Electron impinging on metallic thin film targets

Based on the Vicanek and Urbassek theory [M. Vicanek, H.M. Urbassek, Phys. Rev. B 44 (1991) 7234] combined to a home-made Monte Carlo simulation, the present work deals with backscattering coefficients, mean penetration depths and stopping profiles for 1-4 keV electrons normally incident impinging on Al and Cu thin film targets. The cross-sections used to describe the electron transport are calculated via the appropriate analytical expression given by Jablonski [A. Jablonski, Phys. Rev. B 58 (1998) 16470] whose new improved version has been recently given [Z. Rouabah, N. Bouarissa, C. Champion, N. Bouaouadja, Appl. Surf. Sci. 255 (2009) 6217]. The behavior of the backscattering coefficient, mean penetration depth and stopping profiles versus the metallic film thickness at the nanometric scale and beyond is here analyzed and discussed.     

Electron range-energy relationships for calculating backscattering coefficients in elemental and compound semiconductors

Backscattering coefficients for electrons normally impinging on Si, Ge, GaN, GaAs and InSb targets have been calculated by using the Vicanek and Urbassek theory [M. Vicanek, H.M. Urbassek, Phys. Rev. B 44 (1991) 7234] for incident energies ≤5 keV. Electron range has been calculated from various semi-empirical analytical expressions. The cross-sections used to describe the electron transport are determined via the appropriate analytical expression given by Jablonski [A. Jablonski, Phys. Rev. B 58 (1998) 16470] whose new improved version has been recently reported by Rouabah et al. [Z. Rouabah, N. Bouarissa, C. Champion, N. Bouaouadja, Appl. Surf. Sci. 255 (2009) 6217]. The results may be seen as the first predictions for low-energy electron backscattering coefficients impinging on GaN, GaAs and InSb semiconductors. The models used in the calculation of the electron range affect both the accuracy and behaviour of the electron backscattering coefficients.     

Penetration range and backscattering ratio of low-energy electrons in metals

The accuracy of the Ashley's optical-data model [J.C. Ashley, J. Electron Spectrosc. Relat. Phenom. 50 (1990) 323] for describing the energy loss rate and inelastic mean free path of low-energy electrons in Al and Cu has been examined through the use of Ashley's model for calculating the electron penetration range. The latter has also been calculated using the empirical expressions reported by Iskef et al. [Phys. Med. Biol. 28 (1983) 535] and Gryzinski excitation function. The resulting range of penetration is used for determining the electron backscattering coefficients via the use of Vicanek and Urbassek theory [Phys. Rev. B 44 (1991) 7234] where the transport cross-sections are obtained from Rouabah et al. [Appl. Surf. Sci. 255 (2009) 6217] approximation. Besides the electron backscattering coefficients have been calculated from Monte Carlo simulation in which the inelastic scattering processes are handled using Ashley dielectric approach. It is found that the use of Ashley's optical model via Monte-Carlo method gives backscattering coefficients more accurate than those obtained via Vicanek and Urbassek theory. This confirms the accuracy of Ashley's optical model and shows that the Monte-Carlo method is much more accurate and precise than the Vicanek and Urbassek semi-empirical approach.     

Improved expression for calculating electron transport cross sections

We report a simplified correction for the electron transport cross sections (TCSs) for a number of selected atomic targets ranging from H to U and electron energies between 50 and 4000 eV. The correction has been made to the approximate analytical expression of transport cross sections derived by Jablonski [A. Jablonski, Phys. Rev. B 58 (1998) 16470] where an argued parameter is introduced. The latter is obtained from a polynomial fit. The energy dependence of the percentage deviation between TCSs from the corrected expression and those obtained from other sources is presented. The TCSs calculated in the present work showed better agreement with accurate values of TCSs than those reported in earlier publications. This may facilitate the evaluation of parameters needed for quantitative Auger-electron spectroscopy and X-ray photoelectron spectroscopy.     

Calculation of transport cross sections for positrons in solid targets via improved expressions

Transport cross sections (TCS) for positron impinging on a number of selected atomic targets have been calculated in the energy range 1–4 keV. The calculations were performed by using a simplified correction of the approximate analytical expression of transport cross sections reported by Jablonski [A. Jablonski, Phys. Rev. B 58 (1998) 16470]. An argued parameter–obtained from both an exponential and polynomial fits–was then introduced. The agreement generally observed between our results and those previously reported in the literature is quite good, especially for positron energies greater than 2 keV. Furthermore, an attempt has been made for scaling the TCS with the atomic number of solid targets of interest. Such a scaling was found to be possible for both exponential and third order polynomial fits. The information gathered by the present study may be useful for the evaluation of parameters needed for quantitative low-energy positron annihilation spectroscopy.     

Universal Scaling Law for Atomic Diffusion and Viscosity in Liquid Metals

The recently proposed scaling law relating the diffusion coefficient and the excess entropy of liquid [Samanta A et al. 2004 Phys. Reu. Lett. 92 145901; Dzugutov M 1996 Nature 381 137], and a quasi-universal relationship between the transport coefficients and excess entropy of dense fluids [Rosenfeld Y 1977 Phys. Rev. A 15 2545],are tested for diverse liquid metals using molecular dynamics simulations. Interatomic potentials derived from the glue potential and second-moment approximation of tight-binding scheme are used to study liquid metals.Our simulation results give sound support to the above-mentioned universal scaling laws. Following Dzugutov,we have also reached a new universal scaling relationship between the viscosity coefficient and excess entropy.The simulation results suggest that the reduced transport coefficients can be expressed approximately in terms of the corresponding packing density.     

Backscattering coefficients and mean penetration depths of 1–4 keV electron scattering in solids

In the present study, we have performed Monte Carlo simulation of 1–4 keV electrons impinging on semi-infinite Al and Au to determine the transport cross-section, the backscattering coefficient and the mean penetration depth using a new approximation of the differential elastic scattering cross section. The mean number of the wide angle collisions suffered by the electron before slowing down to rest, and the backscattering coefficient are analytically calculated using Vicanek and Urbassek theory. The analytical results are compared with the numerical ones obtained from Monte Carlo simulation. The present results are found to reveal good agreement with experimental results. PACS 68.37.-d; 68.37.Nq     

Simple approach to the mesoscopic open electron resonator: quantum current oscillations

The open electron resonator, described by Duncan et al. [D.S. Duncan, M.A. Topinka, R.M. Westervelt, K.D. Maranowski, A.C. Gossard, Phys. Rev. B 64 (2001) 033310. [1]], is a mesoscopic device that has attracted considerable attention due to its remarkable behaviour (conductance oscillations), which has been explained by detailed theories based on the behaviour of electrons at the top of the Fermi sea. In this work, we study the resonator using the simple quantum quantum electrical circuit approach, developed recently by Li and Chen [Y.Q. Li, B. Chen, Phys. Rev. B 53 (1996) 4027. [2]]. With this approach, and considering a very simple capacitor-like model of the system, we are able to theoretically reproduce the observed conductance oscillations. A very remarkable feature of the simple theory developed here is the fact that the predictions depend mostly on very general facts, namely, the discrete nature of electric charge and quantum mechanics; other detailed features of the systems described enter as parameters of the system, such as capacities and inductances.     

Bi- and Au-Induced Reconstructions on GaAs（001）-2 × 4 Surface

Submonolayer Bi and Au adsorptions on the GaAs（001）-2× 4 surface are investigated by scanning tunnelling microscopy, low energy electron diffraction and first-principles calculations. The 1 ×4 and 3 × 4 reconstructed surface induced by Bi and Au, respectively, are revealed and their structural models are proposed based on experiments and first-principles calculations. Moreover, the validity of the recently proposed generalized electron counting （GEC） model [Phys. Rev. Lett. 97 （2006） 126103] is examined in detail by using the two surfaces. The GEC model perfectly explains the structural features, such Bi-1 × 4 surface and the 3x arrangement of four-atom Au as the characteristic short double-line structure in the clusters.     

Angular Distributions for

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Mixing,ergodicity and slow relaxation phenomena

Investigations on diffusion in systems with memory [I.V.L. Costa, R. Morgado, M.V.B.T. Lima, F.A. Oliveira, Europhys. Lett. 63 (2003) 173] have established a hierarchical connection between mixing, ergodicity, and the fluctuation–dissipation theorem (FDT). This hierarchy means that ergodicity is a necessary condition for the validity of the FDT, and mixing is a necessary condition for ergodicity. In this work, we compare those results with recent investigations using the Lee recurrence relations method [M.H. Lee, Phys. Rev. B 26 (1982) 2547; M.H. Lee, Phys. Rev. Lett. 87 (2001) 250601; M.H. Lee, J. Phys. A: Math. Gen. 39 (2006) 4651]. Lee shows that ergodicity is violated in the dynamics of the electron gas [M.H. Lee, J. Phys. A: Math. Gen. 39 (2006) 4651]. This reinforces both works and implies that the results of [I.V.L. Costa, R. Morgado, M.V.B.T. Lima, F.A. Oliveira, Europhys. Lett. 63 (2003) 173] are more general than the framework in which they were obtained. Some applications to slow relaxation phenomena are discussed.     

Study of Strange Quark Mass in CFL Phase

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The determination of the lifetime of hot electrons in metals by time-resolved two-photon photoemission: the role of transport effects, virtual states, and transient excitons

Experience has shown that theoretically determined lifetimes of bulk states of hot electrons in real metals agree quantitatively with the experimental ones, if theory fully takes into account the crystal structure and many-body effects of the investigated metal, i.e., if the Dyson equation is solved at the ab initio level and the effective electron–electron interaction is determined beyond the plasmon-pole approximation. Therefore the hitherto invoked transport effect [Knoesel et al.: Phys. Rev. B 57, 12812 (1998)] does not seem to exist. In this paper we show that likewise neither virtual states [Hertel: et al. Phys. Rev. Lett. 76, 535 (1996)] nor damped band-gap states [Ogawa: et al.: Phys. Rev. B 55, 10869 (1997)] exist, but that the hitherto unexplained d-band catastrophe in Cu [Cu(111), Cu(110)] can be naturally resolved by the concept of the transient exciton. This is a new quasiparticle in metals, which owes its existence to the dynamical character of dielectric screening at the microscopic level. This means that excitons, though they do not exist under stationary conditions, can be observed under ultrafast experimental conditions. Received: 30 March 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Structure of the hydrogen stabilized MgO(1 1 1)-(1 × 1) surface from low energy electron diffraction (LEED)

A structural study has been performed on the MgO(1 1 1)-(1 × 1) surface by low energy electron diffraction (LEED) using experimental data obtained with a delay-line-detector LEED (DLD-LEED) system to minimize electron damage. It was found that the surface is terminated by a hydroxide layer with the top O-Mg interlayer spacing equal to 1.02 Å, which is close to the spacings between Mg and O planes in bulk brucite crystals (Mg(OH)₂). This is in good agreement with a recent study using photoelectron diffraction (PhD) spectroscopy and density functional theory calculation (DFT) [V.K. Lazarov, R. Plass, H.-C. Poon, D.K. Saldin, M. Weinert, S.A. Chambers, M. Gajdardziska-Josifovska, Phys. Rev. B 71 (2005) 115434]. The second interlayer spacing shows a small expansion of 3% and the third is bulk-like, while the DFT calculation predicted that the spacings below the top one are all bulk-like. This result clearly favors hydroxylation [K. Refson, R.A. Wogelius, D.G. Fraser, M.C. Payne, M.H. Lee, V. Milman, Phys. Rev. B 52 (1995) 10823] as a way of stabilizing the MgO(1 1 1) surface at low temperature over metallization, which has a top layer spacing of 0.86 Å for O termination and 1.25 Å for Mg termination [Lazarov et al. 2005; T. Tsukada, T. Hoshino, Phys. Soc. Jpn. 51 (1982) 2562, J. Goniakowski, C. Noguera, Phys. Rev. B 60 (1999) 16120].     

Top Quark Flavor Changing Decay t

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Dynamics of Two-Photon Lasers with

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Re-study of Nucleon Pole Contribution in J/

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Sigma Terms and Strangeness Contents of Baryon Octet in Modified Chiral Perturbation Theory

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Electronic structure and shearing in nanolaminated ternary carbides

We have studied shearing in M₂AlC phases (M=Sc,Y,La,Ti,Zr,Hf,V,Nb,Ta,Cr,Mo,W) using ab initio calculations. We propose that these phases can be classified into two groups based on the valence electron concentration induced changes in C₄₄. One group comprises M=V B and VIB, where the C₄₄ values are approximately 170 GPa and independent of the corresponding MC. The other group includes M=IIIB and IVB, where the C₄₄ shows a linear dependency with the corresponding MC. This may be understood based on the electronic structure: shear resistant bands are filled in M₂AlC phases with M=V B and VIB, while they are not completely filled when M=IIIB and IVB. This notion is also consistent with our stress-strain analysis. These valence electron concentration induced changes in shear behaviour were compared to previously published valence electron concentration induced changes in compression behaviour [Z. Sun, D. Music, R. Ahuja, S. Li, J.M. Schneider, Phys. Rev. B 70 (2004) 092102]. These classification proposals exhibit identical critical valence electron concentration values for the group boundary. However, the physical mechanisms are not identical: the classification proposal for the bulk modulus is based on MC-A coupling, while shearing is based on MC-MC coupling.     

Bouncing branes

We investigate (4+1)- and (5+0)-dimensional gravity coupled to a non-compact scalar field sigma-model in the context of a single-brane-world scenario with separable metric and a bulk fluid. We briefly discuss the standard cosmological solutions and the family of warp factors (which includes both the original Randall–Sundrum [Phys. Rev. Lett. 83 (1999) 3370, hep-ph/9905221; Phys. Rev. Lett. 83 (1999) 4690, hep-th/9906064] solution and the solution of Kachru, Schulz and Silverstein [H.A. Chamblin, H.S. Reall, Nucl. Phys. B 562 (1999) 133, hep-th/9903225; S. Kachru, M. Schulz, E. Silverstein, Phys. Rev. D 62 (2000) 045021, hep-th/0001206]) for the case of a rolling fifth radius [C. Kennedy, E.M. Prodanov, Phys. Lett. B 488 (2000) 11, hep-th/0003299]. We show how this model can be adjusted so that it describes the standard cosmology on a self-tuning domain wall (with static fifth radius) [C. Kennedy, E.M. Prodanov, hep-th/0010202] and we discuss the solutions. Searching for a possible relation to the negative Euclidean stress energy, appearing in the Giddings and Strominger's axion induced topology change in quantum gravity and string theory [S.B. Giddings, A. Strominger, Nucl. Phys. B 306 (1988) 890], we modify the non-compact sigma-model into a single-field model (with a rolling fifth radius, separable metric, and no bulk fluid) for the more general case of a brane with non-zero curvature parameter. We find a solution (with a Kachru–Schulz–Silverstein warp factor [Phys. Rev. D 62 (2000) 045021, hep-th/0001206]), representing a Tolman wormhole for a brane with Lorentz metric and for a brane with positive definite metric.