Andreev reflection between a normal metal and the FFLO superconductor II: A self-consistent approach

We consider Andreev reflection in a two dimensional junction between a normal metal and a heavy fermion superconductor in the Fulde–Ferrell (FF) type of the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state. We assume s-wave symmetry of the superconducting gap. The parameters of the superconductor: the gap magnitude, the chemical potential, and the Cooper pair center-of-mass-momentum Q, are all determined self-consistently within a mean-field (BCS) scheme. The Cooper pair momentum Q is chosen as perpendicular to the junction interface. We calculate the junction conductance for a series of barrier strengths. In the case of incoming electron with spin σ = ↑ only for magnetic fields close to the upper critical field H_c2, we obtain the so-called Andreev window, i.e. the energy interval in which the reflection probability is maximal, which in turn is indicated by a peak in the conductance. The last result differs with other non-self-consistent calculations existing in the literature.     

Light scattering by electrons in osmium: Effect of pressure

The inelastic electron scattering of light and phonons in single crystals of 5d osmium transition metal has been analyzed at pressures up to 60 GPa and temperatures of 10–300 K. An anomalous increase in the intensity of the spectra of the electron scattering of light with the appearance of pronounced continua at ～580 and 350 cm^?1 for q ‖ [0001] and q ‖ [10\(\overline 1 \)0], respectively, has been observed in a pressure range of 20–30 GPa at an excitation energy of 2.41 eV. The comparison of the q dependences measured and calculated in the framework of the band theory of the spectra implies the significant renormalization of energies and relaxation of electronic states near the Fermi energy and the dependence of the Fermi energy on the pressure and temperature.     

Sonochemical temperature controlled synthesis of pellet-, laminate- and rice grain-like morphologies of a Cu(II) porous metal–organic framework nano-structures

Nano-structures of the Cu(II) metal–organic framework, {Cu(BDT)(DMF)·CH₃OH·0.25DMF}_n (1), which BDT²⁻ is 1,4-benzeneditetrazolate, have been synthesized by the reaction of H₂BDT with Cu(NO₃)₂·6H₂O via ultrasonic irradiation in three different temperatures, which causes different morphologies. The products were characterized by IR spectroscopy, elemental analysis, scanning electron microscopy and X-ray powder diffraction. This study demonstrates that sonochemistry is a suitable method for preparation of metal–organic framework nano-structures and temperature is an effective parameter on morphologies of Cu(II) metal–organic framework nano-structures.     

Oxygen adsorption on Ge(111) surface: II. Alkali metal-covered surfaces

Oxygen adsorption on an alkali metal (a.m.)-covered Ge(111) surface has been studied by means of Auger electron spectroscopy (AES), electron energy loss spectroscopy (ELS), thermal desorption (TD), and work function measurements (WF). It was found that the presence of a.m. results in enhancement of the oxygen adsorption rate. The initial values of the sticking coefficient, S₀, are exponential functions of the work function changes caused by the a.m. adsorption. It was shown that no germanium oxide phases are formed on an alkali-covered Ge surface at 300 K. The oxidation rate at high temperatures is limited by the rearrangement processes taking place in the surface GeO layer. The results obtained show that the alkali metal perturbs the GeO bond to a certain extent but no alkali oxide formation was observed at a.m. covertages under investigation.     

2,7-Carbazole and thieno[3,4-c]pyrrole-4,6-dione based copolymers with deep highest occupied molecular orbital for photovoltaic cells

Three kinds of donor–acceptor (D–A) type photovoltaic polymers were synthesized based on 2,7-carbazole and thieno[3,4-c]pyrrole-4,6-dione (TPD). The conjugation of weakly electron (e)-donating 2,7-carbazole and strongly e-accepting TPD moieties yielded a deep highest occupied molecular orbital (HOMO) and its energy level was fine-controlled to be −5.72, −5.67 and −5.57 eV through the incorporation of thiophene (T), thieno[3,2-b]thiophene (TT) and bithiophene (BT) as a π-bridge. Polymer:[6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM) based bulk heterojunction solar cells exhibited a high open-circuit voltage (V_OC) in the range, 0.86–0.94 V, suggesting good agreement with the measured HOMO levels. Despite the high V_OC, the thiophene (or thienothiophene)-containing PCTTPD (or PCTTTPD) showed poor power conversion efficiency (PCE, 1.14 and 1.25%) because of the very low short-circuit current density (J_SC). The voltage-dependent photocurrent and photoluminescence quenching measurements suggested that hole transfer from PC₇₁BM to polymer depends strongly on the HOMO level of the polymer. The PCTTPD and PCTTTPD devices suffered from electron–hole recombination at the polymer/PC₇₁BM interfaces because of the insufficient energy offset between the HOMOs of the polymer and PC₇₁BM. The PCBTTPD:PC₇₁BM device showed the best PCE of 3.42% with a V_OC and J_SC of 0.86 V and 7.79 mA cm⁻², respectively. These results show that photovoltaic polymers should be designed carefully to have a deep HOMO level for a high V_OC and sufficient energy offset for ensuring efficient hole transfer from PC₇₁BM to the polymer.     

Plasma oscillations in two-dimensional electron systems with a superstructure under Stark quantization conditions

The effect of quantizing electric field on plasma oscillations of two-dimensional electron gas in a system with a periodic potential has been theoretically investigated. The coupled-plasmon spectrum ω(q) is calculated for high temperatures (Δ ? T, where Δ is the conduction miniband width and T is temperature in energy units). The calculations are based on the quantum theory of plasma oscillations in the random-phase approximation, with allowance for the umklapp processes.     

The LEP collider

The LEP collider and the performances which have been achieved are presented in simple terms. Some basic facts of electron circular machine physics are recalled. The ambitious and very successful programmes undertaken to maximize LEP luminosity and energy are described in detail. To cite this article: R. Bailey et al., C. R. Physique 3 (2002) 1107–1120.     

Screening in a δ-doped semiconductor

The screening of an impurity in the quasi-two-dimensional (2D) electron gas in a δ-doped semiconductor structure is investigated. The screened impurity matrix elements are calculated and compared using three different approaches: the 2D random phase approximation (RPA), the corresponding 2D Thomas–Fermi theory and a quasi-three-dimensional (3D) Yukawa-like screening model. It is found that the 2D Thomas–Fermi theory differs from the RPA result, even in the limit of low q vectors, if more than one subband is occupied. This result is explained analytically by closely examining theq → 0 limit of the dielectric tensor. The 2D Thomas–Fermi theory is shown to represent a poor approximation to the RPA whereas the quasi-3D screening model agrees well with the RPA results for not too smallqvectors. Furthermore, this model reduces computing times by orders of magnitude in comparison with the RPA. Thus, our 3D screening model considerably simplifies the calculation of impurity scattering rates in the investigation of the electron mobility in a δ-doping layer.     

New theoretical results about the anisotropy of the secondary emission of Al(001) and Al(110)

The source function giving the number of electrons acquiring the energy E and the direction of propagation Ω at the depth z in a target of aluminium submitted to a bombardment of electrons of an energy of 350 eV is evaluated by a Monte Carlo calculation. This source function is used to study the anisotropy of the secondary electron emission. The influences of the depth dependence and of the anisotropy of the source function on the anisotropy of the secondary electron emission on Al(001) and Al(110) are negligible. These results are considered as an evidence that the observed anisotropy in the emitted beam is due to electron final state effects.     

Excitons in organic semiconductors

In this article, excitonic effects in organic semiconductors investigated within the framework of many-body perturbation theory are reviewed. As an example for this technologically relevant class of materials the oligoacene series is studied. The electron–hole interaction is included by solving the Bethe–Salpeter equation for the electron–hole Green's function. This approach allows for the evaluation of the exciton binding energies, which are of major interest concerning the application in organic opto-electronic devices. We start the discussion with a comparison of the Kohn–Sham band structure with recent angular resolved photo-emission data. Starting from this one-electron band structure we focus on the impact of the electron–hole interactions on the optical properties by solving the Bethe–Salpeter equation. We demonstrate the dependence of the exciton binding energy on the molecular size and emphasize the effect of the intermolecular interaction on the exciton binding energies by means of pressure investigations. To cite this article: P. Puschnig, C. Ambrosch-Draxl, C. R. Physique 10 (2009).     

Theory of dielectric screening and electron energy loss spectroscopy at surfaces

We present an overview of theoretical techniques for describing electron energy loss processes in a reflection geometry. We start from a fundamental representation of the dielectric susceptibility tensor of the semi-infinite crystal, and illustrate how the screening becomes modified by the presence of the surface. A new formalism is also presented which improves upon existing techniques for modeling energy loss, is fully q-dependent, and accounts for nonlocality. The impact of nonlocality, local field effects and other many-body effects is discussed. The theory is supported by some explicit calculations on the GaAs(001)–$c(4\times 4)$ surface. To cite this article: C. Hogan et al., C. R. Physique 10 (2009).     

Interference effects in the electrical conductivity tensors of doped polar semiconductors

In this paper, we develop a general procedure, based on the Kubo formula, for finding the frequency- and wavevector-dependent electrical conductivity tensor for arbitrary polarizations of the applied electric field. This procedure gives careful consideration to both the electron and the ion contributions to the current density. We find that, in addition to the standard electron and ion contributions to the conductivity tensor, there are contributions arising from the quantum interference between the electrons and the ions. These interference effects, which will affect the infrared absorption of a material, are similar to the interference effects observed by Cerdeira, Fjeldly, and Cardona (Solid State Commun.13 (1973), 325–328; Phys. Rev. B8 (1973), 4734–4745; 9 (1974), 4344–4350) in Raman scattering from p-type Si. We then evaluate these cross terms for simple models of a variety of semiconductors. We find that these interference effects are finite-q effects, with the actual dependence of the cross terms on the wavevector q being sensitive to the symmetry of the crystal. We also find that, in principle, these cross terms may be quite large near the TO-phonon frequency ω_TO. The cross terms are evaluated for both n-type and p-type semiconductors, and it is suggested that they are probably most important for p-type materials. However, we also find that the basic structure of these terms is very similar in the two cases.     

Structural,luminescence and biological studies of trivalent lanthanide complexes with N,N′-bis(2-hydroxynaphthylmethylidene)-1,3-propanediamine Schiff base ligand

New eight lanthanide metal complexes were prepared. These complexes were characterized by elemental analysis, molar conductivity measurements, spectral analysis (¹H NMR, FT-IR, UV–vis), luminescence and thermal gravimetric analysis. All Ln(III) complexes were 1:1 electrolytes as established by their molar conductivities. The microanalysis and spectroscopic analysis revealed eight-coordinated environments around lanthanide ions with two nitrate ligands behaving in a bidentate manner. The other four positions were found to be occupied with tetradentate L_III ligand. Tb–L_III and Sm–L_III complexes exhibited characteristic luminescence emissions of the central metal ions and this was attributed to efficient energy transfer from the ligand to the metal center. The L_III and Ln–L_III complexes showed antibacterial activity against a number of pathogenic bacteria.     

Electronic excitation energy transfer and nonstationary processes in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript>:Tl crystals

We report the results of our experimental study and numerical simulation of the electronic excitation energy transfer to impurity centers under conditions where nonstationary processes take place in the hydrogen sublattice of potassium dihydrogen phosphate (KH₂PO₄) single crystals doped with mercury-like Tl⁺ ions (KDP:Tl). We present the experimental results of our investigation of the decay kinetics of the transient optical absorption (100 ns–50 s) of intrinsic defects in the hydrogen sublattice of KDP:Tl obtained by pulsed absorption spectroscopy and the results of our study of the dynamics of the change in steady-state luminescence intensity with irradiation time (1–5000 s). To explain the transfer of the energy being released during electron recombination involving intrinsic KDP:Tl lattice defects, we formulate a mathematical model for the transfer of this energy to impurity Tl⁺ luminescence centers. Within the model being developed, we present the systems of differential balance equations describing the nonstationary processes in the electron subsystem and the hydrogen sublattice; provide a technique for calculating the pair correlation functions Y(r, t) of dissimilar defects based on the solution of the Smoluchowski equation for the system of mobile hydrogen sublattice defects; calculate the time-dependent reaction rate constants K(t) for various experimental conditions; and outline the peculiarities and results of the model parametrization based on our experimental data. Based on our investigation, the dramatic and significant effect of a gradual inertial increase by a factor of 50–100 in steady-state luminescence intensity in the 4.5-eV band in KDP:Tl crystals due to the luminescence of mercury-like Tl⁺ ions has been explained qualitatively and quantitatively.     

Lithium absorption on single-walled boron nitride,aluminum nitride,silicon carbide and carbon nanotubes: A first-principles study

Using the DFT-B3LYP calculations we investigate the adsorption of Li atom on CNT, BNNT, AlNNT and SiCNT. We found that Li atom can be chemisorbed on zig-zag SiCNT with binding energy of −2.358 eV and charge transfer of 0.842 |e|, which are larger than the results of other nanotubes. The binding energy of Li on SiCNT is foun to be stronger than activation energy barrier indicating that Li metal could be well dispersed on SiCNTs. Furthermore, the average voltage caused by the lithium adsorption on SiCNT demonstrated that SiCNTs could exhibit as a stable anode similar to the lithium metal anode. The binding nature has been rationalized by analyzing the electronic structures. Our findings demonstrate that Li-BNNT, Li-SiCNT and Li-AlNNT systems exhibit spin polarized behaviors and can fascinating potential application in future spintronics. Also, Li-SiCNT system with rather small band gap might be a promising material for optical applications and active molecule in its environment.     

Analysis of dispersion of long-wavelength optical phonons in zinc with the help of light scattering

The temperature dependences (5–300 K) of the Raman spectra of E _2g phonons and optical constants in zinc single crystals are measured in the excitation energy range 1.4–2.54 eV. It is found that phonon damping decreases upon an increase in the wavelength of exciting radiation. The obtained results are compared with the dependence of the phonon width on the excitation energy (the probed wave vector of the excitations under investigation), which are presented for the first time for the transition metal osmium, as well as with the calculated electron-phonon renormalization of damping, taking into account the actual distribution of wave vectors.     

Atom scattering as a probe of the surface electron-phonon interaction at conducting surfaces

An atomic projectile colliding with a surface at kinetic energies in the thermal or hyperthermal range interacts with and is reflected by the electronic density well in front of the first layer of target atoms, and it is generally accepted that the repulsive interaction potential is proportional to the density of electrons extending outside the surface. This review develops a complete treatment of the elastic and inelastic scattering of atoms from a conducting surface in which the interaction with the electron density and its vibrations is treated using electron-phonon coupling theory. Starting from the basic principles of formal scattering theory, the elastic and inelastic scattering intensities are developed in a manner that identifies the small overlap region in the surface electron density where the projectile atom is repelled. The effective vibrational displacements of the electron gas, which lead to energy transfer through excitation of phonons, are directly related to the vibrational displacements of the atomic cores in the target crystal via electron-phonon coupling. The effective Debye-Waller factor for atom-surface scattering is developed and related to the mean square displacements of the atomic cores. The complex dependence of the Debye-Waller factor on momentum and energy of the projectile, including the effects of the attractive adsorption well in the interaction potential, are clearly defined. Applying the standard approximations of electron-phonon coupling theory for metals to the distorted wave Born approximation leads to expressions which relate the elastic and inelastic scattering intensities, as well as the Debye-Waller factor, to the well known electron-phonon coupling constant λ. This treatment reproduces the previously obtained result that the intensities for single phonon inelastic peaks in the scattered spectra are proportional to the mode specific mass correction components λ_Q,ν defined by the relationship λ = 〈λ_Q,ν〉. The intensities of elastic diffraction peaks are shown to be a weighted sum over the λ_Q,ν, and the Debye-Waller factor can also be expressed in terms of a similar weighted summation. In the simplest case the Debye-Waller exponent is shown to be proportional to λ and for simple metals, metal overlayers, and other kinds of conducting surfaces values of λ are extracted from available experimental data. This dependence of the elastic and inelastic scattering, and that of the Debye-Waller factor, on the electron-phonon coupling constant λ shows that measurements of elastic and inelastic spectra of atomic scattering are capable of revealing detailed information about the electron-phonon coupling mechanism in the surface electron density.     

Theoretical treatment of excited electronic states of adsorbates on metals: Electron attachment to CO2 adsorbed on Pt(1 1 1)

Photochemistry involving adsorbates on metals often proceeds by photoexcitation of the metal followed by transient attachment of photoemitted electrons to the adsorbate. First principles theoretical methods suitable for describing electronic states embedded in a near continuum of metal to metal excitations are described and an application to electron attachment to CO₂ adsorbed on Pt(1 1 1) is reported. Wavefunctions are constructed by ab initio configuration interaction methods which allow a rigorous resolution of states and differentiation between competing pathways of molecular desorption and dissociation. An embedding theory is used to achieve high accuracy in the adsorbate-surface region. The energy required to form the electron attached state is 5.2 eV for excitation to bent CO₂ and 6.8 eV for excitation to linear CO₂, hence both energies are near the work function of the metal (5.7 eV). The process also involves localization of the metal hole and attraction of the charged adsorbate to the metal. Optimum geometries are calculated and pathways that results in desorption, dissociation by bond rupture directly in the excited electronic state, or dissociation after return to the ground state potential energy surface via vibrational processes are explored.     

Trinal Nature of the Excitation of Bulk Plasmons by a Fast Charged Particle at Grazing Incidence on the Interface between Vacuum and a Metal with Strong Spatial Dispersion

The excitation and propagation of bulk and surface (surface waves of transition radiation of plasmons at frequencies above the plasma frequency) plasma waves by incident electrons moving both in vacuum toward the surface of a metal and inside the metal whose boundary elastically and spectacularly reflects internal nonequilibrium electrons have been analyzed. In contrast to work [B. N. Libenson, J. Exp. Theor. Phys. 113, 553 (2011)], attention in this work is focused on the influence of surface effects on bulk-plasmon excitation by incident electrons. The probabilities and spectra of single characteristic energy loss of an intermediate- energy electron (50–500 eV) moving at an angle to the surface of the medium in three regions: in vacuum, in the medium, and again in vacuum after the electron leaves the medium are calculated. The kinetic approximation is used for the dielectric function, where the entire range of the plasmon spectrum is taken into account correctly for the problem under consideration. In the indicated energy range of incident electrons, surface effects, on the one hand, significantly reduces the probability of excitation of bulk plasma waves in the medium with strong spatial dispersion, in particular, as compared to the results obtained in [B.N. Libenson, J. Exp. Theor. Phys. 113, 553 (2011)], where surface effects were disregarded and the probability of bulkplasmon excitation by a 200-eV electron incident and emitted perpendicularly to the boundary is about one third of that in the unbounded medium. On the other hand, at grazing incidence from vacuum, the probability of transition radiation of bulk plasmons increases significantly and can lead to a change in the character of the angular dependence of the intensity of bulk plasma energy loss. Thus, the main result of this work is that a decrease in the glancing angle of the fast electron with respect to the vacuum–metal interface is accompanied both by an increase in the contribution from the transition mechanism to the probability of bulk-plasmon excitation in the vacuum region and by a decrease in the contribution from Cherenkov and bremsstrahlung mechanisms of excitation in the medium. The probability of bulk-plasmon excitation in the vacuum region exceeds the probability of excitation at the further motion of the electron in metallic aluminum at angles of incidence larger than 65°, 70°, and 75° at the energies E = 200, 350, and 500 eV, respectively.