Resonance split of ballistic conductance peaks in electric and magnetic superlattices

Resonant peak splitting for ballistic conductance in finite electric superlattices (ES) and magnetic superlattices (MS) was investigated theoretically. It is shown that, for electron tunneling through the ES (MS) of identical n electric (magnetic) barriers, the resonance split of the conductance peak is (n–1)-fold; while for electron tunneling through the ES (MS) made of two different barriers, one resonant window of the former splits into two subwindows, within each of which the resonance split is (m–1)-fold, where m is the number of the renormalized building blocks consisting of two different barriers of the latter. Received 15 February 2000     

The mystery of the alkali metals; the induced anomalous Hall effect in thin Cs films

Sandwiches made from Fe and Cs films are investigated as a function of the magnetic field and the Cs thickness. Conduction electrons which cross from the Fe to the Cs are marked by a drift velocity component perpendicular to the electric field. The anomalous Hall effect in the Fe provides this “non-diagonal” kick to the electrons that cross from the Fe into the Cs. The ballistic propagation of the conduction electrons can be monitored as a function of the Cs film thickness. The free propagation into the Cs is measured in terms of the non-diagonal conductance L_xy which we denote as the “induced anomalous Hall conductance”L _xy ⁰. For a normal (non-magnetic) metal in contact with Fe, L_xy increases with the thickness of the normal metal until the film thickness exceeds (half) the mean free path of the conduction electrons. For Cs on top of Fe the induced anomalous Hall conductance increases up to a Cs coverage of about 100 A, then, in contrast to other non-magnetic metals, L _xy ⁰ decreases for larger Cs coverage and approaches zero. This behavior cannot be explained with the free electron model. The strange behavior of the induced AHC in Cs films adds an even more challenging mystery to the already poorly understood properties of thin Cs films. These results defy explanation in the free electron model. Received 29 April 1999 and Received in final form 10 July 1999     

Single electron tunneling at large conductance: The semiclassical approach

We study the linear conductance of single electron devices showing Coulomb blockade phenomena. Our approach is based on a formally exact path integral representation describing electron tunneling nonperturbatively. The electromagnetic environment of the device is treated in terms of the Caldeira-Leggett model. We obtain the linear conductance from the Kubo formula leading to a formally exact expression which is evaluated in the semiclassical limit. Specifically we consider three models. First, the influence of an electromagnetic environment of arbitrary impedance on a single tunnel junction is studied focusing on the limits of large tunneling conductance and high to moderately low temperatures. The predictions are compared with recent experimental data. Second, the conductance of an array of N tunnel junctions is determined in dependence on the length N of the array and the environmental impedance. Finally, we consider a single electron transistor and compare our results for large tunneling conductance with experimental findings. Received 2 February 2000     

Two-dimensional electron systems in inversion layers of p-type Hg0.8Zn0.2Te metal-insulator-semiconductor structures

Strong oscillations on capacitance and conductance have been observed in p-type Hg0.8Zn0.2Te metal-insulator-semiconductor structures, made by using a recent process for the interface passivation. This behaviour is attributed to a two-dimensional electron gas in the n-inversion layer and the variation of the conductance maximums with temperature indicates that the dominant perpendicular transport mechanism for electrons is an incoherent two-step tunnelling through deep levels in the gap. Three models have been used to describe the quantum confinement: the simple variational method, the triangular potential approximation and the propagation matrix method. The later approach takes into account the non parabolicity of the conduction band structure and uses a finite height barrier at the insulator-semiconductor interface. A very good agreement between experimental and calculated values for the two lowest subband energy is obtained. Received 9 February 1999     

Interlayer exchange coupling: Preasymptotic corrections

In the asymptotic limit, the interlayer exchange coupling decays as D^-2, where D is the spacer thickness. A systematic procedure for calculating the preasymptotic corrections, i.e., the terms of order D^-n with ,is presented. The temperature dependence of the preasymptotic corrections is calculated. The results are used to discuss the preasymptotic corrections for the Co/Cu/Co(001) system. Received 7 January 1999     

X-ray emission from aluminium under intense, ultrashort irradiation

We present studies of X-ray emission from aluminium under picosecond and femtosecond irradiation in the intensity range 10¹²-10¹⁵ W cm^-2. We use a new and simple method to measure spectrally resolved absolute X-ray yields. It is shown that the X-ray yields can be obtained for arbitrary levels of X-ray flux. We present details of the variation of the absolute yields as a function of wavelength, intensity, polarization and pulse duration of the incident laser radiation. Electron temperatures in the keV range are observed at 10¹⁵ W cm^-2 with femtosecond laser pulses. Received 12 August 1999 and Received in final form 29 November 1999     

Heat transport through a quantum dot with one-dimensional interacting leads under Coulomb blockade regime

The peculiarities of a low temperature heat transfer through a ballistic quantum dot (a double potential barrier) with interacting leads due to a long-range Coulomb interaction (in the geometrical capacitance approach) are considered. It is found that the thermal conductance K shows periodic peaks as a function of the electrostatic potential of a dot at low temperatures. At the peak maximum it is whereas near the minimum it is . Near the peak maximum the dependence K(T) is essentially nonmonotonic at the temperatures correspondent to the level spacing in the quantum dot. Received 20 October 1999 and Received in final form 20 January 2000     

Universal conductance in quantum wires in the presence of Umklapp scattering

The effects of Umklapp scattering on the zero-temperature conductance in one-dimensional quantum wires are reexamined by taking into account both the screening of external potential and the non-uniform chemical potential shift due to electron-electron interaction. It is shown that in the case away from half-filling the conductance is given by the universal value, 2e ² /h, even in the presence of Umklapp scattering, owing to these renormalization effects of external potential. The conclusion is in accordance with the recent claim obtained for the system with non-interacting leads being attached to a quantum wire. Received: 5 February 1998 / Received in final form: 16 March 1998 / Accepted: 17 April 1998     

Influence of the nematic order on the rheology and conformation of stretched comb-like liquid crystalline polymers

We have studied the rheology and the conformation of stretched comb-like liquid-crystalline polymers. Both the influence of the comb-like structure and the specific effect of the nematic interaction on the dynamics are investigated. For this purpose, two isomers of a comb-like polymetacrylate polymer, of well-defined molecular weights, were synthesized: one displays a nematic phase over a wide range of temperature, the other one has only an isotropic phase. Even with high degrees of polymerization N, between 40 and 1000, the polymer chains studied were not entangled. The stress-strain curves during the stretching and relaxation processes show differences between the isotropic and nematic comb-like polymers. They suggest that, in the nematic phase, the chain dynamics is more cooperative than for a usual linear polymer. Small-angle neutron scattering has been used in order to determine the evolution of the chain conformation after stretching, as a function of the duration of relaxation t _r. The conformation can be described with two parameters only: , the global deformation of the polymer chain, and p, the number of statistical units of locally relaxed sub-chains. For the comb-like polymer, the chain deformation is pseudo-affine: is always smaller than (the deformation ratio of the whole sample). In the isotropic phase, has a constant value, while pincreases as tr. This latter behavior is not that expected for non-entangled chains, in which p varies as t _r ^1/2 (Rouse model). In the nematic phase, decreases as a stretched exponential function of t _r, while p remains constant. The dynamics of the comb-like polymers is discussed in terms of living clusters from which junctions are produced by interactions between side chains. The nematic interaction increases the lifetime of these junctions and, strikingly, the relaxation is the same at all scales of the whole polymer chain. Received 5 May 1999 and Received in final form 18 October 1999     

Free electron-positron pair production in collisions of Au (10.8 GeV/u) with Cu , Ag and Au

Total and differential cross-sections for the production of free electron-positron pairs are calculated for the reactions Au⁷⁹⁺ (10.8 GeV/u) on Cu²⁹⁺, Ag⁴⁷⁺ and Au⁷⁹⁺. The methods used are the semiclassical approach and the solution of the time-dependent Dirac equation by a coupled channel method with free wavepakets describing the created fermions. The obtained total cross-sections are in good agreement with the experimental data. The differential cross-sections give informations about the correlation between the electron and positron. Received 21 December 1999 and Received in final form 6 March 2000     

The interplay between the surface band structure and possible surface reconstructions of Mo(112)

The experimental band structure of Mo(112) and the effects by temperature and adsorbate are presented. A surface resonance, identified as crossing the Fermi level at about 1/3 from to of surface Brillouin zone, was observed to be very sensitive to both contamination and temperature. We find evidence of adsorbate and temperature induced reconstruction of the Mo(112) surface. Examination of low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) data provides evidence for an adsorbate induced reconstruction of the Mo(112) surface with periodicities consistent with the Fermi level crossing of the surface resonance. The reconstruction is found to occur at coverages as low as 0.03 Langmuirs of oxygen or carbon. The reconstruction and/or adsorbate affects the density of states and bands near the Fermi level of a ₁ symmetry. Received 3 March 1999 and Received in final form 1 October 1999     

Hall effect plateaus and giant Shubnikov-de Haas oscillations in (BiSb)Telayered single crystals near the quantum limit

On bulk layered single crystals (Bi_0.25Sb_0.75)₂Te₃ with a hole concentration cm^-3 and a mobility cm²/Vs magnetoresistance and Hall effect investigations were performed in the temperature range T = 1.4 K ... 20 K in magnetic fields up to 18 T. For the magnetic field perpendicular to the layered structure giant Shubnikov-de Haas oscillations are measured; the positions of the maxima are triplets in the reciprocally scaled magnetic field. From the damping of the amplitudes with increasing temperature the cyclotron mass m _c = 0.12m ₀ is evaluated. Correlated with the SdH oscillations doublets of Hall effect plateaus (or kinks in low fields) are found. The weak well known Shubnikov-de Haas oscillations from the generally accepted multivallied highest valence band can be detected as a modulation on the giant oscillation. The high anisotropy of the SdH oscillations and their triplet structure in connection with the layered crystal structure lead us to suggest that the effects are caused by hole carrier pairing (mediated by the bipolaron mechanism) in quasi 2D sheets parallel to the crystal layer stacks. The measured Hall plateau resistances coincide with the quantum Hall effect values considering the number of layer stacks and the valley degeneracy of the 3D hole carrier reservoir. The ratio of spin splitting to Landau (cyclotron) splitting is observed to be . Received: 12 September 1997 / Revised: 8 January 1998 / Accepted: 22 January 1998     

Structural characterization of Rh/pumice SMAD catalysts

The structure of Rh/pumice catalysts prepared by the SMAD (Solvated Metal Atoms Dispersion) technique at different metal loadings has been investigated by EXAFS (Extended X-ray Absorption Fine Structure Spectroscopy), XPS (X-ray Photoelectron Spectroscopy), SAXS (Small-Angle X-ray Scattering), WAXS (Wide-Angle X-ray Scattering) and TEM (Transmission Electron Microscopy). According to EXAFS and XPS, a fraction of the Rh atoms is oxidised, but a noticeable part is also present as Rh ⁰. The Rh oxidation is attributed to the interaction of the Rh atoms with the hydroxyls of the support; after the formation of the oxide, the nucleation of metallic rhodium becomes possible. The WAXS data do not show evidence of rhodium fcc crystallites; the metal-bearing particles are probably amorphous and/or very small, as results from the SAXS and TEM data analysis. The disagreement between the latter two techniques, resulting in a small-angle determination of the average size of the particles that is about half that of TEM in the catalyst with the higher Rh loading, is acknowledged and discussed. Preliminary catalytic tests are described, demonstrating the suitability of using a low surface area support for the preparation of SMAD catalysts. Received 2 February 1999     

Fluid and plastic flow dynamics of the critical state for a strongly pinned 2D superconductor

The frequency of the Raman active A_1g radial breathing mode has been widely used as a tool to estimate the distribution of diameters of single wall carbon nanotubes (SWNT). However, the relation between frequency and diameter is not straightforward and results are model-dependent. Because most of the experiments are performed on bundles and not on isolated tubes, the model should especially take into account the van der Waals intertube interactions. Here, we use a pair-potential approach to account for such interactions and we derive a nonlinear relation between the SWNT diameter and the frequency of the A_1g radial breathing modes. We demonstrate a good agreement between calculations and the diameters derived from diffraction experiments on the same samples. Received 22 December 1999 and Received in final form 17 July 2000     

Coherent resonant transport through a mesoscopic system with quantum ac microwave field

The time-dependent transport through an ultrasmall quantum dot coupling to two electron reservoirs is investigated. The quantum dot is perturbed by a quantum microwave field (QMF) through gate. The tunneling current formulae are obtained by taking expectation values over coherent state (CS), and SU(1,1) CS. We derive the transport formulae at low temperature by employing the nonequilibrium Green function technique. The currents exhibit coherent behaviors which are strongly associated with the applied QMF. The time-dependent currents appear compound effects of resonant tunneling and time-oscillating evolution. The time-averaged current and differential conductance are calculated, which manifest photon-assisted behaviors. Numerical calculations reveal the similar properties as those in classical microwave field (CMF) perturbed system for the situations concerning CS and squeezed vacuum SU(1,1) CS. But for other squeezed SU(1,1) CS, the tunneling behavior is quite different from the system perturbed by a single CMF through gate. Due to the quantum signal perturbation, the measurable quantities fluctuate fiercely. Received 28 May 1998     

Andreev bound states in high temperature superconductors

Andreev bound states at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of Andreev bound states in high temperature superconductors (HTS) employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBa₂Cu₃O (YBCO), Bi₂Sr₂CaCu₂O(BSCCO), and La_1.85Sr_0.15CuO₄ (LSCO) show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS Nd_1.85Ce_0.15CuO_4-y (NCCO) no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented. Received: 17 February 1998 / Revised: 27 April 1998 / Accepted: 23 June 1998     

Modeling and simulations of the behavior of glass particles in a rotating drum in heptane and water vapor atmospheres

We have performed noise measurements on suspended ropes of single wall carbon nanotubes (SWNT) between 1 and 300 K for different values of dc current through the ropes. We find that the shot noise is suppressed by more than a factor 100 compared to the full shot noise 2eI. We have also measured an individual SWNT and found a level of noise which is smaller than the minimum expected. Another finding is the very low level of 1/f noise, which is significantly lower than previous observations. We propose two possible interpretations for the strong shot noise reduction: i) Transport within a rope takes place through few nearly ballistic tubes within a rope and possibly involves non integer effective charges with e ^*∼ 0.3e. ii) A substantial fraction of the tubes conduct with a strong reduction of effective charge (by more than a factor 50). Received 25 January 2002 Published online 19 July 2002     

Simulation of Raman spectra of C and C by non-orthogonal tight-binding molecular dynamics

A non-orthogonal tight-binding molecular-dynamics formalism is used to simulate Raman spectra of the fullerene molecules C₆₀ and C₇₀. Two parametrization schemes for the Hamiltonian and the overlap matrix elements are investigated. The considered molecules are excited randomly and the Fourier transform of the displacement autocorrelation function is employed to extract the vibrational properties. Fair agreement with experiment and with force-constant and ab initio calculations is achieved, with comparatively smaller maximum errors in the frequencies than for other molecular dynamics or semi-empirical calculations from the literature. Received 4 February 1999 and Received in final form 28 November 1999     

Electron excitation of neutral and ionic levels in Townsend discharges of argon at high E/N

We have measured the emission coefficients of the 3p levels of ArI: 3p₁, 3p₅, 3p₆, 3p₇, 3p₈, and 3p₁₀. The data for the 3p₅, 3p₆, 3p₇, 3p₈ and 3p₁₀ levels were converted to excitation coefficients by using quenching coefficients from the literature. Measurements were performed in the range of E/N between to above except for the 3p₇ level where measurements were done only up to . The data for the emission coefficients for Ar II levels include two 4p' levels with terms ² P ⁰ _1/2 and ² F ⁰ _7/2, and three 4p levels with terms ² P ⁰ _1/2, ⁴ P ⁰ _5/2 and ² D ⁰ _5/2. The measurements for the ionic levels were done for E/N above up to nearly . The absolute values of the coefficients were obtained from the intensity of the light emitted at the anode in the parallel plate self-sustained Townsend argon discharges. For low E/N the apparent emission coefficients (i.e. the normalized spatial profile of emission) for both neutral and ionic levels increase exponentially in almost the entire discharge gap. At about the exponentially increasing signal was obtained only near the anode, while at the spatial dependence was flat throughout the electrode gap. Received: 18 January 1999 / Received in final form: 12 April 1999     

Time-evolution of cascade processes of muonic atoms in hydrogen-helium mixtures

The time-dependence of the population of muonic hydrogen states in hydrogen-helium mixtures is calculated for principal quantum number n. The number of muons transferred to helium nuclei is also determined. The dependence of the population of the ground state of muonic hydrogen on time and target density and the helium concentration is also considered. The results are in agreement with recent experimental data. The comparison of the calculated yield of K lines of X-ray in pure hydrogen and deuterium with experimental data indicates the essential role of the Coulomb deexcitation process. Possible Stark mixing is also analysed. Received 17 February 1999 and Received in final form 9 June 1999