Showing papers by "Fernando Flores published in 1993"

Electron correlation resonances in the transport through a single quantum level.

Abstract: Correlation effects in the transport properties of a single quantum level coupled to electron reservoirs are discussed theoretically using a nonequilibrium Green function approach Our method is based on the introduction of a second-order self-energy associated with the Coulomb interaction that consistently eliminates the pathologies of previous perturbative calculations We present results for the current-voltage characteristic illustrating the different correlation effects that may be found in this system, including the Kondo anomaly and Coulomb blockade We discuss the experimental conditions for the simultaneous observation of these effects in an ultrasmall quantum dot

Molecular-orbital theory for the stopping power of atoms in the low-velocity regime : the case of helium in alkali metals

Abstract: A free-parameter linear-combination-of-atomic-orbitals approach is presented for analyzing the stopping power of slow ions moving in a metal. The method is applied to the case of He moving in alkali metals. Mean stopping powers for He present good agreement with local-density-approximation calculations. Our results show important variations in the stopping power of channeled atoms with respect to their mean values.

Model calculation of the noise characteristic in double-barrier heterostructures.

Abstract: Using a nonequilibrium Green's-function formalism, we perform model calculations for the current-noise characteristic of a double-barrier heterostructure. A simplified scheme is proposed to include self-consistency between the electronic charge inside the well and the electrostatic potential along the device. We obtain a general expression for the frequency-dependent current fluctuations that reduces to well-known results given by the transmission approach in the zero-frequency limit. Our numerical results are in good agreement with recent experimental data. By means of an approximate calculation, we show that phase coherence has a negligible effect in the noise characteristic of a double-barrier heterostructure

Self-consistent calculation of the intrinsic bistability in double-barrier heterostructures.

Abstract: The intrinsic bistability of double-barrier heterostructures has been analyzed by using the nonequi- librium Keldysh formalism. In this approach, the induced electron charge in the well is calculated as a function of the applied bias and the electrostatic potential induced in the heterostructure. Self-consistency is achieved by relating this induced potential to the electron charge. Detailed results for different AlAs/GaAs/AlAs double barriers have been analyzed and show good agreement with the experimental results. A bistability behavior is found for high dopings.

Electron States at Semiconductor Interfaces: The Intrinsic and Extrinsic Charge Neutrality Levels

Abstract: A discussion is presented on the semiconductor interface barrier formation. Schottky barriers and heterojunction band offsets are analysed by means of the intrinsic and extrinsic charge neutrality levels. These levels are shown to be controlled by the interface geometry and its local chemistry. The chemical properties and the charge neutrality levels of different Schottky barriers are presented. Heterojunction band offsets are also analysed and are shown to depend on the electronegativity of the metal intralayers deposited at the interface: more electronegative metal atoms tend to reduce the hetero junction band offsets.

Theory of light rectification in scanning tunneling microscopy in the presence of an adsorbed molecule

Abstract: We consider the problem of the rectified current induced by laser radiation in the STM junction when the tip is placed above a small molecule like CO or NO. This is calculated assuming a simple tight-binding model for the tunneling junction including the adsorbate and using non-equilibrium Green's functions techniques. The coupling between tunneling electrons and the molecule vibrational modes is taken into account by a local electron-phonon interaction term. In a second step we estimate the excitation rate of the molecule vibrations for a given laser power. This value is then used to obtain the relative change in the rectified current when the laser is in resonance with a molecule vibration. For a moderate laser power of 2 kW/cm 2 a relative change of 1 to 3% is predicted

The transport and switching of Xe atoms on the Ni-W interface of a scanning tunnelling microscope

Abstract: The transport and switch of Xe atoms on the Ni-W interface of a scanning tunnelling microscope have been analysed for different geometries as a function of the tip position with respect to the Ni(110) surface. Our results show that the control of the different experimental results can only be achieved by a precise control of the position of the tip on the sample. In particular, the tip-sample Xe switch can only be obtained within tip heights of 0.2A.

Dynamic Interaction of Ions with Condensed Matter Using a LCAO Approach

Abstract: A free-parameter LCAO method is used to calculate: (a) the stopping power for He moving in alkali metals in the low velocity limit; (b) the dynamical charge transfer processes between He+ and He* with Al-metal. Our approach allows us to calculate the stopping power dependence of He on the impact parameter for channeling conditions. The dynamical charge transfer processes are discussed as a function of the resonance between the He-2s level and the metal.