Observation of a degenerate Fermi gas confined by a standing electromagnetic wave

We have observed a gas of Fermi atoms confined in the antinodes of a standing electromagnetic wave. The standing wave is formed by two counter-propagating beams with the wavelength of 10.6 μm focused on the same spot. Each antinode confines a pancake-shaped cloud of 7500 lithium-6 atoms in two equally populated spin states at the temperature T = 0.1E _F, where E _F is the Fermi energy. The system is in the regime beyond the local density approximation: Only the 3 lowest energy states of the axial motion are populated. The system may become an instrument for the study of 2D Fermi physics and 3D effects beyond the local density approximation.     

B（N）掺杂单壁碳纳米管的Al原子吸附性能的第一性原理研究

采用基于密度泛函理论的平面波赝势方法和广义梯度近似,对未掺杂、掺B、掺N的碳纳米管(CNT)不同位置上Al原子的吸附进行了几何优化,计算了吸附Al、掺杂前后CNT的能带结构、态密度、差分电荷密度、电荷布居数和吸附能.计算结果表明,掺B使CNT形成缺电子状态,利于具有自由电子的Al原子的吸附结合,可显著提高Al在金属性的(5,5)CNT和半导性的(8,0)CNT外壁的吸附能;掺杂N形成多电子状态,在费米能级附近半满的施主能级也利于填充Al的价电子,改善Al在(5,5)CNT和(8,0)CNT外壁的吸附结合性 关键词： 密度泛函理论 单壁碳纳米管 B(N)掺杂 Al原子吸附     

Conductance through atomic point contacts between fcc(100) electrodes of gold

Electrical conductance through various nanocontacts between gold electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 gold atoms between the tips. Upon elongation of the contact by adding gold atoms between the tips, the conductance at Fermi energy E_F evolves from G ≈ 3G₀ to G ≈ 1G₀ (G₀ = 2e/h²). Formation of a true one-atom point contact, with G ≈ 1G₀ and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 ≤ E-E_F ≤ 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear “gold molecule" between the electrodes, and a weak coupling of this “molecule" to the tips.     

Electronic properties of orthorhombic LiGaS<Subscript>2</Subscript> and LiGaSe<Subscript>2</Subscript>

We report theoretical calculations of the band structure and density of states for orthorhombic LiGaS₂ (LGS) and LiGaSe₂ (LGSe). These calculations are based on the full potential linear augmented plane wave (FP-LAPW) method within a framework of density functional theory. Our calculations show that these crystals have similar band structures. The valence band maximum (VBM) and the conduction band minimum (CBM) are located at Γ, resulting in a direct energy band gap. The VBM is dominated by S/Se-p and Li-p states, while the CBM is dominated by Ga-s, S/Se-p and small contributions of Li-p and Ga-p. From the partial density of states we find that Li-p hybridizes with Li-s below the Fermi energy (E _F), while Li-s/p hybridizes with Ga-p below and above E _F. Also, we note that S/Se-p hybridizes with Ga-s below and above E _F.     

Spin-polarized metastable deexcitation spectroscopy study of iron films

A spin-polarized metastable deexcitation spectroscopy apparatus using a nozzle-skimmer pulsed discharge metastable atom source was developed. The oxygen adsorption dependence of the surface magnetism of thin iron films deposited on MgO(100) was investigated using this apparatus. The surface local density of states spilling towards the vacuum (SDOS) at around the Fermi energy, E_F, for the clean surface shows a negative polarization to the bulk. The oxygen derived SDOS for the lightly oxygen adsorbed surfaces (2–10 L) also shows a negative polarization while the SDOS at E_F changes its polarity to positive. The polarization of SDOS is not detected for the heavily oxidized surfaces (20–100 L).     

Comparative photoemission study of Bi2(Sr,Ca)3Cu2O8+y and Bi1.7Pb0.3Sr2Ca2Cu3O10+y

The electronic structure of Bi-based 2223 single phase superconductor has been studied by ultraviolet and X-ray photoemission. By comparison with that of 2212 phase superconductor, we find a higher density of states nearE _F for 2223 phase. From analysis of the Cu 2p core-level spectroscopy, we obtain a relatively smaller charge transfer energy between copper and oxygen as well as the Coulomb repulsion energy on Cu site for 2223 phase. We relate these changes to the increase of hole concentration from 2212 to 2223 phase. The experimental results support the viewpoint that the transition temperature should be correlated with the density of states atE _F .     

First-principles study on the electronic structures of iron phthalocyanine monolayer

The electronic band structure and magnetic properties of iron phthalocyanine (FePc) monolayer were investigated by using the first-principles all-electron full-potential linearized augmented plane wave energy band method. It is found that the ferromagnetic FePc monolayer is energetically more stable than the paramagnetic one. The exchange interaction, which splits the majority and minority bands, influences strongly on the electronic structure near the Fermi level (E_F). Magnetic moment of the central Fe atom is calculated to 1.95 μ_B. The range of the positive polarization of Fe site is larger in the out-of-plane than in the in-plane direction. The FePc ligand remains paramagnetic. The presence of states at E_F indicates the metallic character of FePc monolayer both for the paramagnetic and ferromagnetic states. However, the large density of states at E_F of the majority spins in the ferromagnetic state is expected to cause a phase transition to insulating antiferromagnetic state from the metallic ferromagnetic one.     

Adsorption of SO2 on Li atoms deposited on MgO (1 0 0) surface: DFT calculations

The adsorption of sulfur dioxide molecule (SO₂) on Li atom deposited on the surfaces of metal oxide MgO (1 0 0) on both anionic and defect (F_s-center) sites located on various geometrical defects (terrace, edge and corner) has been studied using density functional theory (DFT) in combination with embedded cluster model. The adsorption energy (E_ads) of SO₂ molecule (S-atom down as well as O-atom down) in different positions on both of O⁻² and F_s sites is considered. The spin density (SD) distribution due to the presence of Li atom is discussed. The geometrical optimizations have been done for the additive materials and MgO substrate surfaces (terrace, edge and corner). The oxygen vacancy formation energies have been evaluated for MgO substrate surfaces. The ionization potential (IP) for defect free and defect containing of the MgO surfaces has been calculated. The adsorption properties of SO₂ are analyzed in terms of the E_ads, the electron donation (basicity), the elongation of S-O bond length and the atomic charges on adsorbed materials. The presence of the Li atom increases the catalytic effect of the anionic O⁻² site of MgO substrate surfaces (converted from physisorption to chemisorption). On the other hand, the presence of the Li atom decreases the catalytic effect of the F_s-site of MgO substrate surfaces. Generally, the SO₂ molecule is strongly adsorbed (chemisorption) on the MgO substrate surfaces containing F_s-center.     

Energy gap modulation of graphene nanoribbons by F termination

Under the generalized gradient approximation (GGA), the electronic properties are studied for the F-terminated graphene nanoribbons (GNRs) with either zigzag edge (ZGNRs) or armchair edge (AGNRs) by using the first-principles projector augmented wave potential within the density function theory (DFT) framework. The results show that an edge state appears at the Fermi level E_F in the broader F-terminated ZGNRs, but does not appear in all the F-terminated AGNRs due to their dimerized C-C bonds at edge. The density of states (DOS) and projected DOS (PDOS) analyses show that the F-terminated ZGNRs are metallic and have a sharp peak at the Fermi level when the width is large enough. In contrast, the AGNRs are always semiconductors independent of their width. The charge density contours analyses shows that the C-F bond is an ionic bond due to a much stronger electronegativity of the F atom than that of the C atom. However, all kinds of the C-C bonds display a typical nonpolar covalent bonding feature.     

Electronic properties of TiC(100) and polar TiC(111) surfaces

The energy bands of films of TiC have been calculated using the linear-combination-of-atomic-orbitals method with parameters obtained by a fit to the bulk band structure. The Madelung potentials and charge redistribution have been determined self-consistently. For the neutral TiC(100) surface, the density of states (DOS) is similar to that of the bulk. For the non-neutral Ti-covered TiC(111) surface, Ti 3d-derived surface states appear around the Fermi energy E_F. The long-range electric field produced by the polar surfaces is screened by the charge redistribution, and the polar surfaces are stabilized. Characteristic features of TiC(111) compared to other surfaces of TiC are attributed to the high surface DOS at E_F.     

Density functional theory study on the structural and electronic properties of Ag-adsorbed (MgO)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> clusters

Equilibrium geometries, charge distributions, stabilities and electronic properties of the Ag-adsorbed (MgO) _n (n = 1–8) clusters have been investigated by density functional theory (DFT) with generalized gradient approximation (GGA) for exchange-correlation functional. The results show that hollow site is energetically preferred for n⩾4, and the incoming Ag atoms tend to cluster on the existing Ag cluster. The Mulliken populations indicate that the interaction between the Ag atom and Magnesia clusters is mainly induced by a weak atomic polarization. The adsorbed Ag atom only causes charge redistributions of the atoms near itself. The effect of the adsorbed Ag atom on the bonding natures and structural features of Magnesia clusters is minor. Furthermore, the investigations on the first energy difference, fragmentation energies and electron affinities show that the Ag(MgO)₄ and Ag(MgO)₆ are the most stable among studied clusters. Supported by the Startup Fund of High-level Personal for Shihezi University (Grant No. RCZX200747)     

An investigation of the adsorption of oxygen and oxygen containing species on platinum by photoelectron spectroscopy

It is shown that XPS can detect 0.01 monolayers of adsorbed carbon or oxygen and can identify the chemical state of the adsorbed atom(s). Two states of adsorbed oxygen were resolved by thermal desorption spectroscopy and by XPS. The O 1s binding energies (^FE_B) were 530.2 and 533 eV below the platinum Fermi level for the strongly and weakly adsorbed states respectively. (^FE_B) did not vary with coverage. The resulting apparent variation of (^VE_B), the vacuum level referenced value, is discussed in terms of a simple model for the work function Φ which was measured in situ. UPS indicated that the weakly adsorbed state is probably molecular, with levels at 6.1, 9.3, 10.4 and l2.4 eV below the Fermi level. The main change in the UPS spectra produced by the strongly adsorbed state was a reduction of a peak close to the Fermi level.     

ZnO(0001)表面吸附B的电子结构和光学性质研究

采用基于密度泛函理论的总体能量平面波超软赝势方法,结合广义梯度近似,对清洁ZnO(0001)表面及B/ZnO(0001)吸附体系进行了几何结构优化,计算了B/ZnO(0001)吸附体系的吸附能、能带结构、电子态密度和光学性质.计算结果表明:B在ZnO(0001)表面最稳定的吸附位置是T₄位.吸附后B/ZnO(0001)吸附体系表面带隙有所减小,表面态的组成发生变化,n型导电特性有一定程度的减弱,同时,对紫外光的吸收能力显著增强. 关键词： ZnO(0001)表面 B吸附 电子结构 光学性质     

Fractionally charged excitations in the charge density wave state of a quarter-filled t-J chain with quantum phonons

Elementary excitations of the 4k _F charge density wave state of a quarter-filled strongly correlated electronic one-dimensional chain are investigated in the presence of dispersionless quantum optical phonons using Density Matrix Renormalization Group techniques. Such excitations are shown to be topological solitons carrying charge e/2 and spin zero. Relevance to the 4k _F charge density wave instability in (DI - DCNQI)₂ A g or recently discovered in (TMTTF)₂X ( X=PF ₆, AsF₆) is discussed. Received 30 March 2001 and Received in final form 11 May 2001     

Abnormal electronic transport in disordered graphene nanoribbon

We investigate the conductivity σ of graphene nanoribbons with zigzag edges as a function of Fermi energy E_F in the presence of the impurities with different potential range. The dependence of σ(E_F) displays four different types of behavior, classified to different regimes of length scales decided by the impurity potential range and its density. Particularly, low density of long range impurities results in an extremely low conductance compared to the ballistic value, a linear dependence of σ(E_F) and a wide dip near the Dirac point, due to the special properties of long range potential and edge states. These behaviors agree well with the results from a recent experiment by Miao et al. [Science 317 (2007) 1530 (SOM)].     

SnSe分子外场下的基态性质和激发态性质

对Sn原子使用SDB-cc-pVTZ基组, Se原子采用6-311++G^**基组, 利用密度泛函中的B3LYP方法研究了电场强度为-0.04–0.04 a.u.的外电场对SnSe基态分子的几何结构、 电荷布居分布、 HOMO能级、 LUMO能级、 能隙、 费米能级、 谐振频率和红外光谱强度的影响. 继而使用含时密度泛函(TD-B3LYP) 方法研究了SnSe分子在外场下的激发特性. 结果表明, 外电场的大小和方向对SnSe分子基态的这些性质有明显影响. 在所加的电场范围内(-0.04 a.u.–0.04 a.u.), 随着正向电场的增大, 核间距先减小后增大, 在F=0.03 a .u.时取得最小值0.2317 nm; 分子电偶极矩μ近似线性地增大; E_L, E_H、 费米能级E_F和能隙E_g均减小. 随着正向电场逐渐增大, 分子总能量和谐振频率均先增大后减小; 红外谱强度则先减小后增大, 在F=0.03 a.u.时, 取得最小值 0.1138 km·mol^-1. 由基态到第1–10个单重激发态的波长均随着正向电场的增大而增大. 激发能均随着正向电场的增大而减小. 电场的引入可改变SnSe分子激发态出现的顺序并使得一些禁止的跃迁变得可能. 关键词： SnSe 外电场 能隙 激发特性     

分层掺B和吸附H2O碳纳米管的结构稳定性及电子场发射性能

运用基于第一性原理的密度泛函理论,系统研究了处于外电场中分层掺B并吸附不同数目H₂O碳纳米管体系的结构稳定性和电子场发射性能. 研究表明:第3层掺B并吸附5个H₂O的B₃CNT+5H₂O体系结构最稳定,管帽处Mulliken电荷最密集,尤其与单独掺B的B₃CNT和单独吸附H₂O的B₃CNT+5H₂O相比,其Fermi能级处态密度分别     

Optical properties and electronic structure of US

The optical properties of uranium sulphide single crystals have been determined for the first time. An excellent agreement is found between the structure in the optical spectrum and the results of a self-consistent cellular multiple scattering calculation. The results evidence that the 5? electrons form a resonance state at E_F and that the 5??6d coupling produces a dip in the 6d density of states near E_F which is responsible for many peculiar properties of US.     

Ab initio study of the diffusion mechanisms of gallium in a silicon matrix

We present the results of a study into the diffusion mechanisms of Ga defects in crystalline Si using ab initio techniques. Five stable neutral configurations for single and multi-atom defects are identified by density-functional theory (DFT) calculations within the local density approximation and using a localized basis set as implemented in the SIESTA package. Formation energy (E _F ) calculations on these stable structures show the most likely neutral single-atom defect to be the Ga substitutional, with an E _F of 0.7 eV in good agreement with previous work. Charge state studies show the Ga tetrahedral interstitial defect to be in a + 1 state for most doping conditions. They also indicate the possibility for a gallium substitutional-tetrahedral interstitial complex to act as a deactivating center for the Ga dopants except in n-doped regime, where the complex adopts a − 1 charge state. Migration pathway calculations using SIESTA coupled with the activation relaxation technique (ART nouveau) allow us to determine possible migration paths from the stable configurations found, under various charge states. In general, diffusion barriers decrease as the charge state becomes more negative, suggesting that the presence of Si self-interstitials can enhance diffusion through the kicking out of substitutional Si and by adding negative charge carriers to the system. An overall picture of a possible Ga diffusion and complex formation mechanism is presented based on these results.     

Single atom chemisorption on a bcc metal

The effect of chemisorption of a single atom on the (001) surface of a bcc metal is investigated via the Green's function and the phase shift techniques using the LCAO method and the tight-binding approximation. In particular, we obtain the change in the electronic density of states Δn due to two different binding sites, the on-site and the centered four-fold-site. For each site, the adatom energy level E_a is placed both inside and outside the band, and several adatom-substrate interaction strengths σ are considered. By varying these parameters we obtain a qualitative understanding of the effects on Δn due to either the adsorption of different atoms or a readjustment of E_a arising from a flow of charge onto or away from the adsorbed atom. We compare our results to previous studies of chemisorption on the (001) surface of an s-band simple cubic crystal and find that, although some details distinctive of the bcc metal arise, the overall qualitative features are the same.