long time slower diffusion, and the mean displacements for fixed initial orientation are non-zero and saturate out at long time. The measured anisotropic diffusion coefficients versus the tracking point position indicate that there exists one unique point, i.e. the center of hydrodynamic stress (CoH), at which all coupled diffusion coefficients vanish. This implies that in contrast to motion in 3D where the CoH only exists for high symmetry particles, the CoH always exists for Brownian motion in 2D. We develop an analytical model based on Langevin theory to explain the experimental results and show that among the 6 anisotropic diffusion coefficients only 5 are independent because the translation-translation coupling originates from the translation-rotation coupling. Finally we classify the behavior of 2D Brownian motion of arbitrarily shaped particles into four groups based on the particle shape symmetry group.

cond-mat.soft↗

arXiv2014

A survey and a molecular dynamics study on the (central) hydrophobic region of prion proteins

Prion diseases are invariably fatal neurodegenerative diseases that affect humans and animals. Unlike most other amyloid forming neurodegenerative diseases, these can be highly infectious. Prion diseases occur in a variety of species. They include the fatal human neurodegenerative diseases Creutzfeldt-Jakob Disease (CJD), Fatal Familial Insomnia (FFI), Gerstmann-Straussler-Scheinker syndrome (GSS), Kuru, the bovine spongiform encephalopathy (BSE or 'mad-cow' disease) in cattle, the chronic wasting disease (CWD) in deer and elk, and scrapie in sheep and goats, etc. Transmission across the species barrier to humans, especially in the case of BSE in Europe, CWD in North America, and variant CJDs (vCJDs) in young people of UK, is a major public health concern. Fortunately, scientists reported that the (central) hydrophobic region of prion proteins (PrP) controls the formation of diseased prions. This article gives a detailed survey on PrP hydrophobic region and does molecular dynamics studies of human PrP(110-136) to confirm some findings from the survey. The structural bioinformatics presented in this article can be helpful as a reference in three-dimensional images for laboratory experimental works to study PrP hydrophobic region.

physics.bio-ph↗

arXiv2014

Observation of giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor

Two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit novel electrical and optical properties and are emerging as a new platform for exploring 2D semiconductor physics. Reduced screening in 2D results in dramatically enhanced electron-electron interactions, which have been predicted to generate giant bandgap renormalization and excitonic effects. Currently, however, there is little direct experimental confirmation of such many-body effects in these materials. Here we present an experimental observation of extraordinarily large exciton binding energy in a 2D semiconducting TMD. We accomplished this by determining the single-particle electronic bandgap of single-layer MoSe2 via scanning tunneling spectroscopy (STS), as well as the two-particle exciton transition energy via photoluminescence spectroscopy (PL). These quantities yield an exciton binding energy of 0.55 eV for monolayer MoSe2, a value that is orders of magnitude larger than what is seen in conventional 3D semiconductors. This finding is corroborated by our ab initio GW and Bethe Salpeter equation calculations, which include electron correlation effects. The renormalized bandgap and large exciton binding observed here will have a profound impact on electronic and optoelectronic device technologies based on single-layer semiconducting TMDs.

cond-mat.mes-hall↗

arXiv2014

Modified Futaki invariant and equivariant Riemann-Roch formula

In this paper, we give a new version of the modified Futaki invariant for a test configuration associated to the soliton action on a Fano manifold. Our version will naturally come from toric test configurations defined by Donaldson for toric manifolds. As an application, we show that the modified $K$-energy is proper for toric invariant Kähler potentials on a toric Fano manifold.

math.DG↗

arXiv2014

Characterising motion types of G-band bright points in the quiet Sun

We study the motions of G band bright points (GBPs) in the quiet Sun to obtain the characteristics of different motion types. A high resolution image sequence taken with the Hinode/Solar Optical Telescope (SOT) is used, and GBPs are automatically tracked by segmenting 3D evolutional structures in a space time cube. After putting the GBPs that do not move during their lifetimes aside, the non stationary GBPs are categorized into three types based on an index of motion type. Most GBPs that move in straight or nearly straight lines are categorized into a straight motion type, a few moving in rotary paths into a rotary motion, and the others fall into a motion type we called erratic. The mean horizontal velocity is 2.18 km/s, 1.63 km/s and 1.33 km/s for straight, erratic and rotary motion type, respectively. We find that a GBP drifts at a higher and constant velocity during its whole life if it moves in a straight line. However, it has a lower and variational velocity if it moves in a rotary path. The diffusive process is ballistic, super and sub diffusion for straight, erratic and rotary motion type, respectively. The corresponding diffusion index and coefficients are 2.13 and 850 km2/s, 1.82 and 331 km2/s, 0.73 and 13 km2/s. In terms of direction of motion, it is homogeneous and isotropical, and usually persists between neighbouring frames, no matter what motion type a GBP belongs to.

astro-ph.SR↗

arXiv2014

Colloquium: Graphene spectroscopy

Spectroscopic studies of electronic phenomena in graphene are reviewed. A variety of methods and techniques are surveyed, from quasiparticle spectroscopies (tunneling, photoemission) to methods probing density and current response (infrared optics, Raman) to scanning probe nanoscopy and ultrafast pump-probe experiments. Vast complimentary information derived from these investigations is shown to highlight unusual properties of Dirac quasiparticles and many-body interaction effects in the physics of graphene.

cond-mat.mes-hall↗

arXiv2014

Ultrafast Charge Transfer in Atomically Thin MoS2/WS2 Heterostructures

Van der Waals heterostructures have recently emerged as a new class of materials, where quantum coupling between stacked atomically thin two-dimensional (2D) layers, including graphene, hexagonal-boron nitride, and transition metal dichalcogenides (MX2), give rise to fascinating new phenomena. MX2 heterostructures are particularly exciting for novel optoelectronic and photovoltaic applications, because 2D MX2 monolayers can have an optical bandgap in the near-infrared to visible spectral range and exhibit extremely strong light-matter interactions. Theory predicts that many stacked MX2 heterostructures form type-II semiconductor heterojunctions that facilitate efficient electron-hole separation for light detection and harvesting. Here we report the first experimental observation of ultrafast charge transfer in photo-excited MoS2/WS2 heterostructures using both photoluminescence mapping and femtosecond (fs) pump-probe spectroscopy. We show that hole transfer from the MoS2 layer to the WS2 layer takes place within 50 fs after optical excitation, a remarkable rate for van der Waals coupled 2D layers. Such ultrafast charge transfer in van der Waals heterostructures can enable novel 2D devices for optoelectronics and light harvesting.

cond-mat.mes-hall↗

arXiv2014

Molecular dynamics studies on the NMR structures of rabbit prion protein wild-type and mutants: surface electrostatic charge distributions

Prion is a misfolded protein found in mammals that causes infectious diseases of the nervous system in humans and animals. Prion diseases are invariably fatal and highly infectious neurodegenerative diseases that affect a wide variety of mammalian species such as sheep and goats, cattle, deer, elk and humans etc. Recent studies have shown that rabbits have a low susceptibility to be infected by prion diseases with respect to other animals including humans. The present study employs molecular dynamics (MD) means to unravel the mechanism of rabbit prion proteins (RaPrPC) based on the recently available rabbit NMR structures (of the wild-type and its two mutants of two surface residues). The electrostatic charge distributions on the protein surface are the focus when analysing the MD trajectories. It is found that we can conclude that surface electrostatic charge distributions indeed contribute to the structural stability of wild-type RaPrPC; this may be useful for the medicinal treatment of prion diseases.

physics.bio-ph↗

arXiv2014

Ultrafast Generation of Pseudo-magnetic Field for Valley Excitons in WSe2 Monolayers

A new degree of freedom, the valley pseudospin, emerges in atomically thin two-dimensional transition metal dichalcogenides (MX2) and has attracted great scientific interest. The capability to manipulate the valley pseudospin, in analogy to the control of spin in spintronics, can open up exciting opportunities in valleytronics. Here we demonstrate that an ultrafast and ultrahigh valley pseudomagnetic field can be generated using circularly polarized femtosecond pulses to selectively control the valley degree of freedom in monolayer MX2. Employing ultrafast pump-probe spectroscopy, we observed a pure and valley-selective optical Stark effect in WSe2 monolayers from the non-resonant pump, which instantaneously lift the degeneracy of valley exciton transitions without any dissipation. The strength of the optical Stark effect scales linearly with both the pump intensity and the inverse of pump detuning. An energy splitting more than 10 meV between the K and K_prime valley transitions can be achieved, which corresponds to an effective pseudomagnetic field over 170 Tesla. Our study demonstrates efficient and ultrafast control of the valley excitons with optical light, and opens up the possibility to coherent manipulate the valley polarization for quantum information applications.

cond-mat.mes-hall↗

arXiv2014

Evolution of Interlayer Coupling in Twisted MoS2 Bilayers

Van der Waals (vdW) coupling is emerging as a powerful method to engineer and tailor physical properties of atomically thin two-dimensional (2D) materials. In graphene/graphene and graphene/boron-nitride structures it leads to interesting physical phenomena ranging from new van Hove singularities1-4 and Fermi velocity renormalization5, 6 to unconventional quantum Hall effects7 and Hofstadter's butterfly pattern8-12. 2D transition metal dichalcogenides (TMDCs), another system of predominantly vdW-coupled atomically thin layers13, 14, can also exhibit interesting but different coupling phenomena because TMDCs can be direct or indirect bandgap semiconductors15, 16. Here, we present the first study on the evolution of interlayer coupling with twist angles in as-grown MoS2 bilayers. We find that an indirect bandgap emerges in bilayers with any stacking configuration, but the bandgap size varies appreciably with the twist angle: it shows the largest redshift for AA- and AB-stacked bilayers, and a significantly smaller but constant redshift for all other twist angles. The vibration frequency of the out-of-plane phonon in MoS2 shows similar twist angle dependence. Our observations, together with ab initio calculations, reveal that this evolution of interlayer coupling originates from the repulsive steric effects, which leads to different interlayer separations between the two MoS2 layers in different stacking configurations.

cond-mat.mes-hall↗

arXiv2014

Observation of an intrinsic bandgap and Landau level renormalization in graphene/boron-nitride heterostructures

Van der Waals heterostructures formed by assembling different two-dimensional atomic crystals into stacks can lead to many new phenomena and device functionalities. In particular, graphene/boron-nitride heterostructures have emerged as a very promising system for band engineering of graphene. However, the intrinsic value and origin of the bandgap in such heterostructures remain unresolved. Here we report the observation of an intrinsic bandgap in epitaxial graphene/boron-nitride heterostructures with zero crystallographic alignment angle. Magneto-optical spectroscopy provides a direct probe of the Landau level transitions in this system and reveals a bandgap of ~ 38 meV (440 K). Moreover, the Landau level transitions are characterized by effective Fermi velocities with a critical dependence on specific transitions and magnetic field. These findings highlight the important role of many body interactions in determining the fundamental properties of graphene heterostructures.

cond-mat.mes-hall↗

arXiv2014

Bergman Kernels and algebraic structure of limit space for a sequence of almost Kähler-Ricci solitons

In this paper, we give a lower bound of Bergman kernels for a sequence of almost Kähler-Einstein Fano manifolds, or more general, a sequence of Fano manifolds with almost Kähler-Ricci solitons. This generalizes a result by Donaldson-Sun, Tian for Kähler-Einstein manifolds sequence with positive scalar curvature. As an application of our result, we prove that the Gromov-Hausdorff limit of sequence is homomorphic to a log terminal $Q$-Fano variety which admits a Kähler-Ricci soliton on its smooth part.

math.DG↗

arXiv2014

Gate-dependent Pseudospin Mixing in Graphene/Boron Nitride Moire Superlattices

Electrons in graphene are described by relativistic Dirac-Weyl spinors with a two-component pseudospin1-12. The unique pseudospin structure of Dirac electrons leads to emerging phenomena such as the massless Dirac cone2, anomalous quantum Hall effect2, 3, and Klein tunneling4, 5 in graphene. The capability to manipulate electron pseudospin is highly desirable for novel graphene electronics, and it requires precise control to differentiate the two graphene sub-lattices at atomic level. Graphene/boron nitride (graphene/BN) Moire superlattice, where a fast sub-lattice oscillation due to B-N atoms is superimposed on the slow Moire period, provides an attractive approach to engineer the electron pseudospin in graphene13-18. This unusual Moire superlattice leads to a spinor potential with unusual hybridization of electron pseudospins, which can be probed directly through infrared spectroscopy because optical transitions are very sensitive to excited state wavefunctions. Here, we perform micro-infrared spectroscopy on graphene/BN heterostructure and demonstrate that the Moire superlattice potential is dominated by a pseudospin-mixing component analogous to a spatially varying pseudomagnetic field. In addition, we show that the spinor potential depends sensitively on the gate-induced carrier concentration in graphene, indicating a strong renormalization of the spinor potential from electron-electron interactions. Our study offers deeper understanding of graphene pseudospin structure under spinor Moire potential, as well as exciting opportunities to control pseudospin in two-dimensional heterostructures for novel electronic and photonic nanodevices.

cond-mat.mes-hall↗

arXiv2014

Wide-Gap Semiconducting Graphene from Nitrogen-Seeded SiC

All carbon electronics based on graphene has been an elusive goal. For more than a decade, the inability to produce significant band-gaps in this material has prevented the development of semiconducting graphene. While chemical functionalization was thought to be a route to semiconducting graphene, disorder in the chemical adsorbates, leading to low mobilities, have proved to be a hurdle in its production. We demonstrate a new approach to produce semiconducting graphene that uses a small concentration of covalently bonded surface nitrogen, not as a means to functionalize graphene, but instead as a way to constrain and bend graphene. We demonstrate that a submonolayer concentration of nitrogen on SiC is sufficient to pin epitaxial graphene to the SiC interface as it grows, causing the graphene to buckle. The resulting 3-dimensional modulation of the graphene opens a band-gap greater than 0.7eV in the otherwise continuous metallic graphene sheet.

cond-mat.mtrl-sci↗

arXiv2014

First-principles study of Carbz-PAHTDDT dye sensitizer and two Carbz-derived dyes for dye sensitized solar cells

Two new carbazole-based organic dye sensitizers are designed and investigated in silico. These dyes are designed through chemical modifications of the conjugated bridge of a reference organic sensitizer known as Carbz-PAHTDDT (S9) dye. The aim of designing these dyes was to reduce the energy gap between their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and to red-shift their absorption response compared to those of the reference S9 dye sensitizer. This reference dye has a reported promising efficiency when coupled with ferrocene-based electrolyte composition. To investigate geometric and electronic structure, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were conducted on the new dyes as well as the reference dye. The present study indicated that the long-range correction to the theoretical model in the TD-DFT simulation is important to produce accurate absorption wavelengths.The theoretical studies have shown a reduced HOMO-LUMO gap and red-shifted absorption spectra for both of the new candidate dyes. Such findings suggest that the new dyes are promising and suitable for optoelectronic applications.

physics.chem-ph↗

arXiv2014

From building blocks of proteins to drugs: A quantum chemical study on structure-property relationships of phenylalanine, tyrosine and dopa

Density functional theory and ab initio methods have been employed to address the impacts of hydroxyl (OH) group substitutions on the physico-chemical properties of levodopa (or L-dopa) against the natural amino acids, phenylalanine and tyrosine. L-dopa, which is an important therapeutic drug for Parkinson's disease, shares structural homology with the amino acids, whose structures differ only by OH substitutions in their phenyl side chains. It is revealed that the backbone geometries of the aromatic molecules do not show apparent OH-dependent differences; however, their other molecular-level properties, such as molecular dipole moment, electronic properties and aromaticity, change significantly. The core binding energy spectra indicate that the atom sites that undergo modifications exhibit large energy shifts, so as to accommodate the changes in the intra-molecular chemical environment of the molecules. The binding energies of the modified C 1s sites in the molecules shift as much as 1.8 eV, whereas the electronic changes in their O 1s spectra happen in the higher energy region (ca. 536 eV). The valence spectra provide enhanced insights on the reactivity and chemical properties of the aromatic molecules. The impacts of OH moieties on the valence spectra are predominantly focussed in the energy band<16 eV, where the frontier molecular orbitals display much reorganization and energy shifts from the amino acids to L-dopa. Of the three molecules, L-dopa also has the least HOMO-LUMO energy gap, which can readily explain its proactivity as a drug compound. Furthermore, the nuclear independent chemical shift calculations suggest that L-dopa also has more aromaticity features than those of the amino acids. The OH groups, therefore, play a more prominent role in shaping the physico-chemical properties of L-dopa, which significantly improve its drug potency.

physics.chem-ph↗