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Search Results: 1 - 10 of 27576 matches for " Wenmei Ming "
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Tuning Fe Nucleation Density with Charge Doping of Graphene Substrate
Wenmei Ming,Feng Liu
Physics , 2014, DOI: 10.1063/1.4893947
Abstract: We have demonstrated that the island nucleation in the initial stage of epitaxial thin film growth can be tuned by substrate surface charge doping. This charge effect was investigated using spin density functional theory calculation in Fe-deposition on graphene substrate as an example. It was found that hole-doping can apparently increase both Fe-adatom diffusion barrier and Fe inter-adatom repulsion energy occurring at intermediate separation, whereas electron-doping can decrease Fe-adatom diffusion barrier but only slightly modify inter-adatom repulsion energy. Further kinetic Monte Carlo simulation showed that the nucleation island density can be increased up to ten times larger under hole-doping and can be decreased down to ten times smaller than that without doping. Our findings indicates a new route to tailoring the growth morphology of magnetic metal nanostructure for spintronics applications via surface charge doping.
Effects of Li doping on H-diffusion in MgH$_2$: a first-principles study
Wenmei Ming,Zhigang Zak Fang,Feng Liu
Physics , 2013, DOI: 10.1063/1.4853055
Abstract: The effects of Li doping in MgH$_2$ on H-diffusion process are investigated, using first-principles calculations. We have identified two key effects: (1) The concentration of H vacancy in the $+1$ charge state (V$_H^{+1}$) can increase by several orders of magnitude upon Li doping, which significantly increases the vacancy mediated H diffusion rate. It is caused by the preferred charge states of substitutional Li in the $-1$ state (Li$_{Mg}^{-1}$) and of interstitial Li in the $+1$ state (Li$_i^{+1}$), which indirectly reduce the formation energy of V$_H^{+1}$ by up to 0.39 eV depending on the position of Fermi energy. (2) The interaction between V$_H^{+1}$ and Li$_{Mg}^{-1}$ is found to be attractive with a binding energy of 0.55 eV, which immobilizes the V$_H^{+1}$ next to Li$_{Mg}^{-1}$ at high Li doping concentration. As a result, the competition between these two effects leads to large enhancement of H diffusion at low Li doping concentration due to the increased H-vacancy concentration, but only limited enhancement at high Li concentration due to the immobilization of H vacancies by too many Li.
Giant Rashba-Spin Splitting of Bi(111) Bilayer on Large Band Gap $β-$In$_2$Se$_3$
Wenmei Ming,Z. F. Wang,Feng Liu
Physics , 2014,
Abstract: Experimentally it is still challenging to epitaxially grow Bi(111) bilayer (BL) on conventional semiconductor substrate. Here, we propose a substrate of $\beta-$In$_2$Se$_3$(0001) with van der Waals like cleavage and large band gap of 1.2~eV. We have investigated the electronic structure of BL on one quintuple-layer (QL) $\beta-$In$_2$Se$_3$(0001) using density functional theory calculation. It is found that the intermediate hybridization between BL and one QL $\beta-$In$_2$Se$_3$(0001) results in the formation of bands with giant Rashba spin splitting in the large band gap of the substrate. Furthermore the Rashba parameter $\alpha_R$ can be increased significantly by tensile strain of substrate. Our findings provide a good candidate substrate for BL growth to experimentally realize spin splitting Rashba states with insignificant effect of spin degenerate states from the substrate.
Quantum Size Effect on Dielectric Properties of Ultrathin Metallic Film: A First-Principles Study of Al(111)
Wenmei Ming,Steve Blair,Feng Liu
Physics , 2014, DOI: 10.1088/0953-8984/26/50/505302
Abstract: Quantum manifestations of various properties of metallic thin films by quantum size effect (QSE) have been studied intensively. Here, using first-principles calculations, we show quantum manifestation in dielectric properties of Al(111) ultrathin films. The QSE on the dielectric function is revealed, which arises from size dependent contributions from both intraband and interband electronic transitions. More importantly, the in-plane interband transitions in the films thinner than 15 monolayers are found to be smaller than the bulk counterpart in the energy range from 1.5~eV to 2.5~eV. This indicates less energy loss with plasmonic material of Al in the form of ultrathin film. Our findings may shed light on searching for low-loss plasmonic materials via quantum size effect.
sd2 Graphene: Kagome Band in Hexagonal lattice
Miao Zhou,Zheng Liu,Wenmei Ming,Zhengfei Wang,Feng Liu
Physics , 2014, DOI: 10.1103/PhysRevLett.113.236802
Abstract: Graphene, made of sp2 hybridized carbon, is characterized with a Dirac band, representative of its underlying 2D hexagonal lattice. Fundamental understanding of graphene has recently spurred a surge of searching for 2D topological quantum phases in solid-state materials. Here, we propose a new form of 2D material, consisting of sd2 hybridized transition metal atoms in hexagonal lattice, called sd2 graphene. The sd2 graphene is characterized with bond-centered electronic hopping, which transforms the apparent atomic hexagonal lattice into the physics of kagome lattice that may exhibit a wide range of topological quantum phases. Based on first-principles calculations, room temperature quantum anomalous Hall states with an energy gap of 0.1 eV are demonstrated for one such lattice made of W, which can be epitaxially grown on a semiconductor surface of 1/3 monolayer Cl-covered Si(111), with high thermodynamic and kinetic stability.
Doping-dependent Phase Diagram of LaO{\it M}As ({\it M}=V--Cu) and Electron-type Superconductivity near Ferromagnetic Instability
Gang Xu,Wenmei Ming,Yugui Yao,Xi Dai,Shouchen Zhang,Zhong Fang
Physics , 2008, DOI: 10.1209/0295-5075/82/67002
Abstract: By first-principles calculations, we present a doping-dependent phase diagram of LaO{\it M}As ({\it M}=V--Cu) family. It is characterized as antiferromagnetic semiconductor around LaOMnAs side and ferromagnetic metal around LaOCoAs. Both LaOFeAs and LaONiAs, where superconductivity were discovered, are located at the borderline of magnetic phases. Extensive Fermi surface analysis suggests that the observed superconductivity is of electron-type in its origin. We discuss possible pairing mechanisms in the context of competing ferromagnetic phases found in this work and the ferromagnetic spin fluctuations.
Formation of quantum spin Hall state on Si surface and energy gap scaling with strength of spin orbit coupling
Miao Zhou,Wenmei Ming,Zheng Liu,Zhengfei Wang,Yugui Yao,Feng Liu
Physics , 2014, DOI: 10.1038/srep07102
Abstract: For potential applications in spintronics and quantum computing, it is desirable to place a quantum spin Hall insulator [i.e., a 2D topological insulator (TI)] on a substrate while maintaining a large energy gap. Here, we demonstrate a unique approach to create the large-gap 2D TI state on a semiconductor surface, based on first-principles calculations and effective Hamiltonian analysis. We show that when heavy elements with strong spin orbit coupling (SOC) such as Bi and Pb atoms are deposited on a patterned H-Si(111) surface into a hexagonal lattice, they exhibit a 2D TI state with a large energy gap of over 0.5 eV. The TI state arises from an intriguing substrate orbital filtering effect that selects a suitable orbital composition around the Fermi level, so that the system can be matched onto a four-band effective model Hamiltonian. Furthermore, it is found that within this model, the SOC gap does not increase monotonically with the increasing strength of SOC. These interesting results may shed new light in future design and fabrication of large-gap topological quantum states.
Epitaxial growth of large-gap quantum spin Hall insulator on semiconductor surface
Miao Zhou,Wenmei Ming,Zheng Liu,Zhengfei Wang,Ping Li,Feng Liu
Physics , 2014, DOI: 10.1073/pnas.1409701111
Abstract: Formation of topological quantum phase on conventional semiconductor surface is of both scientific and technological interest. Here, we demonstrate epitaxial growth of 2D topological insulator, i.e. quantum spin Hall (QSH) state, on Si(111) surface with a large energy gap, based on first-principles calculations. We show that Si(111) surface functionalized with 1/3 monolayer of halogen atoms [Si(111)-sqrt(3) x sqrt(3)-X (X=Cl, Br, I)] exhibiting a trigonal superstructure, provides an ideal template for epitaxial growth of heavy metals, such as Bi, which self-assemble into a hexagonal lattice with high kinetic and thermodynamic stability. Most remarkably, the Bi overlayer is "atomically" bonded to but "electronically" decoupled from the underlying Si substrate, exhibiting isolated QSH state with an energy gap as large as 0.8 eV. This surprising phenomenon is originated from an intriguing substrate orbital filtering effect, which critically select the orbital composition around the Fermi level leading to different topological phases. Particularly, the substrate-orbital-filtering effect converts the otherwise topologically trivial freestanding Bi lattice into a nontrivial phase; while the reverse is true for Au lattice. The underlying physical mechanism is generally applicable, opening a new and exciting avenue for exploration of large-gap topological surface/interface states.
Formation of Ideal Rashba States on Layered Semiconductor Surfaces Steered by Strain Engineering
Wenmei Ming,Z. F. Wang,Miao Zhou,Mina Yoon,Feng Liu
Physics , 2015, DOI: 10.1021/acs.nanolett.5b04005
Abstract: Spin splitting of Rashba states in two-dimensional electron system provides a promising mechanism of spin manipulation for spintronics applications. However, Rashba states realized experimentally to date are often outnumbered by spin-degenerated substrate states at the same energy range, hindering their practical applications. Here, by density functional theory calculation, we show that Au one monolayer film deposition on a layered semiconductor surface beta-InSe(0001) can possess "ideal" Rashba states with large spin splitting, which are completely situated inside the large band gap of the substrate. The position of the Rashba bands can be tuned over a wide range with respect to the substrate band edges by experimentally accessible strain. Furthermore, our nonequilibrium Green's function transport calculation shows that this system may give rise to the long-sought strong current modulation when made into a device of Datta-Das transistor. Similar systems may be identified with other metal ultrathin films and layered semiconductor substrates to realize ideal Rashba states.
Research Progress on Aging Mechanisms  [PDF]
Qiang Wang, Fengjuan Zhou, Wenmei Xie, Xiaorong Zhao, Xuan Liu
Advances in Aging Research (AAR) , 2016, DOI: 10.4236/aar.2016.52005
Abstract: Aging refers to a gradual process of functional and organic recession with the age increased after the organism maturing. Many researchers have tried to elucidate aging mechanism which includes the free radical theory: free radicals can become lipofuscin and cause the mutations of mitochondrial DNA (mtDNA), and the damage of the nuclear DNA. From the view of heredity, aging is the results of the activation and inhibition of a series of genes as well as the products of their interaction. From the changes of immune function theory, it points out that aging is attributed to the decline of immune responses and the increase of autoimmune reactions.
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