Approximation of photonuclear interaction cross-sections

Photonuclear interaction cross-sections from the GEANT4 database are approximated for all nuclei and all energies (from the hadron production threshold to about 40 TeV). The approximation methods in the giant-dipole resonance region, nucleon resonance region, and high-energy region are improved with respect to existing approximations. As an application of the approximation for photonuclear cross-sections, an improved method of calculating electronuclear cross-sections is developed. The interaction cross-section of virtual photons with nuclei at high Q² are approximated and a simple algorithm for describing the electronuclear reactions, including high-Q² scattering, is proposed. Received: 22 February 2002 / Accepted: 6 May 2002     

Quantum fluctuations in a single electron box

Recent experiments have demonstrated that the numbern of additional electrons on a small metallic island is a staircase function of a continuous external chargen _x for temperaturesT small compared to the single electron charging energyU. We show that the finite conductanceg of the tunnel barrier connecting the island to the external gate gives rise to quantum fluctuations inn which lead to a smearing of the staircase even at zero temperature. In the experimentally relevant case of wide junctions and in the limit of small conductanceg1 the slope <n>/n _x at the turning point between two plateaus saturates at a finite value of order 1/g asT0 instead of diverging likeU/T as predicted with thermal fluctuations only. The experimentally observed broadening however is still much larger which is probably due to extrinsic effects.     

Measurement of RLT and ATL in the 4He(e, e’p)3H reaction at pmiss of 130-300 MeV/ c

We have measured the ⁴He(e, ep)³H reaction at missing momenta of 130-300 MeV/c using the three-spectrometer facility at the Mainz microtron MAMI. Data were taken in perpendicular kinematics to allow us to determine the response function R_LT and the asymmetry term A_TL. The data are compared to both relativistic and non-relativistic calculations.     

The conductance and capacitance-frequency characteristics of Au/pyronine-B/p-type Si/Al contacts

The rectifying junction characteristics of the organic compound pyronine-B (PYR-B) film on a p-type Si substrate have been studied. The PYR-B has been evaporated onto the top of p-Si surface. The barrier height and ideality factor values of 0.67 ± 0.02 eV and 2.02 ± 0.03 for this structure have been obtained from the forward bias current-voltage (I-V) characteristics. The energy distribution of the interface states and their relaxation time have been determined from the forward bias capacitance-frequency and conductance-frequency characteristics in the energy range of ((0.42 ± 0.02) − E_v)-((0.66 ± 0.02) − E_v) eV. The interface state density values ranges from (4.21 ± 0.14) × 10¹³ to (3.82 ± 0.24) × 10¹³ cm⁻² eV⁻¹. Furthermore, the relaxation time ranges from (1.65 ± 0.23) × 10⁻⁵ to (8.12 ± 0.21) × 10⁻⁴ s and shows an exponential rise with bias from the top of the valance band towards the midgap.     

Electrical Characterization of Metal-Insulator-Metal Capacitors with Atomic-Layer-Deposited HfO2 Dielectrics for Radio Frequency Integrated Circuit Application

Metal-insulator-metal （MIM） capacitors with atomic-layer-deposited HfO2 dielectric and TaN electrodes are investigated for rf integrated circuit applications. For 12nm HfO2, the fabricated capacitor exhibits a high capacitance density of 15.5fF/μm2 at 100kHz, a small leakage current density of 6.4 × 10^-9 A/cm^2 at 1.8V and 125℃, a breakdown electric field of 2.6 MV//cm as well as voltage coefficients of capacitance （VCCs） of 2110ppm/V^2 and -824 ppm/V at 100kHz. Further, it is deduced that the conduction mechanism in the high field range is dominated by the Poole-Frenkel emission, and the conduction mechanism in the low field range is possibly related to trap-assisted tunnelling. Finally, comparison of various HfO2 MIM capacitors is present, suggesting that the present MIM capacitor is a promising candidate for future rf integrated circuit application.     

Ultrathin oxide films and interfaces for electronics and spintronics

Oxides have become a key ingredient for new concepts of electronic devices. To a large extent, this is due to the profusion of new physics and novel functionalities arising from ultrathin oxide films and at oxide interfaces. We present here a perspective on selected topics within this vast field and focus on two main issues. The first part of this review is dedicated to the use of ultrathin films of insulating oxides as barriers for tunnel junctions. In addition to dielectric non-magnetic epitaxial barriers, which can produce tunneling magnetoresistances in excess of a few hundred percent, we pay special attention to the possibility of exploiting the multifunctional character of some oxides in order to realize ‘active’ tunnel barriers. In these, the conductance across the barrier is not only controlled by the bias voltage and/or the electrodes magnetic state, but also depends on the barrier ferroic state. Some examples include spin-filtering effects using ferro- and ferrimagnetic oxides, and the possibility of realizing hysteretic, multi-state junctions using ferroelectric barriers. The second part of this review is devoted to novel states appearing at oxide interfaces. Often completely different from those of the corresponding bulk materials, they bring about novel functionalities to be exploited in spintronics and electronics architectures. We review the main mechanisms responsible for these new properties (such as magnetic coupling, charge transfer and proximity effects) and summarize some of the most paradigmatic phenomena. These include the formation of high-mobility two-dimensional electron gases at the interface between insulators, the emergence of superconductivity (or ferromagnetism) at the interface between non-superconducting (or non-ferromagnetic) materials, the observation of magnetoelectric effects at magnetic/ferroelectric interfaces or the effects of the interplay and competing interactions at all-oxide ferromagnetic/superconducting interfaces. Finally, we link up the two reviewed research fields and emphasize that the tunneling geometry is particularly suited to probe novel interface effects at oxide barrier/electrode interfaces. We close by giving some directions toward tunneling devices exploiting novel oxide interfacial phenomena.     

Giant magnetoresistance in Co–Al2O3 granular films prepared by self-organized growth

Co–Al₂O₃ granular films with a narrow distribution in cluster size of Co clusters embedded in Al₂O₃ matrix were prepared by sequential deposition based on self-organized growth. Resistivity dependence of giant magnetoresistance (GMR) was studied. The GMR takes a maximum of 5.2% at room temperature and 9.4% at 13 K and 5700 Gs when the resistivity of the sample is 4×10⁵–7×10⁵ μΩ cm. The temperature dependence of resistivities and GMR were discussed especially. A temperature dependence of conductance ρ∼exp[T₁/(T+T₀)] was found, which indicates the dominant conduction mechanism is fluctuation-induced tunneling. A linear relationship of GMR versus T was observed, GMR=a–kT, in applied magnetic field 5700 Gs. The remarkable character of temperature dependence of GMR should be due to the special microstructure that the clusters are monodispersed in the films.     

Reproducible resistive switching effect for memory applications in heterocontacts based on strongly correlated electron systems

The transitional processes in heterocontacts based on strongly correlated electron systems (SCES) are studied for analyzing of the effect of resistive switching (ERS). It has been shown that the process is asymmetric with respect to switching into “on” and “off” states, the switching time is controlled by a voltage level, this time can be less than microseconds, on the other hand, relaxation processes can reach tens seconds. The switching is controlled by two processes: a change in the resistance state of the normal metal/SCES interface under effect of electric current field and by electrodiffusion of oxygen to vacancies, at that the doping level of the contact area and resistive properties of the heterocontact change. In particular, electrodiffusion of mobile oxygen induced by the electric field makes it possible to use a device with ERS as a memristor. On the other hand, a possibility to control the switching time and ON and OFF parameters show the possibilities to use these devices as memory elements “RAM”.     

Study on the device characteristics of FePc and FePcCl organic thin film Schottky diodes: Influence of oxygen and post deposition annealing

Device characteristics of Al/FePc/Au and Al/FePcCl/Au are performed and found to show rectification properties. The basic diode parameters of the device are determined. The electrical conductivity has been measured both after exposure to oxygen for 20 days and after annealing at temperature up to 473 K. Current density-voltage characteristics under forward bias are found to be due to ohmic conduction at lower voltage regions. At higher voltage regions there is space charge limited conductivity (SCLC) controlled by a discrete trapping level above the valance edge. The electrical parameters of oxygen doped and annealed samples in the ohmic and SCLC region are determined. The reverse bias curves are interpreted in terms of a transition from electrode-limited Schottky emission to the bulk-limited the Poole-Frenkel effect. The Schottky barrier parameters of oxygen doped and annealed structures of FePc and FePcCl are determined from the C²-V characteristics.     

Experimental evidence and consequences of rare events in quantum tunneling

Tiny spatial fluctuations of tunnel barrier parameters are shown to have dramatic consequences on the statistical properties of quantum tunneling. A direct experimental evidence is provided that the tunnel current through metal-oxide junctions, imaged at a nanometric scale, exhibits broad statistical distributions extending over more than 4 orders of magnitude. Striking effects of broad current distributions are shown: the total tunnel transmission is dominated by few highly transmitting sites and the typical current density varies strongly with the size of the junction. Moreover, self-averaging of the tunnel current fluctuations occurs only for unexpectedly large junction areas. Received 1 April 1999