arXiv subjects

Jinlong Yang

Publications and source records attributed to Jinlong Yang.

At least 163 records · Page 9Linked to original sources

arXiv2009

Chemical functionalization on planar polysilane and graphane

Two dimensional materials are important for electronics applications. A natural way for electronic structure engineering on two dimensional systems is on-plane chemical functionalization. Based on density functional theory, we study the electronic structures of fluorine substituted planar polysilane and graphane. We find that carbon and silicon present very different surface chemistry. The indirect energy gap of planar polysilane turns to be direct upon fluorine decoration, and the gap width is mainly determined by fluorine coverage regardless of its distribution on the surface. However, electronic structure of fluorine doped graphane is very sensitive to the doping configuration, due to the competition between antibonding states and nearly-free-electron (NFE) states. With specific fluorine distribution pattern, zero-dimensional and one-dimensional NFE states can be obtained. We have also studied the chemical modification with -OH or -NH$_2$ group. Carbon seems to be too small to accommodate big functional groups on its graphane skeleton with a high concentration.

cond-mat.mtrl-sci↗

arXiv2009

A first principles study on FeAs single layers

FeAs- single layer is tested as a simple model for LaFeAsO and BaFe2As2 based on first-principles calculations using generalized gradient approximation (GGA) and GGA+U. The calculated single- layer geometric and electronic structures are inconsistent with that of bulk materials. The bulk collinear antiferromagnetic ground state is failed to be obtained in the FeAs- single layer. The monotonous behavior of the Fe-As distance in z direction upon electron or hole doping is also in contrast with bulk materials. Our results indicate that, in LaFeAsO and BaFe2As2, interactions between FeAs layer and other layers beyond simple charge doping are important, and a single FeAs layer may not represent a good model for Fe based superconducting materials.

cond-mat.supr-con↗

arXiv2008

Origin of the Ising Ferrimagnetism and Spin-Charge Coupling in LuFe2O4

The spin ordering and spin-charge coupling in LuFe2O4 were investigated on the basis of density functional calculations and Monte Carlo simulations. The 2:1 ferrimagnetism arises from the strong antiferromagnetic intra-sheet Fe3+-Fe3+ and Fe3+ -Fe2+ as well as some substantial antiferromagnetic Fe2+-Fe3+ inter-sheet spin exchange interactions. The giant magnetocapacitance at room temperature and the enhanced electric polarization at 240 K of LuFe2O4 are explained by the strong spin-charge coupling.

cond-mat.mtrl-sci↗

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

Magnetism in Graphene Systems

Graphene has attracted a great interest in material science due to its novel electronic structrues. Recently, magnetism discovered in graphene based systems opens the possibility of their spintronics application. This paper provides a comprehensive review on the magnetic behaviors and electronic structures of graphene systems, including 2-dimensional graphene, 1-dimensional graphene nanoribbons, and 0-dimensional graphene nanoclusters. Theoretical research suggests that such metal-free magnetism mainly comes from the localized states or edges states. By applying external electric field, or by chemical modification, we can turn the zigzag nanoribbon systems to half metal, thus obtain a perfect spin filter.

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

Steric Effect in Threshold Photoionization Dissociations of Serine Conformers

Steric effect in the threshold dissociative ionizations of serine conformers [CH2OH-C\alphaH(NH2)-C\betaOOH] is revealed by high-level ab initio calculations combined with our newly developed infrared laser desorption / tunable VUV photoionization mass spectrometry. We find that near the ionization thresholds the Cα-Cβand Cα-C bonds are selectively broken for the respective cationic conformers, yielding the different fragments. Novel dynamic processes, proton transfer and reorientation between the predissociative fragments, are involved in the threshold photoionization dissociations.

physics.chem-ph↗

arXiv2008

Electronic structures of defective BN nanotubes under transverse electric fields

We investigate the electronic structures of some defective boron nitride nanotubes (BNNTs) under transverse electric fields within density-functional theory. (16,0) BNNTs with antisite, carbon substitution, single vacancy, and Stone-Wales 5775 defects are studied. Under transverse electric fields, the band gaps of the defective BNNTs are reduced, similar to the pristine ones. The energy levels of the defect states vary with the transverse electric field directions, due to the different electrostatic potential shift at the defect sites induced by the electric fields. Therefore, besides electronic structure and optical property engineering, the transverse electric field can be used to identify the defect positions in BNNTs.

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

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

Edge Effects on the Electronic Structures of Chemically Modified Armchair Graphene Nanoribbons

In this paper, we apply the first-principle theory to explore how the electronic structures of armchair graphene nanoribbons (AGNRs) are affected by chemical modifications. The edge addends include H, F, N, NH$_{2}$, and NO$_{2}$. Our theoretical results show that the energy gaps are highly tunable by controlling the widths of AGNRs and addends. The most interesting finding is that N-passivated AGNRs with various widths are metallic due to the unique electronic features of N-N bonds. This property change of AGNRs (from semiconducting to metallic) is important in developing graphene-based devices.

cond-mat.mtrl-sci↗

arXiv2007

Average Density of States in Disordered Graphene systems

In this paper, the average density of states (ADOS) with a binary alloy disorder in disordered graphene systems are calculated based on the recursion method. We observe an obvious resonant peak caused by interactions with surrounding impurities and an anti-resonance dip in ADOS curves near the Dirac point. We also find that the resonance energy (Er) and the dip position are sensitive to the concentration of disorders (x) and their on-site potentials (v). An linear relation, not only holds when the impurity concentration is low but this relation can be further extended to high impurity concentration regime with certain constraints. We also calculate the ADOS with a finite density of vacancies and compare our results with the previous theoretical results.

cond-mat.mes-hall↗

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↗

arXiv2007

Switching mechanism of photochromic diarylethene derivatives molecular junctions

The electronic transport properties and switching mechanism of single photochromic diarylethene derivatives sandwiched between two gold surfaces with closed and open configurations are investigated by a fully self-consistent nonequilibrium Green's function method combined with density functional theory. The calculated transmission spectra of two configurations are strikingly distinctive. The open form lacks any significant transmission peak within a wide energy window, while the closed structure has two significant transmission peaks on the both sides of the Fermi level. The electronic transport properties of the molecular junction with closed structure under a small bias voltage are mainly determined by the tail of the transmission peak contributed unusually by the perturbed lowest perturbed unoccupied molecular orbital. The calculated on-off ratio of currents between the closed and open configurations is about two orders of magnitude, which reproduces the essential features of the experimental measured results. Moreover, we find that the switching behavior within a wide bias voltage window is extremely robust to both substituting F or S for H or O and varying end anchoring atoms from S to Se and Te.

cond-mat.other↗

arXiv2007

Strain Effect on Energy Gaps of Armchair Graphene Nanoribbons

We report a first-principles study on electronic structures of the deformed armchair graphene nanoribbons (AGNRs). The variation of the energy gap of AGNRs as a function of uniaxial strain displays a zigzag pattern, which indicates that the energy gaps of AGNRs can be effectively tuned. The spatial distributions of two occupied and two empty subbands close to the Fermi level are swapped under different strains. The tunable width of energy gaps becomes narrower as increasing the width of AGNRs. Our simulations with tight binding approximation, including the nearest neighbor hopping integrals between $π$- orbitals of carbon atoms, reproduce these results by first-principles calculations. One simple empirical formula is obtained to describe the scaling behavior of the maximal value of energy gap as a function of the width of AGNRs.

cond-mat.mtrl-sci↗

arXiv2007

An insight into the electronic structure of graphene: from monolayer to multi-layer

In this paper, we analytically investigate the electronic structure of Bernal stacking (AB stacking) graphene evolving from monolayer (a zero-gap semiconductor with a linear Dirac-like spectrum around the Fermi energy) to multi-layer (semi-metal bulk graphite). We firstly derive a real space analytical expression for the free Green's function (propagator) of multi-layer graphene based on the effective-mass approximation. The simulation results exhibit highly spatial anisotropy with three-fold rotational symmetry. By combining with the STM measurement of d2I/dV2 (the second derivative of current), we also provide a clear high-throughput and non-destructive method to identify graphene layers. Such a method is lacking in the emerging graphene research.

cond-mat.mes-hall↗

arXiv2006

Electronic Structure of Bilayer Graphene: A Real-space Green's Function Study

In this paper, a real-space analytical expression for the free Green's function (propagator) of bilayer graphene is derived based on the effective-mass approximation. Green's function displays highly spatial anisotropy with three-fold rotational symmetry. The calculated local density of states (LDOS) of a perfect bilayer graphene produces the main features of the observed scanning tunneling microscopy (STM) images of graphite at low bias voltage. Some predicted features of the LDOS can be verified by STM measurements. In addition, we also calculate the LDOS of bilayer graphene with vacancies by using the multiple-scattering theory (scatterings are localized around the vacancy of bilayer graphene). We observe that the interference patterns are determined mainly by the intrinsic properties of the propagator and the symmetry of the vacancies.

cond-mat.mtrl-sci↗