arXiv subjects

Zhi Zeng

Publications and source records attributed to Zhi Zeng.

126 records · Page 7Linked to original sources

arXiv2012

Patterning graphene nanostripes in substrate-supported functionalized graphene: A promising route to integrated, robust, and superior transistors

It is promising to apply quantum-mechanically confined graphene systems in field-effect transistors. High stability, superior performance, and large-scale integration are the main challenges facing the practical application of graphene transistors. Our understandings of the adatom-graphene interaction combined with recent progress in the nanofabrication technology indicate that very stable and high-quality graphene nanostripes could be integrated in substrate-supported functionalized (hydrogenated or fluorinated) graphene using electron-beam lithography. We also propose that parallelizing a couple of graphene nanostripes in a transistor should be preferred for practical application, which is also very useful for transistors based on graphene nanoribbon.

cond-mat.mes-hall↗

arXiv2011

Constructing silicon nanotubes by assembling hydrogenated silicon clusters

The search or design of silicon nanostructures similar to their carbon analogues has attracted great interest recently. In this work, density functional calculations are performed to systematically study a series of finite and infinite hydrogenated cluster-assembled silicon nanotubes (SiNTs). It is found that stable one-dimensional SiNTs with formula $Si_{m(3k+1)}H_{2m(k+1)}$ can be constructed by proper assembly of hydrogenated fullerene-like silicon clusters $Si_{4m}H_{4m}$. The stability is first demonstrated by the large cohesive energies and HOMO-LUMO gaps. Among all such silicon nanotubes, the ones built from $Si_{20}H_{20}$($m=5$) and $Si_{24}H_{24}$($m=6$) are the most stable due to the silicon bond angles that are most close to the bulk $sp^{3}$ type in these structures. Thermostability analysis further verifies that such tubes may well exist at room temperature. Finally, both finite nanotubes and infinite nanotubes show a large energy gap. A direct-indirect-direct band gap transition has been revealed with the increase of the tube radius. The existence of direct band gap may make them potential building blocks for electronic and optoelectronic devices.

cond-mat.mes-hall↗

arXiv2011

A Three-Dimensional Tight-Binding Model and Magnetic Instability of KFe2e2

For a newly discovered iron-based high T_c superconducting parent material KFe2Se2, we present an effective three-dimensional five-orbital tight-binding model by fitting the band structures. The three t2g-symmetry orbitals of the five Fe 3d orbitals mainly contribute to the electron-like Fermi surface, in agreement with recent angle-resolved photoemission spectroscopy experiments. To understand the groundstate magnetic structure, the two- and three-dimensional dynamical spin susceptibilities within the random phase approximation are investigated. It obviously shows a sharp peak at wave vector $\mathbf{Q}$ $\thicksim$ ($π$, $π$), indicating the magnetic instability of {\it N$\acute{e}$el}-type antiferromagnetic rather than ($π$/2, $π$/2)-type antiferromagnetic ordering. While along $\emph{c}$ axis, it exhibits a ferromagnetic coupling between the nearest neighboring FeSe layers. The difference between the present results and the experimental observation in KxFe2-ySe2 is attributed to the presence of Fe vacancy in the latter.

cond-mat.supr-con↗

arXiv2011

KAgF3: quasi-one-dimensional magnetism in three-dimensional magnetic ions sublattice

The electronic structure and magnetic properties of the Jahn-Teller-distorted perovskite KAgF3 have been investigated using the full-potential linerized aug- mented plane-wave method. It is found that KAgF3 exhibits significant quasi-one- dimensional antiferromagnetism with the ratio of exchange constant jJ?j (perpen- dicular to the z axis) and J (along the z axis) about 0.04, although the sublattice of magnetic ion is three-dimensional. The strong quasi-one-dimensional antiferromag- netism originates from the C-antiferro-distortive orbital ordering of the Ag2+ 4d9 ions. The orbital ordered antiferromagnetic insulating state in KAgF3 is determined by on-site Coulomb repulsion to a large extent.

cond-mat.str-el↗

arXiv2011

Ab initio study of the giant ferroelectric distortion and pressure induced spin-state transition in BiCoO3

Using configuration-state-constrained electronic structure calculations based on the generalized gradient approximation plus Hubbard U method, we sought the origin of the giant tetragonal ferroelectric distortion in the ambient phase of the potentially multiferroic material BiCoO3 and identified the nature of the pressure induced spin-state transition. Our results show that a strong Bi-O covalency drives the giant ferroelectric distortion, which is further stabilized by an xy-type orbital ordering of the high-spin (HS) Co3+ ions. For the orthorhombic phase under 5.8 GPa, we find that a mixed HS and low-spin (LS) state is more stable than both LS and intermediate-spin (IS) states, and that the former well accounts for the available experimental results. Thus, we identify that the pressure induced spin-state transition is via a mixed HS+LS state, and we predict that the HS-to-LS transition would be complete upon a large volume decrease of about 20%.

cond-mat.str-el↗

arXiv2010

Titanium-capped carbon chains as promising new hydrogen storage media

The capacity of Ti-capped sp carbon atomic chains for use as hydrogen storage media is studied using first-principles density functional theory. The Ti atom is strongly attached at one end of the carbon chains via d-p hybridization, forming stable TiCn complexes. We demonstrate that the number of adsorbed H2 on Ti through Kubas interaction depends upon the chain types. For polyyne (n even) or cumulene (n odd) structures, each Ti atom can hold up to five or six H2 molecules, respectively. Furthermore, the TiC5 chain effectively terminated on a C20 fullerene can store hydrogen with optimal binding of 0.52 eV/H2. Our results reveal a possible way to explore high-capacity hydrogen storage materials in truly one-dimensional carbon structures.

cond-mat.mtrl-sci↗

arXiv2010

Modulation of the thermodynamic, kinetic and magnetic properties of the hydrogen monomer on graphene by charge doping

The thermodynamic, kinetic and magnetic properties of the hydrogen monomer on doped graphene layers were studied by ab initio simulations. Electron doping was found to heighten the diffusion potential barrier, while hole doping lowers it. However, both kinds of dopings heighten the desorption potential barrier. The underlying mechanism was revealed by investigating the effect of doping on the bond strength of graphene and on the electron transfer and the coulomb interaction between the hydrogen monomer and graphene. The kinetic properties of H and D monomers on doped graphene layers during both the annealing process (annealing time $t_0 =$300 s) and the constant-rate heating process (heating rate $α=$1.0 K/s) were simulated. Both electron and hole dopings were found to generally increase the desorption temperatures of hydrogen monomers. Electron doping was found to prevent the diffusion of hydrogen monomers, while the hole doping enhances their diffusion. Macroscopic diffusion of hydrogen monomers on graphene can be achieved when the doping-hole density reaches $5.0\times10^{13}$ cm$^{-2}$. The magnetic moment and exchange splitting were found to be reduced by both electron and hole dopings, which was explained by a simple exchange model. The study in this report can further enhance the understanding of the interaction between hydrogen and graphene and is expected to be helpful in the design of hydrogenated-graphene-based devices.

cond-mat.mtrl-sci↗

arXiv2010

The thermodynamic and kinetic properties of hydrogen dimers on graphene

The thermodynamic and kinetic properties of hydrogen adatoms on graphene are important to the materials and devices based on hydrogenated graphene. Hydrogen dimers on graphene with coverages varying from 0.040 to 0.111 ML (1.0 ML $= 3.8\times10^{15}$cm$^{-2}$) were considered in this report. The thermodynamic and kinetic properties of H, D and T dimers were studied by ab initio simulations. The vibrational zero-point energy corrections were found to be not negligible in kinetics, varying from 0.038 (0.028, 0.017) to 0.257 (0.187, 0.157) eV for H (D, T) dimers. The isotope effect exhibits as that the kinetic mobility of a hydrogen dimer decreases with increasing the hydrogen mass. The simulated thermal desorption spectra with the heating rate $α= 1.0$ K/s were quite close to experimental measurements. The effect of the interaction between hydrogen dimers on their thermodynamic and kinetic properties were analyzed in detail.

cond-mat.mtrl-sci↗

arXiv2010

The spin states of Co ions in La1.5Ca0.5CoO4 from first-principles

The spin states and electronic structure of layered perovskite La1.5Ca0.5CoO4 are investigated using fullpotential linearized augmented plane-wave method. All the computational results indicate that the Co2+ ion is in a high-spin state and the Co3+ in a low-spin state. The Co2+ t2g orbitals with a small crystal-field splitting are mixed by spin-orbit coupling, which accounts for the observed easy in-plane magnetism. The nonmagnetic LS-Co3+ state, which is stabilized by a strong crystal field, provides a natural explanation for the observed low magnetic ordering temperature and a spin-blockade phenomenon of the electron hopping. Furthermore, we find that the intermediate-spin state of Co3+ has a large multiplet splitting. But the lowest-lying IS state of Co3+ is still higher in energy than the LS ground state by a few hundred millielectron volts and the HS state of Co3+ is even less stable, both in sharp contrast to a recent experimental study which suggested the HS+IS mixed Co3+ ground state. We note that either the IS-Co3+ or HS-Co3+ states or their mixture would produce a wrong out-of-plane magnetic anisotropy and a much higher magneticordering temperature than observed. Thus, the present work sheds light on this material concerning its electronic and magnetic structure, and it would stimulate different experiments to settle this intriguing spin-state issue.

cond-mat.str-el↗

arXiv2010

Realization of high-capacity hydrogen storage using carbon atomic chains: the role of terminations

The capacity of carbon atomic chains with different terminations for hydrogen storage is studied using first-principles density functional theory calculations. Unlike the physisorption of H2 on the H-terminated chain, we show that two Li (Na) atoms each capping one end of the carbon chain can hold ten H2 molecules with optimal binding energies for room temperature storage. The hybridization of the Li 2p states with the H2 sigma orbitals contributes to the H2 adsorption. However, the binding mechanism of the H2 molecules on Na arises only from the polarization interaction between the charged Na atom and the H2. Moreover, additional H2 molecules can be bound to the carbon atoms at the chain ends due to the charge transfer between Li 2s2p (Na 3s) and C 2p states. Importantly, dimerization of these isolated metal-capped chains does not affect the hydrogen binding energy significantly. In addition, a single chain can be stabilized effectively by termination on the C60 clusters. With a hydrogen uptake of > 10 wt % on Li-coated C60-Cn-C60 (n = 5, 8), the Li12C60-Cn-Li12C60 complex, without reducing the number of adsorbed H2 molecules per Li, can serve as better building blocks of polymers than the (Li12C60)2 dimer. These findings suggest a new route to design cluster-assembled storage materials based on terminated sp carbon chains.

cond-mat.mes-hall↗

arXiv2010

An ab initio design of cluster-assembled silicon nanotubes

Density functional calculations were performed to systematically study a series of finite and infinite cluster-assembled silicon nanotubes (SiNTs). One-dimensional SiNTs can be prepared by proper assembly of hydrogenated cage-like silicon clusters to form semiconductors with a large band gap, and their electronic properties can be accurately tuned by transition metal doping in the center of the tubes. Specifically, doping with Fe made the SiNTs metallic and magnetic materials. More interestingly, a metal to half-metal transition was observed with increasing tube radius in Fe-doped SiNTs, which demonstrates that SiNTs doped with magnetic elements may find important applications in spintronics.

cond-mat.mes-hall↗

arXiv2010

Organometallic Wires Constructed from Transitional Metals and Anthracene: A Theoretical Study

The properties of organometallic wires [TM2(Ant)] constructed with transitional metals (TM = Sc, Ti, V, Cr, Mn and Fe) and anthracene (Ant) are investigated by first-principles calculations. As the gap between HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) of Ant is much smaller than that of benzene (Bz), much larger charge transfer (CT) occurs between TMs and Ant, which results in much more diverse magnetic properties in [TM2(Ant)] than in [TM2(Ant)]. Particularly, [V2(Ant)] and [Cr2(Ant)] are found to be half-metallic ferromagnets. As a result of this and the better structural stability, compared with [TM(Bz)], [TM2(Ant)] (like [V2(Ant)] and [Cr2(Ant)]) may be better candidates of spintronic devices. Furthermore, as the HOMO-LUMO gap of small pieces of graphene (SPG), such as pentacene and coronene, decreases with the increase of polycyclic number, the CT effects may also fit for the TM-SPG sandwich polymers which can also act as good spintronic materials.

cond-mat.mtrl-sci↗

arXiv2009

Orbitally relieved magnetic frustration in NaVO2

The magnetic properties of NaVO2 are investigated using full-potential linearized augmented plane wave method. We perform calculations for three structures. For the rhombohedral structure at 100 K, the t2g orbitals of V ions are split into upper a1g and lower e'g orbitals by a trigonal distortion of compression. For the monoclinic structure at 91.5 K, the system behaves like a frustrated spin lattice with spatially anisotropic exchange interactions. For another monoclinic structure at 20 K, the magnetic frustration is relieved by a lattice distortion which is driven by a certain orbital ordering, and the long-range magnetic ordering is thus formed. Moreover, the small magnetic moment originates from the compensation oforbital moment for the spin moment.

cond-mat.str-el↗

arXiv2009

New potential super-incompressible phase of ReN$_{2}$

The structural, elastic, and electronic properties of ReN$_{2}$ are investigated by first-principles calculations with density functional theory. The obtained orthorhombic $Pbcn$ structure is energetically the most stable structure at ambient pressure. ReN$_{2}$ is a metallic, superincompressible solid and presents a rather elastic anisotropy. The estimated Debye temperature and hardness are 735 K and 17.1 GPa, respectively. Its estimated hardness is comparative to that of Si$_{3}$N$_{4}$.

cond-mat.mtrl-sci↗

arXiv2008

Elastic properties of transition metal dioxides: XO$_{2}$(X=Ru, Rh, Os, Ir)

The elastic properties of rutile transition metal dioxides XO$_{2}$ (X=Ru, Rh, Os, and Ir) are investigated using first-principles calculations based on density functional theory. Elastic constants, bulk modulus, shear modulus, and Young's modulus as well as Possion ratio are given. OsO$_2$ and IrO$_2$ show strong incompressibility. The hardness estimated for these dioxides shows that they are not superhard solids. The obtained Debye temperatures are comparative to those of transition metal dinitrides or diborides.

cond-mat.mtrl-sci↗

arXiv2008

Potential ultra-incompressible material ReN: first-principles prediction

The structural, elastic and electronic properties of ReN are investigated by first-principles calculations based on density functional theory. Two competing structures, i.e., CsCl-like and NiAs-like structures, are found and the most stable structure, NiAs-like, has a hexagonal symmetry which belongs to space group P63/mmc with a=2.7472 and c=5.8180 Å. ReN with hexagonal symmetry is a metal ultra-incompressible solid and has less elastic anisotropy. The ultra-incompressibility of ReN is attributed to its high valence electron density and strong covalence bondings. Calculations of density of states and charge density distribution, together with Mulliken atomic population analysis, show that the bondings of ReN should be a mixture of metallic, covalent, and ionic bondings. Our results indicate that ReN can be used as a potential ultra-incompressible conductor. In particular, we obtain a superconducting transition temperature T$_c$=4.8 K for ReN.

cond-mat.mtrl-sci↗

arXiv2006

Conductance oscillation and quantization in monoatomic Al wires

We present first-principles calculations for the transport properties of monoatomic Al wires sandwiched between Al(100) electrodes. The conductance of the monoatomic Al wires oscillates with the number of the constituent atoms as a function of the wire length, either with a period of four-atom for wires with the typical interatomic spacing or a period of six-atom with the interatomic spacing of the bulk fcc aluminum, indicating a dependence of the period of conductance oscillation on the interatomic distance of the monoatomic Al wires.

cond-mat.mes-hall↗

arXiv2006

Effects of relative orientation of the molecules on electron transport in molecular devices

Effects of relative orientation of the molecules on electron transport in molecular devices are studied by non-equilibrium Green's function method based on density functional theory. In particular, two molecular devices, with the planer Au$_{7}$ and Ag$_{3}$ clusters sandwiched between the Al(100) electrodes are studied. In each device, two typical configurations with the clusters parallel and vertical to the electrodes are considered. It is found that the relative orientation affects the transport properties of these two devices completely differently. In the Al(100)-Au$_7$-Al(100) device, the conductance and the current of the parallel configuration are much larger than those in the vertical configuration, while in the Al(100)-Ag$_{3}$-Al(100) device, an opposite conclusion is obtained.

cond-mat.mes-hall↗