Electron-phonon interaction at the Si(111)-7 x 7 surface

It is shown that electron-phonon interaction provides a natural explanation for the unusual band dispersion of the metallic surface states at the Si(111)-(7 x 7) surface. Angle-resolved photoemission reveals a discontinuity of the adatom band at a binding energy close to the dominant surface phonon mode at h(omega0) = 70 meV. This mode has been assigned to adatom vibrations by molecular dynamics calculations. A calculation of the spectral function for electron-phonon interaction with this well-defined Einstein mode matches the data. Two independent determinations of the electron-phonon coupling parameter from the band dispersion and from the temperature-dependent phonon broadening yield similar values of lambda = 1.09 and lambda = 1.06.     

Microscopic structure of Er-related optically active centers in crystalline silicon

A successful observation and analysis of the Zeeman effect on the lambda approximately 1.54 microm photoluminescence band in Er-doped crystalline MBE-grown silicon are presented. The symmetry of the dominant optically active centers is conclusively established as orthorhombic I(C(2v)) with g axially approximately 18.39 and g radially approximately 0. In this way the long standing puzzle as regards the paramagnetism of optically active Er-related centers in silicon is settled. Preferential generation of a single type of an optically active Er-related center confirmed in this study is essential for photonic applications of Si:Er.     

Divergence of perturbation theory for bosons

Perturbation theory is studied in two dimensional space-time. There all non-derivative boson self-interactions are renormalizable and in each order of perturbation theory there are no divergences, that is all renormalizations are finite in perturbation theory. Thus the unrenormalized perturbation series may be studied and it is shown that any interaction of the general form leads to Green's functions which are not analytic in at =0. This result holds in momentum space at a large set of points, enough to show that the Green's functions are not distributions in the momenta which are analytic in at =0. Furthermore the proper self energy and the two-particle scattering amplitude are shown not to be analytic in at =0 for certain momenta on or below the bare mass shell. In the course of this analysis we use the integral representations for Feynman graphs to derive a minorization of the form |I)p ₁,...,p _e )|>A B ⁿ for the contribution from alln ^th order connected graphs in a theory with an interaction of the form . Then the constantsA andB depend only on the momentap _i , and not on the structure of a particular graph.     

Interacting Spinor and Scalar Fields in Bianchi Type I Universe Filled with Perfect Fluid: Exact Self-Consistent Solutions

In the framework of Bianchi I (BI) cosmological models a self-consistent system of interacting spinor and scalar fields has been considered. We introduced an interaction function F(I, J) which is an arbitrary function of invariants I and J, generated from the real bilinear forms of the spinor field. Exact self-consistent solutions to the field equations have been obtained for the cosmological model filled with perfect fluid. The initial and the asymptotic behavior of the field functions and of the metric one has been thoroughly studied.     

Spin states of holes in Ge/Si nanowire quantum dots

We investigate tunable hole quantum dots defined by surface gating Ge/Si core-shell nanowire heterostructures. In single level Coulomb-blockade transport measurements at low temperatures spin doublets are found, which become sequentially filled by holes. Magnetotransport measurements allow us to extract a g factor g approximately 2 close to the value of a free spin-1/2 particle in the case of the smallest dot. In less confined quantum dots smaller g factor values are observed. This indicates a lifting of the expected strong spin-orbit interaction effects in the valence band for holes confined in small enough quantum dots. By comparing the excitation spectrum with the addition spectrum we tentatively identify a hole exchange interaction strength chi approximately 130 microeV.     

Improving the charge transfer performance of Si nanomaterial through C surface modification: A first-principles study

Models of oxidized Si (111) surface under different C coverage were established to study the charge transfer ability of Si nanomaterial from strategy of C surface modification using first-principles calculation. The calculated formation energies show that structures of C surface-modified oxidized Si (111) surface are stable. The electronic properties present that the interaction between C and Si atoms is mainly contributed by the hybridization of C-2p and Si-3p states. And the interaction between C and Si atoms increases firstly and then decreases with the increasing C coverage rate. The transfer charge between C and Si reaches a maximum when C coverage rate is 0.5. We speculate that the conductivity of experimentally prepared Si nanomaterial does not monotonously enhance with the increasing C concentration, which would reach a maximum at a certain C concentration, and then decreases.     

Evidence for the two-pole structure of the lambda(1405) resonance

The K- p --> pi0pi0sigma0 reaction is studied within a chiral unitary model. The distribution of pi0sigma0 states forming the lambda(1405) shows, in agreement with a recent experiment, a peak at 1420 MeV and a relatively narrow width of gamma = 38 MeV. The mechanism for the reaction is largely dominated by the emission of a pi0 prior to the K- p interaction leading to the lambda(1405). This ensures the coupling of the lambda(1405) to the K- p channel, thus maximizing the contribution of the second state found in chiral unitary theories, which is narrow and of higher energy than the nominal lambda(1405). This is unlike the pi- p --> K0pisigma reaction, which gives more weight to the pole at lower energy and with a larger width. The data of these two experiments, together with the present theoretical analysis, provide firm evidence of the two-pole structure of the lambda(1405).     

Relative enhancement of photoluminescence intensity of passivated silicon nanocrystals in a silicon dioxide matrix

Photoluminescence (PL) intensity of passivated silicon nanocrystals (Si NCs) embedded in a SiO 2 matrix is compared with that of unpassivated Si NCs. We investigate the relative enhancement of PL intensity (I R ) as a function of annealing temperature and implanted Si ion dose. The I R increases simultaneously with the annealing temperature. This demonstrates an increase in the number of dangling bonds (DBs) with the degree of Si crystallization varying via the annealing temperature. The increase in I R with implanted Si ion dose is also observed. We believe that the near-field interaction between DBs and neighboring Si NCs is an additional factor that reduces the PL efficiency of unpassivated Si NCs.     

Whereabouts of missing atoms in a laser-injected Si: Part 1

In a stealth dicing of Si wafers, voids are formed in laser-induced modified volumes (LIMVs). Most of the voids are free from apparent defects such as dislocations and cracks. Needless to say, in what will become a void (pre-void) upon laser injection, Si atoms are present prior to the laser injection. The critical issue is where these missing atoms are after the laser injection. Two obvious possibilities are that (1) they remain inside the Si wafer as interstitials (I’s) or (2) these I’s reach the surface of the wafer to disappear. If (1) is the case, I’s are to coagulate to form dislocation loops of I-type upon post laser-injection annealing. However, it has been shown that this is not the case. In order to see whether (2) is the case, surfaces of a laser-injected Si wafer were studied by a scanning electron microscope in detail. No evidence of I’s having reached the surfaces was obtained.     

碳纳米管-硅纳米线复合结构的形成和热稳定性

通过分子动力学方法模拟了在碳纳米管内填充一定数目的半导体元素硅形成碳纳米管-硅纳米线复合结构的过程,并采用Lindemann指数研究了这种复合结构的热稳定性．计算结果表明,当考虑碳纳米管和硅纳米线轴向方向的周期性边界条件之后,在C（13,0）和C （14,0）碳纳米管内能够形成亚稳结构的硅纳米线Si₁₆NW和Si₂₀NW,从而获得一种碳纳米管-硅纳米线的新型复合结构．通过计算这种复合结构的Lindemann指数,可以看到由于碳纳米管的保护作用,碳纳米管包裹的硅纳 关键词： 复合结构 纳米线 碳纳米管 分子动力学     

Interpreting measures of frequency selectivity: is forward masking special?

In a previous article [Lutfi, J. Acoust. Soc. Am. 76, 1045-1050 (1984)], the following relation was used to predict measures of frequency selectivity obtained in forward masking from measures obtained in simultaneous masking: F(g) = G + H(g) - H(0), where, for a given masker level, F is the amount of forward masking (in dB) as a function of signal-masker frequency separation (g), H is the amount of simultaneous masking, and G is the amount of forward masking for g = 0. In the present study, the relation was tested for a wider range of signal and masker frequencies, masker levels, and signal delays. The relation described thresholds from all conditions well with the inclusion of one free parameter lambda corresponding to a constant frequency increment, F(g) = G + H(g + lambda) - H(lambda). The parameter lambda was required to account for observed shifts in the frequency of maximum forward masking. It is argued that a single tuning mechanism can account for commonly observed differences between simultaneous- and forward-masked measures of frequency selectivity.     

Work function, valence band and secondary ion intensity variations noted during the initial stages of SIMS depth profiling of Si and SiO2 by Cs

Work function, valence band and ²⁸Si⁻ secondary ion intensity variations from various Si substrates sputtered by 1 keV Cs⁺ at 60° were measured. Oxide free Si wafers and native oxide terminated wafers did not reveal any appreciable valence band variations close to the Fermi edge. Their work functions however, decreased substantially with an exponential trend noted between this and Si⁻ secondary ion intensities from the O free Si wafer. This is consistent with the electron tunneling model which assumes a resonance charge transfer process. Native oxide terminated wafers exhibited deviations from this exponential trend, while Si wafers with thicker oxides revealed the growth of sub-band features in the valence band spectra on sputtering with Cs⁺. These features, may partially, if not fully, explain the Cs⁺ induced enhancement effect noted on SiO₂ substrates where work function based models are not applicable.     

Si(001)面上外延生长的Ru2 Si3电子结构及光学性质研究

基于第一性原理的赝势平面波方法,对异质外延关系为Ru2Si3(100)//Si(001),取向关系为Ru2Si3[010]//Si[110]正交相的Ru2Si3平衡体系下能带结构、态密度和光学性质等进行了理论计算.计算结果表明:当1.087 nm≤a≤1.099 nm时,正交相Ru2Si3的带隙值随着晶格常数a取值的增大而增大.当a取值为1.093 nm时,体系处于稳定状态,此时Ru2Si3是具有带隙值为0.773 eV的直接带隙半导体.Ru2Si3价带主要是由Si的3p,3s态电子及Ru 4d态电子构成;导带主要由Ru的4d及Si的3p态电子构成.外延稳定态及其附近各点处Ru2Si3介电函数的实部和虚部变化趋势基本一致,但外延稳定态Ru2Si3介电函数的曲线相对往低能区漂移,出现的介电峰减少且峰的强度明显增强.     

Carrier injection at silicide/silicon interfaces in nanowire based-nanocontacts

In this article, we investigate the silicide/Si nanowire (Si NW) interface properties based on a detailed characterization of PtSi/NW nanocontacts. For that purpose, we fabricate two-terminal structures implemented on vertical Si NWs arrays defined by a top-down approach with an ultra-high density. Each termination of Si NWs is silicided and contacted to an external metal line. The temperature dependence and the non-linearity of current–voltage (I–V) characteristics are identified as a clear signature indicating that contacts dominate the overall resistance of the Si NW arrays. It is demonstrated that this trend remains valid in the limit of extremely small NW radii and that trap-induced surface depletion also reduces the contact injection cross-section. In this context, the electrostatic landscape at the vicinity of the silicide-to-semiconductor contact interface is dominated by the field effect imposed by peripheral surface states and not by the Schottky barrier height.     

Origin of p(2 x 1) phase on Si(001) by noncontact atomic force microscopy at 5 k

The controversial issue of the origin of the p(2 x 1) reconstruction of the Si(001) surface observed in recent low temperature scanning tunneling microscopy experiments is clarified here using 5 K noncontact atomic force microscopy. The c(4 x 2) phase is observed at separations corresponding to weak tip-surface interactions, confirming that it is the ground state of the surface. At larger frequency shifts the p(2 x 1) phase of symmetric dimers is observed. By studying the interaction of a reactive Si tip with the c(4 x 2) Si(001) surface using an ab initio method, we find that the observed change in the surface reconstruction is an apparent effect caused by tip induced dimer flipping resulting in a modification of the surface structure and appearance of the p(2 x 1) phase in the image. Using an appropriate scanning protocol, one can manipulate the surface reconstruction at will, which has significance in nanotechnology.     

ZnGeP2 optical parametric oscillator with 3.8-12.4-mum tunability

A ZnGeP>(2) (ZGP) optical parametric oscillator (OPO) with wide mid-IR tunability has been demonstrated. The singly resonant angle-tuned ZGP OPO was pumped by 100-ns erbium laser pulses at lambda =2.93mum and yielded output that was continuously tunable from 3.8 to 12.4 mum (type I phase matching) and from 4 to 10 mum (type II phase matching). An OPO pump threshold was less than 1 mJ in the whole 4-12 mum range of the output, and the quantum conversion efficiency reached 35%. An OPO linewidth was typically a few wave numbers; however, with a single intracavity etalon (uncoated Si plate) in a type II OPO it was narrowed to <0.5cm(-1). We demonstrate the sensitive detection of N(2)O gas with the narrow-linewidth OPO.     

Electron emission from metal surfaces by ultrashort pulses: determination of the carrier-envelope phase

The phase varphi of the field oscillations with respect to the peak of a laser pulse influences the light field evolution as the pulse length becomes comparable to the wave cycle and, hence, affects the interaction of intense few-cycle pulses with matter. We theoretically investigate photoelectron emission induced by an intense, few-cycle laser pulse from a metal surface (jellium) within the framework of time-dependent density functional theory and find a pronounced varphi dependence of the photocurrent. Our results reveal a promising route to measuring varphi of few-cycle light pulses (tau<6 fs at lambda=0.8 microm) at moderate intensity levels (I(p) approximately 10(12) W/cm(2)) using a solid-state device.     

effects of rattling phonons on the dynamics of quasiparticle excitation in the beta-pyrochlore KOs(2)O(6) Superconductor

Microwave penetration depth lambda and surface resistance at 27 GHz are measured in high quality crystals of KOs(2)O(6). Firm evidence for fully gapped superconductivity is provided from lambda(T). Below the second transition at T(p) approximately 8 K, the superfluid density shows a steplike change with a suppression of effective critical temperature T(c). Concurrently, the extracted quasiparticle scattering time shows a steep enhancement, indicating a strong coupling between the anomalous rattling motion of K ions and quasiparticles. The results imply that the rattling phonons help to enhance superconductivity, and that K sites freeze to an ordered state with long quasiparticle mean free path below T(p).     

H2在Li掺杂Al6Si+上吸附的理论研究

本文利用密度泛函理论的B3LYP/6-31G(d, p)和组态相互作用的QCISD/6-31G(d, p)研究了Al6Si+和Al6SiLi+团簇的几何和电子结构及其对H2分子的吸附,两种不同方法计算的H2分子在团簇上的吸附能非常一致。H2分子在Al6Si+团簇上的吸附能仅为-0.018 eV,Al6Si+团簇中掺杂Li原子可以明显增强其对H2分子的吸附。Al6SiLi+团簇吸附一个H2分子的吸附能可以达到-0.157 eV,吸附五个H2分子的平均吸附能为-0.088 eV。态密度和自然键轨道分析表明,电荷从Li原子向Si原子转移,H2分子在带正电的Li离子产生的电场中发生极化,从而在静电相互作用下吸附在Li原子周围。     

有自吸收现象时谱线强度与物质浓度的关系

谱线强度(I)与物质浓度(C)之间的定量关系是定量光谱分析的物理基础。由于实际光源中的谱线或多或少地都有自吸收现象,所以在实际情形下,I与C的关系是较为复杂的。本文中应用迁移取样法进行铸铁中的硅的光谱分析,选择了适当的条件,使Si 2516多重谱线系中的Si 2519线在光源中不发生自吸收,因而可以用它的强度代表光源中的物质浓度,同时测定了多重谱线系中发生自吸收最严重的Si 2516线的定标曲线斜度b与物质浓度C的关系(b=klogC+β+1),从而得出I与C的关系式如下:logI=k/2(logC)²+(β+1)logC+α,这里β与α是试验条件下的两个常数;k是b与logI₂₅₁₉的关系直线的斜度。在理想的情形,当谱线没有自吸收时,b=1,k=0,所以logI与logC之间便有直线的关系;当谱线有自吸收时,谱线的强度与物质的浓度有抛物线的关系。这个关系式所描述的与在较大浓度范围内一般实验中所观察的一致。