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.     

Study on electron scattering in solid targets using accurate transport cross-sections

The Vicanek and Urbassek theory [M. Vicanek, H.M. Urbassek, Phys. Rev. B 44 (1991) 7234] combined with a Monte Carlo simulation are used to investigate the transport of 0.5-4 keV electrons in solid targets. The cross-sections used to describe the electron transport have been calculated via a new improved version of the approximate analytical expression given by Jablonski [A. Jablonski, Phys. Rev. B 58 (1998) 16470]. Some applications are presented here for the calculation of electron backscattering coefficient in semi-infinite Al and Cu targets. The obtained results accord with success with the experiment and clearly represent an improvement with respect to previous theoretical calculations.     

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.     

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     

Reexamining the security of the improved quantum secret sharing scheme

We find that, in the improvement [S.J. Qin et al., Phys. Lett. A 357 (2006) 101] of the multiparty quantum secret sharing [Z.J. Zhang et al., Phys. Rev. A 71 (2005) 044301], Charlie can solely obtain Alice’s secret messages without Bob’s helps. In other words, the improved secret sharing scheme is still insecure. In the end, we further modify Qin et al. improved three-party quantum secret sharing scheme and make it really secure.     

A method for thin foil thickness determination by transmission electron microscopy

With the intention of determining the local thickness within a crystalline thin foil specimen, by means of transmission electron microscopy (TEM), a method previously proposed by Zuo and Shi [J.M. Zuo, Y.F. Shi, Microsc. Microanal. 7 (Suppl. 2) (2001) 224-225] was applied. Using the convergent beam technique, with the incident beam parallel to a zone axis with low indices, diffraction patterns were obtained for some aluminum alloys with low solute content. These patterns were contrasted with those obtained from simulations based on the dynamic theory with Bloch's waves formalism. The local thickness of the thin foil was then obtained by visually comparing the simulated patterns with the experimental one.Comparison of the proposed method with that based on the analysis of two-beam convergent beam patterns [P.M. Kelly, A. Jostsons, R.G. Blake, J.G. Napier, Phys. Stat. Solidi (a) 31 (1975) 771-780] and with that based on the ratio of intensity of the zero loss peak to the total intensity in an electron energy loss spectrum [R.F. Egerton, Electron Energy Loss Spectroscopy in the Electron Microscope, second ed., Plenum Press, New York, 1996] was carried out. A very good agreement between thicknesses determined using the different methods was found. The sensitivity of the method of Zuo et al. was found to be about 1 or 2 nm. The advantages and limitations of the different methods are discussed. The method of Zuo et al. can provide fast and reliable results and can be applied in all modern instruments.     

Monte Carlo calculations of keV electron and positron slowing down in solids. II

An improved Monte-Carlo simulation technique has been used to investigate positron and electron slowing down in solid matter. Elastic scattering is based on exact cross sections of effective crystalline potentials and inelastic processes are described by Gryzinski's semiempirical expression for each core and valence electron excitation. Calculations with normal and oblique angles of incidence have been made for positrons and electrons impinging on semi-infinite aluminium, copper, tungsten, and gold. Interesting differences have been found between positron and electron penetration and backscattering features.     

Angular Distributions for

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Acceleration of ultra-thin electron layer. Analytical treatment compared with 1D-PIC simulation

In this paper, we apply an analytical model [V.V. Kulagin et al., Phys. Plasmas 14, 113101 (2007)] to describe the acceleration of an ultra-thin electron layer by a schematic single-cycle laser pulse and compare with one-dimensional particle-in-cell (1D-PIC) simulations. This is in the context of creating a relativistic mirror for coherent backscattering and supplements two related papers in this EPJD volume. The model is shown to reproduce the 1D-PIC results almost quantitatively for the short time of a few laser periods sufficient for the backscattering of ultra-short probe pulses.     

Magnetic excitations in the low-temperature ferroelectric phase of multiferroic YMn2O5 using inelastic neutron scattering

We studied magnetic excitations in a low-temperature ferroelectric phase of the multiferroic YMn(2)O(5) using inelastic neutron scattering (INS). We identify low-energy magnon modes and establish a correspondence between the magnon peaks observed by INS and electromagnon peaks observed in optical absorption [A. B. Sushkov et al., Phys. Rev. Lett. 98, 027202 (2007).]. Furthermore, we explain the microscopic mechanism, which results in the lowest-energy electromagnon peak, by comparing the inelastic neutron spectral weight with the polarization in the commensurate ferroelectric phase.     

Soft mode dispersion and ‘waterfall’ phenomenon in relaxors revisited

Results of recent inelastic neutron scattering studies of lead-based relaxor ferroelectrics by Gvasaliya et al. [J. Phys.: Condens. Matter 17, 4343 (2005); J. Phys.: Condens. Matter 19, 016219 (2007)] have put in question the existence of the “waterfall” anomaly–an apparent vertical dispersion segment joining the TA and TO branches–observed earlier in low-energy [ξ00] phonon dispersion curves of these materials. In the present article, we review the results of earlier experiments and model calculations together with the outcome of our recent measurements on PMN using the same instrumental set-up as Gvasaliya et al. to conclude that the “waterfall” feature is not an experimental artefact. We also give some hints on a possible explanation of the results of Gvasaliya et al., by exploring the fact that the reported dispersion of the underdamped transverse optic branch follows the longitudinal acoustic (LA) branch dispersion surprisingly closely.     

Simplified models for vibrational energy transfer in proteins

We consider the transfer of vibrational energy in proteins and derive a simplified model for the resonant energy exchange between different vibrational modes. We use the parameters of an earlier study [K. Moritsugu, et al., Phys. Rev. Lett. 85 (2000) 3970 ] to compare our predictions with the results of the molecular dynamics simulations, and reveal an excellent agreement.     

Stabilisation of alkali-adsorbate-induced states on Cu(111) surfaces

The Li and Cs alkali-adsorbate-induced states on a model Cu(111) surface are studied theoretically using the coupled angular mode and the wave packet propagation methods. Their energy position and their decay rate via one-electron transfer into the metal are calculated. An unoccupied alkali-induced state is found with a rather long lifetime, much longer than those found in the case of free-electron metal surfaces. This is attributed to the combined effect of the hybridisation of the alkali levels and of the Cu(111) projected band gap along the normal to the surface that blocks the electron transfer between the adsorbate and the metal. This qualitatively explains the experimental finding of a long-lived state by Bauer et al. [Phys. Rev. B 55 (1997) 10 040] and Ogawa et al. [Phys. Rev. Lett. 82 (1999) 1931].     

Long-lived excited states at surfaces: Cs/Cu(111) and Cs/Cu(100) systems

One-electron and multielectron contributions to the decay of transient states in the Cs/Cu(111) and (100) systems are studied by a joined wave-packet propagation and many-body metal response approach. The long lifetime of these states is due to the Cu L and X band gaps which reduce the electron tunneling between Cs and Cu. In the (111) case, the decay is mainly by inelastic e-e interaction, whereas in the (100) case, electron tunneling is dominating. This accounts very well for the experimental findings [Bauer et al., Phys. Rev. B 55, 10 040 (1997) and Ogawa et al., Phys. Rev. Lett. 82, 1931 (1999)].     

Quasi-one-dimensional In atomic chains on Si(1 1 1) at low temperature studied by reflection high-energy positron diffraction and scanning tunneling microscopy

We have investigated a quasi-one-dimensional structure of Si(1 1 1)-8 × 2-In surface using reflection high-energy positron diffraction (RHEPD) and scanning tunneling microscopy (STM). From the RHEPD rocking curve analyses, we confirmed the formation of the In hexagon structure proposed by González et al. [C. González, F. Flores, J. Ortega, Phys. Rev. Lett. 96 (2006) 136101]. Furthermore, we found that the empty-state STM image at 44 K is consistent with that calculated with the optimum hexagon structure by the RHEPD analyses.     

Electronic transport mechanism in polydiacetylene crystals — variable range hopping or phonon-assisted tunnelling

Experimental results on the current-voltage characteristics of polydiacetylene (PDA) single crystals reported by Aleshin et al [Phys. Rev. Vol. B 69, (2004) art. 214203] are reinterpreted in terms of the phonon-assisted electron tunnelling model. It is shown that the experimental results, measured in the temperature range from 1.8 K to 300 K are consistent with the tunnelling rate dependence on field strength, computed for the same range of temperatures. An advantage of this model over that of Aleshin et al, using the variable range hopping (VRH) model, is the possibility of describing the behaviour of I — V data measured at both high and low temperatures with the same set of parameters characterizing this material. This assertion is confirmed by comparison of the temperature-dependent current-voltage data extracted from Aleshin et al’s work with tunnelling rate dependence on temperature, computed using two different expressions of the phonon-assisted tunnelling theory. The temperature dependence of the conductivity of an ion implanted PDA crystals [B. S. Elman et al, Appl. Phys. Lett., Vol. 46, (1985) p. 100] and polypyrrole [P. Dutta et al, Synth. Met., Vol. 139 (2003) p. 201] are also explained on the basis of this model.      

Diffraction of picosecond bulk longitudinal and shear waves in micron thick films

Investigation of thin metallic film properties by means of picosecond ultrasonics [C. Thomsen et al., Phys. Rev. Lett. 53, 989 (1984)] has been under the scope of several studies. Generation of longitudinal and shear waves [T. Pézeril et al., Phys. Rev. B 73, 132301 (2006); O. Matsuda et al., Phys. Rev. Lett. 93, 095501 (2004)] with a wave vector normal to the film free surface has been demonstrated. Such measurements cannot provide complete information about properties of anisotropic films. Extreme focusing of a laser pump beam (≈0.5 μm) on the sample surface has recently allowed us to provide evidence of picosecond acoustic diffraction in thin metallic films (≈1 μm) [C. Rossignol et al., Phys. Rev. Lett. 94, 166106 (2005)]. The resulting longitudinal and shear wavefronts propagate at group velocity through the bulk of the film. To interpret the received signals, source directivity diagrams are calculated taking into account material anisotropy, optical penetration, and laser beam width on the sample surface. It is shown that acoustic diffraction increases with optical penetration, so competing with the increasing of directivity caused by beam width. Reflection with mode conversion at the film-substrate interface is discussed.     

Comment on breakup densities of hot nuclei

In [V.E. Viola et al., Phys. Rev. Lett. 93 (2004) 132701, D.S. Bracken et al., Phys. Rev. C 69 (2004) 034612] the observed decrease in spectral peak energies of IMFs emitted from hot nuclei was interpreted in terms of a breakup density that decreased with increasing excitation energy. Subsequently, Raduta et al. [Ad. Raduta et al., Phys. Lett. B 623 (2005) 43] performed MMM simulations that showed decreasing spectral peaks could be obtained at constant density. In this Letter we point out that this apparent inconsistency is due to a selective comparison of theory and data that overlooks the evolution of the fragment multiplicities as a function of excitation energy.     

Structure of the SiC (0 0 0 1) 3 × 3 reconstruction studied by surface X-ray diffraction

The surface structure of the 3 × 3 reconstruction of the 4H-SiC (0 0 0 1) surface was investigated with surface X-ray diffraction (SXRD).Of the studied models, the twist model proposed by Starke et al. [U. Starke, J. Schardt, J. Bernhardt, M. Franke, K. Reuter, H. Wedler, K. Heinz, J. Furthmuller, P. Kackell, F. Bechstedt, Phys. Rev. Lett. 80 (1998) 758] gave the best fit to the experimental data. The structural parameters were determined accurately.     

Hosing instability in the blow-out regime for plasma-wakefield acceleration

The electron hosing instability in the blow-out regime of plasma-wakefield acceleration is investigated using a linear perturbation theory about the electron blow-out trajectory in Lu et al. [in Phys. Rev. Lett. 96, 165002 (2006)10.1103/PhysRevLett.96.165002]. The growth of the instability is found to be affected by the beam parameters unlike in the standard theory Whittum et al. [Phys. Rev. Lett. 67, 991 (1991)10.1103/PhysRevLett.67.991] which is strictly valid for preformed channels. Particle-in-cell simulations agree with this new theory, which predicts less hosing growth than found by the hosing theory of Whittum et al.