FOOD DISTRIBUTION RESEARCH PRIORITIES TO ALLOW MAJOR IMPROVEMENTS IN TOTAL SYSTEM PRODUCTIVITY BY 1985: FOOD PROCESSING

Agricultural and Food Policy,

Correlation Effects on Transport Through Few-Electrons Systems

We study lateral tunneling through a quantum box including electron-electron interactions in the presence of a magnetic field which breaks single particle degeneracies. The conductance at zero temperature as a function of the Fermi energy in the leads consists of a set of peaks related to changing by one the electron occupancy in the box. We find that the position and heights of the peaks are controlled by many-body effects. We compute the conductance up to 8 electrons for several cases where correlation effects dominate. In the range of intermediate fields spin selection rules quench some peaks. At low and high fields the behavior of the conductance as a function of the number of electrons is very different due to big changes in the many-body ground state wavefunctions.Comment: 9 pages, 2 postscript figures, Latex 3.1

A simple stochastic model for the evolution of protein lengths

We analyse a simple discrete-time stochastic process for the theoretical modeling of the evolution of protein lengths. At every step of the process a new protein is produced as a modification of one of the proteins already existing and its length is assumed to be random variable which depends only on the length of the originating protein. Thus a Random Recursive Trees (RRT) is produced over the natural integers. If (quasi) scale invariance is assumed, the length distribution in a single history tends to a lognormal form with a specific signature of the deviations from exact gaussianity. Comparison with the very large SIMAP protein database shows good agreement.Comment: 12 pages, 4 figure

Spin-Orbit Assisted Variable-Range Hopping in Strong Magnetic Fields

It is shown that in the presence of strong magnetic fields, spin-orbit scattering causes a sharp increase in the effective density of states in the variable-range hopping regime when temperature decreases. This effect leads to an exponential enhancement of the conductance above its value without spin-orbit scattering. Thus an experimental study of the hopping conductivity in a fixed, large magnetic field, is a sensitive tool to explore the spin-orbit scattering parameters in the strongly localized regime.Comment: 9 pages + 2 figures (enclosed), Revte

Orbital Magnetism and Current Distribution of Two-Dimensional Electrons under Confining Potential

The spatial distribution of electric current under magnetic field and the resultant orbital magnetism have been studied for two-dimensional electrons under a harmonic confining potential V(\vecvar{r})=m \omega_0^2 r^2/2 in various regimes of temperature and magnetic field, and the microscopic conditions for the validity of Landau diamagnetism are clarified. Under a weak magnetic field (\omega_c\lsim\omega_0, \omega_c being a cyclotron frequency) and at low temperature (T\lsim\hbar\omega_0), where the orbital magnetic moment fluctuates as a function of the field, the currents are irregularly distributed paramagnetically or diamagnetically inside the bulk region. As the temperature is raised under such a weak field, however, the currents in the bulk region are immediately reduced and finally there only remains the diamagnetic current flowing along the edge. At the same time, the usual Landau diamagnetism results for the total magnetic moment. The origin of this dramatic temperature dependence is seen to be in the multiple reflection of electron waves by the boundary confining potential, which becomes important once the coherence length of electrons gets longer than the system length. Under a stronger field (\omega_c\gsim\omega_0), on the other hand, the currents in the bulk region cause de Haas-van Alphen effect at low temperature as T\lsim\hbar\omega_c. As the temperature gets higher (T\gsim\hbar\omega_c) under such a strong field, the bulk currents are reduced and the Landau diamagnetism by the edge current is recovered.Comment: 15 pages, 11 figure

A new perturbation treatment applied to the transport through a quantum dot

Resonant tunnelling through an Anderson impurity is investigated by employing a new perturbation scheme at nonequilibrium. This new approach gives the correct weak and strong coupling limit in $U$ by introducing adjustable parameters in the self-energy and imposing self-consistency of the occupation number of the impurity. We have found that the zero-temperature linear response conductance agrees well with that obtained from the exact sum rule. At finite temperature the conductance shows a nonzero minimum at the Kondo valley, as shown in recent experiments. The effects of an applied bias voltage on the single-particle density of states and on the differential conductances are discussed for Kondo and non-Kondo systems.Comment: 4 pages, 4 figures, submitted to PRB-Rapid Comm. Email addresses [email protected], [email protected]

Modified Perturbation Theory Applied to Kondo-Type Transport through a Quantum Dot under a Magnetic Field

Linear conductance through a quantum dot is calculated under a finite magnetic field using the modified perturbation theory. The method is based on the second-order perturbation theory with respect to the Coulomb repulsion, but the self-energy is modified to reproduce the correct atomic limit and to fulfill the Friedel sum rule exactly. Although this method is applicable only to zero temperature in a strict sense, it is approximately extended to finite temperatures. It is found that the conductance near electron-hole symmetry is suppressed by the application of the magnetic field at low temperatures. Positive magnetoconductance is observed in the case of large electron-hole asymmetry.Comment: 4pages, 5 figure

Spin-orbit Scattering and the Kondo Effect

The effects of spin-orbit scattering of conduction electrons in the Kondo regime are investigated theoretically. It is shown that due to time-reversal symmetry, spin-orbit scattering does not suppress the Kondo effect, even though it breaks spin-rotational symmetry, in full agreement with experiment. An orbital magnetic field, which breaks time-reversal symmetry, leads to an effective Zeeman splitting, which can be probed in transport measurements. It is shown that, similar to weak-localization, this effect has anomalous magnetic field and temperature dependence.Comment: 10 pages, RevTex, one postscript figure available on request from [email protected]