Tunneling spectra of submicron Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8+</Subscript><Subscript>δ</Subscript> intrinsic Josephson junctions: evolution from superconducting gap to pseudogap

The electronic, structural, and magnetic properties of Ru and Rh thin films on Ag(001) substrate are investigated by means of density functional calculations. The generalized gradient approximation is used to treat the exchange correlation potential. Alloying, burying, and fully relaxing effects are considered for different degrees of coverage: 0.25, 0.50, 1 and 2 ML. Alloying and burying effects reduce the magnetic moments while fully relaxing effects enlarge them. For Ru, the magnetic moment is high for 0.25 ML and vanishes for 2 ML; however, for Rh, the magnetic moment remains high even for 2 ML. Nevertheless, when cluster formation is analysed we conclude that the absence of magnetism in a number of previous experimental works could be attributed to the formation of big size clusters.     

Argon and krypton ion-induced changes in permalloy thin films

The effects of 75-keV Ar and 100-keV Kr ion irradiations of 72-nm thin DC-sputtered permalloy (Ni₈₁Fe₁₉) films on Si(100) wafers were studied at fluences of up to 10¹⁶ ions/cm². The changes of the structural and magnetic properties were measured via X-ray diffraction, Rutherford backscattering spectroscopy, and magneto-optical Kerr effect. The irradiations increase the lattice constant and improve the crystallinity of the samples. They induce also strong changes of the magnetic polarisation and the coercive field for increasing ion fluence. The hysteresis loops suggest that, with increasing ion fluence, the reversal of the magnetisation changes gradually from rotation-dominated in the as-deposited films to domain-wall-motion dominated at the highest ion fluences. The results are compared with those obtained for Ni-, Cr-and Xe-ion irradiated permalloy films.     

Dynamic behavior of the voter model on fractals: logarithmic-periodic oscillations as a signature of time discrete scale invariance

The understanding of the dynamic behavior of the voter model, in low-dimensional media, is a very interesting open topic. In fact, due to the absence of the interfacial tension, only the interfacial noise becomes relevant during the coarsening processes, bringing the possibility of studing a new physical process. In this way, it is known that below the upper critical dimension (d < 2) and starting from a disordered configuration, a critical coarsening process takes place, and the density of interfaces, ρ(t), decays as a power-law function of time. Recently published numerical studies performed on low-dimensional fractal substrates (d _F < 2) [Physica A 362, 338 (2006)] show the existence of logarithmic-periodic oscillations superimposed on the standard ρ(t) power-law behavior, but the origin of those oscillations remains unclear. In this work, we provide an explanation of these oscillations in terms of the interplay between the dynamics of the voter model and the discrete scale invariance of the underlying fractal substrate. Our arguments are verified by means of extensive numerical simulations carried out on different fractal substrates.     

The study of spin ordering in FM/AFM bilayers with a mixed interface

Monte Carlo simulation is adopted to study the spin ordering in ferromagnet(FM)/antiferromagnet(AFM) bilayers with a mixed interface. We introduce the time-dependent autocorrelation function to describe the thermal stability of the spin ordering for each plane, especially the interface, in FM/AFM bilayers with uncompensated and compensated AFM surfaces. It is found that the thermal decay of spin ordering depends on the plane, the interface coupling and the interface roughness. For the uncompensated AFM surfaces, when the interface coupling is small, the thermal decay of spin ordering is faster at the interface than other planes, while the large interfae coupling makes the spin ordering at the interface become relatively stable. In the case of compensated AFM surfaces, the spin ordering at the interface is thermally disordered much before that at other planes, and the thermal decay becomes slow gradually as the interface coupling increases.     

Dirac magnetic monopole production from photon fusion in proton collisions

We calculate the lowest-order cross-section for Dirac magnetic monopole production from photon fusion ( in p collisions at = 1.96 TeV, pp collisions at = 14 TeV, and we compare with Drell-Yan (DY) production. We find the total cross-section is comparable with DY at = 1.96 TeV and dominates DY by a factor > 50 at = 14 TeV. We conclude that both the and DY processes allow for a monopole mass limit m > 370 GeV based upon the null results of the recent monopole search at the Collider Detector at Fermilab (CDF). We also conclude that production is the leading mechanism to be considered for direct monopole searches at the Large Hadron Collider (LHC).     

Angular dependence of the transverse and vortex modesin magnetic nanotubes

The angular dependence of the transverse and vortex modes in a magnetic nanotube is investigated as a function of the tube geometry, by means of analytical calculations and numerical simulations. A critical radius defining the transition between vortex and transverse reversal modes is determined, leading to low or high coercivity modes just by varying the direction of the external field, in a fixed nanotube.     

Structures and magnetic properties of AunTi2+ (n $\leqslant$ 7) clusters

Base on the density-functional theory, the structural and magnetic properties of Au_nTi₂ ⁺ ( ) clusters are investigated. The two titanium atoms form a dimer in the gold clusters. The second-order energy differences and HOMO-LUMO gap provide a clear explanation of the abundance peaks and odd-even staggering observed recently in photofragmentation experiments. The magnetism of Au_nTi₂ ⁺ cluster shows an odd-even effect when n increases from 1 to 4 and drops to zero at n=5 and 7. The local magnetic moment and charge partition of Ti 4s, 3d orbitals are discussed. The peculiar magnetic properties are related to the structures and the hybridization between the Au 5d, 6s states and Ti 3d, 4s states.     

Transverse energy distributions in participant number model for ultra-relativistic heavy-ion collisions

The transverse energy distribution in ultra-relativistic heavy-ion collisions has been obtained as a convolution over the number of projectile and target participants. The latter is computed using the geometrical overlap model as a function of impact parameter. The data from 10 A GeV to 200 A GeV heavy ion beams on various targets in different pseudo-rapidity domains have been successfully described.     

Momentum distributions of projectile fragments produced in the cold and hot fragmentation of relativistic136Xe and197Au projectiles

The longitudinal-momentum distributions of projectile fragments from 0.8 A GeV¹³⁶Xe and 1 A GeV¹⁹⁷Au projectiles impinging on targets of beryllium and aluminium, respectively, have been measured using the projectile-fragment separator FRS at GSI. Different momentum distributions have been found for two different classes of fragmentation processes: the abundant hot fragmentation with several nucleons evaporated from the prefragments, and the rare cold fragmentation with only protons removed from the projectile, but no nucleons evaporated. The data are compared to model calculations.This article comprises part of the Ph.D. thesis of B. Voss     

Structural and magnetic properties of layered Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript> thin films

Layered cobalt oxides Ca₃Co₄O₉ thin films have been grown directly on c-cut sapphire substrates using pulsed laser deposition. X-ray diffraction and transmission electron microscopy characterizations show that the deposited films present the expected monoclinic structure and a texture along the direction perpendicular to the Al₂O₃(001) plane. The Ca₃Co₄O₉ structure presents six variants in the film plane. Rutherford backscattering spectroscopy shows that the films are stoichiometric and that the film thickness agrees with the nominal value. The susceptibility χ of the films, recorded along the c-axis of the substrate, after field cooling and zero field cooling in an applied field of 1 kOe shows two magnetic transitions at 19 and 370 K which agree well with previous findings on single crystal samples. In turn, at low temperature (5 K), the magnetization curve along the c-axis exhibits coercive field and remanent magnetization much smaller than those reported for bulk samples, which can be related to the influence of structural variants and structural defects.     

Recombination zone and efficiency in bipolar single layer light-emitting devices: a numerical study

The efficiency of organic light-emitting devices (OLEDs) is closely related to the position and width of recombination zone (RCZ) in the emission layer. Based on the drift–diffusion theory of carrier motion in semiconductors, we developed a numerical model for the position and width of the RCZ in bipolar single layer OLEDs. The calculation results show that for a given operation voltage, the position and width of the RCZ are determined by the mobility difference of electrons and holes, and the energy barrier at the two contacts. When the anode and cathode contact are both ohmic, then RCZ will be near the electrode, from which the low-mobility carriers are injected, and the smaller the mobility difference, the wider the RCZ, and the width of RCZ will be maximal when the mobility of holes and electrons are equal. When the anode contact is Schottky, while the cathode contact is ohmic, then the position and width of RCZ will be determined by both the mobility difference and hole–injection energy barrier. When μ _p<μ _n, the RCZ will be at the anode side. When μ _p>μ _n, then RCZ will move away from the anode and become wider, with the increase of the hole injection barrier. For a given hole–injection barrier and mobility of holes and electrons, the position and width of RCZ change with the applied voltage.     

Calculation by plane wave Born approximations of electron-impact ionization of silver and copper

The plane wave Born approximation is used to calculate total electron impact ionization cross section of silver and copper. Wavefunctions of the target and residual ions were modeled by non orthogonal Hartree-Fock and Dirac-Fock orbitals. The wave functions of the atom and residual ion are calculated with allowance for relaxation effects. The one-electron wavefunction of the continuous spectrum for the ejected electron is obtained using single-configuration Hartree-Fock and Dirac-Fock method. The orthogonalization of the ejected electron wave functions to all occupied orbitals of the target atom is performed. Results of calculations are compared to available experimental measurements and theoretical calculations performed by non relativistic one-electron PWBA, where the ejected electrons is modeled by the hydrogenic Coulomb wave function.     

Evidence of glass transition in thin films of maleic anhydride derivatives: Effect of the surfactant coadsorption

The glass transition temperature of poly (maleic anhydride-alt-1-octadecen) and poly (styreneco-maleic anhydride) cumene-terminated thin films has been measured by mechanical relaxation of Langmuir films of these polymers. The dynamical properties show glass-like features (non-Arrhenius relaxation times and non-Debye mechanical response) interpreted by the coupling model. The glass transition temperature values determined by a mechanical relaxation experiment (step-compression) agree very well with those obtained by surface potential measurements. It is found that the glass transition temperature values in thin films decrease by about 100K as compared with those corresponding to the bulk polymers. The coadsorption of the water-insoluble surfactant DODAB decreases the glass transition temperature.     

Excited states populated via nucleon transfer in the reaction32S +208Pb

The population strengths of excited states in nuclei produced via transfer reactions in the 185 MeV³²S +²⁰⁸Pb reaction have been investigated by heavy-ion- coincidence techniques. The cross sections extracted from the spectra, have been analyzed in the framework of the Complex WKB approximation theory.     

On the electronic contribution to the elastic constants in strained quantum wire superlattices of non-parabolic semiconductors with graded structures: Theory and suggestion for experimental determination

Summary  We study the electronic contribution to the second- and third-order elastic constants in strained quantum wire superlattices of non-parabolic semiconductors with graded structures and compare the same with the constituent materials, by formulating the appropriate dispersion laws. It is found, taking InSb/GaSb quantum wire superlattice as an example, that the said contributions increase with decreasing thickness and with increasing electron concentration in oscillatory manners together with the fact that the influence of the finite interface width enhances their numerical values. An experimental method is suggested for determining the electronic contribution to the elastic constants in materials having arbitrary dispersion laws. In addition, the well-known results for constituent semiconductors in the absence of stress have also been obtained as special cases of our generalized formulations.     

On the time evolution of transmittivity in magnetic fluids

When a magnetic fluid is subjected to a magnetic field, a part of the magnetic particles in the fluid agglomerates to form chains. Thus, the ferrofluid becomes optically anisotropic. In this work we describe optically observed patterns in some magnetic fluid films in applied parallel magnetic fields and optical effects of these, especially the optical transmittance. The most interesting experimental observation is that concerning the time dependence of relative transmittivity . For kerosene base ferrofluids relax rapidly at coupling and decoupling magnetic field, but for a transformer-oil magnetic fluid the relaxation times can attain (5–10) minutes, depending on the intensity of applied magnetic field.     

Enhanced magnetization and ferrimagnetic behavior of normal spinel ZnFe2O4 thin film irradiated with femtosecond laser

We report on spatially selective change of magnetism from paramagnetic to ferrimagnetic-like behaviors in normal spinel ZnFe₂O₄ thin film under irradiation with 780 nm femtosecond laser pulses. The distribution of Zn²⁺ and Fe³⁺ ions in the irradiated region on the film surface becomes disordered because of local heating to high temperatures, and the metastable phase of ZnFe₂O₄ is frozen in by the rapid quenching after irradiation, resulting in the formation of the ferrimagnetic phase. The ferrimagnetic phase reverts to the paramagnetic state by annealing at 800°C. The present technique is useful for two-dimensional patterning of magnetic thin films.     

Limited volume thin film deposition on geometrically complicated substrates

In this paper we report a study on the elastic scattering of electrons by lithium and sodium atoms in the presence of circularly polarized resonant laser field within the framework of the two-state rotating wave approximation. The effect of laser on projectile electrons is described by Volkov states. The frequency of the laser field is chosen to match with the 2s–3p (3s–3p) transition frequency in lithium (sodium) atoms. The total and differential elastic cross sections with single photon exchange are calculated for intermediate energies (50–150 eV) and laser intensity (10⁷–10¹¹ W cm^-2). An erratum to this article can be found online at http://dx.doi.org/. An erratum to this article can be found at     

XXZ-like phase in the F-AF anisotropic Heisenberg chain

By means of the Density Matrix Renormalization Group technique, we have studied the region where XXZ-like behavior is most likely to emerge within the phase diagram of the F-AF anisotropic extended (J-J’) Heisenberg chain. We have analyzed, in great detail, the equal-time two-spin correlation functions, both in- and out-of- plane, as functions of the distance (and momentum). Then, we have extracted, through an accurate fitting procedure, the exponents of the asymptotic power-law decay of the spatial correlations. We have used the exact solution of XXZ model (J’ = 0) to benchmark our results, which clearly show the expected agreement. A critical value of J’ has been found where the relevant power-law decay exponent is independent of the in-plane nearest-neighbor coupling.