Alain Nogaret

Electron dynamics in inhomogeneous magnetic fields.

This review explores the dynamics of two-dimensional electrons in magnetic potentials that vary on scales smaller than the mean free path. The physics of microscopically inhomogeneous magnetic fields relates to important fundamental problems in the fractional quantum Hall effect, superconductivity, spintronics and graphene physics and spins out promising applications which will be described here. After introducing the initial work done on electron localization in random magnetic fields, the experimental methods for fabricating magnetic potentials are presented. Drift-diffusion phenomena are then described, which include commensurability oscillations, magnetic channelling, resistance resonance effects and magnetic dots. We then review quantum phenomena in magnetic potentials including magnetic quantum wires, magnetic minibands in superlattices, rectification by snake states, quantum tunnelling and Klein tunnelling. The third part is devoted to spintronics in inhomogeneous magnetic fields. This covers spin filtering by magnetic field gradients and circular magnetic fields, electrically induced spin resonance, spin resonance fluorescence and coherent spin manipulation.

Left and right tunnelling times of electrons from quantum wells in double-barrier heterostructures investigated by the stabilization method

We present a numerical calculation of the tunnelling time of electrons confined in double-barrier structures performed by means of the so-called stabilization method, widely used in quantum chemistry. From the stabilization graphs we find the resonance energy and its width. The method is especially appropriate for treating the case of double-barrier structures (symmetric or non-symmetric) because it allows one to calculate separately the two different tunnelling times (to the left and to the right of the quantum well) contributing to the total lifetime of a resonant level. We use the effective-mass theory. The behaviour of the tunnelling time under applied bias is also investigated and the results are compared with the ones obtained by two alternative approaches, the quasi-classical approximation and the transmission coefficient analysis, respectively. A good agreement between the three methods is obtained for the cases analysed. Finally, the stabilization method as applied here can be employed in the field of scanning tunnelling microscopy of absorbed atoms or molecules where a double-barrier potential also serves as a model for the problem.

Electrical rectification by magnetic edge states

Abstract We investigate electron channelling by inhomogeneous magnetic fields in hybrid semiconductor/ferromagnet structures. A novel type of rectifying behaviour is observed in the current–voltage characteristics measured along the drift path of magnetic edge states. This rectification effect is switched on by the application of the magnetic potential and changes sign with its polarity. We ascribe this non-linear behaviour to the anisotropic electron–phonon interaction arising from the asymmetry of the energy dispersion curves of snake orbits that results in electrons thermalising by emitting phonons along the electron drift direction. We present a theory to evaluate the electromotive force due to this phonon drag effect and compare it to the non-linear behaviour.

Spiking computation and stochastic amplification in a neuron-like semiconductor microstructure

We have demonstrated the proof of principle of a semiconductor neuron, which has dendrites, axon, and a soma and computes information encoded in electrical pulses in the same way as biological neurons. Electrical impulses applied to dendrites diffuse along microwires to the soma. The soma is the active part of the neuron, which regenerates input pulses above a voltage threshold and transmits them into the axon. Our concept of neuron is a major step forward because its spatial structure controls the timing of pulses, which arrive at the soma. Dendrites and axon act as transmission delay lines, which modify the information, coded in the timing of pulses. We have finally shown that noise enhances the detection sensitivity of the neuron by helping the transmission of weak periodic signals. A maximum enhancement of signal transmission was observed at an optimum noise level known as stochastic resonance. The experimental results are in excellent agreement with simulations of the FitzHugh-Nagumo model. Our neuron is therefore extremely well suited to providing feedback on the various mathematical approximations of neurons and building functional networks.

Nanomagnetism and spintronics : fabrication, materials, characterization and applications

Introduction: Spintronics and Nanomagnetism? Fabrication and Growth of Materials: Fabrication of Magnetic Nanostructures Using Electron Beam Induced Chemical Vapor Deposition Preparation of Functional Magnetic Nanoparticles Artificial Magnetic Domain Structures Realized by Focused Ion Beam Irradiation Growth and Properties of Chromium Dioxide (CrO2) A Unique Half-Metallic Ferromagnet for Spintronics Applications Electrodeposition of Magnetic Nanostructures Materials and Characterization: Magnetoelectronic Materials for Spintronics Characterization of Nanostructured Materials by View of Mossbauer Spectroscopy: From Nanoparticles to Superlattices GMR in Electrodeposited Multilayers and Superlattices Spin Transfer Torque Oscillators TMR, FMR and RF: An Overview Electronic and Magnetic Properties of Rare-Earth Nitrides and Related Materials, and Their Potential in Spintronics Devices Diluted Magnetic Semiconductors Applications: Bionanomagnetism Domain Wall for Magnetic Storage Applications.

Tunneling Negative Differential Resistance in a Flexible Active Composite

A novel type of negative differential resistance (NDR) is demonstrated in composites incorporating graphite nanoparticles in a silicone matrix. The NDR occurs as the electric field breaks the π-band of graphite, initiating a semimetal-to-insulator transition. The current peak is robust and tunable with the graphite concentration. This material can produce flexible electronic amplifiers for bioelectronic applications

Molecular scale alignment strategies: An investigation of Ag adsorption on patterned fullerene layers

We have developed a procedure for atomic scale alignment with respect to macroscopic objects. Metallic and etched registration marks on clean reconstructed Si surfaces are used to guide the tip of a scanning tunnelling microscope. The metallic marks are formed from Ta and can withstand thermal cycling up to 1500 K. These procedures have been used to investigate the interaction of Ag with a patterned fullerene multilayer deposited on Si(111)-7×7.

Silicon central pattern generators for cardiac diseases

Cardiac rhythm management devices provide therapies for both arrhythmias and resynchronisation but not heart failure, which affects millions of patients worldwide. This paper reviews recent advances in biophysics and mathematical engineering that provide a novel technological platform for addressing heart disease and enabling beat‐to‐beat adaptation of cardiac pacing in response to physiological feedback. The technology consists of silicon hardware central pattern generators (hCPGs) that may be trained to emulate accurately the dynamical response of biological central pattern generators (bCPGs). We discuss the limitations of present CPGs and appraise the advantages of analog over digital circuits for application in bioelectronic medicine. To test the system, we have focused on the cardio‐respiratory oscillators in the medulla oblongata that modulate heart rate in phase with respiration to induce respiratory sinus arrhythmia (RSA). We describe here a novel, scalable hCPG comprising physiologically realistic (Hodgkin–Huxley type) neurones and synapses. Our hCPG comprises two neurones that antagonise each other to provide rhythmic motor drive to the vagus nerve to slow the heart. We show how recent advances in modelling allow the motor output to adapt to physiological feedback such as respiration. In rats, we report on the restoration of RSA using an hCPG that receives diaphragmatic electromyography input and use it to stimulate the vagus nerve at specific time points of the respiratory cycle to slow the heart rate. We have validated the adaptation of stimulation to alterations in respiratory rate. We demonstrate that the hCPG is tuneable in terms of the depth and timing of the RSA relative to respiratory phase. These pioneering studies will now permit an analysis of the physiological role of RSA as well as its any potential therapeutic use in cardiac disease.

Experimental observation of multistability and dynamic attractors in silicon central pattern generators.

We report on the multistability of chaotic networks of silicon neurons and demonstrate how spatiotemporal sequences of voltage oscillations are selected with timed current stimuli. A three neuron central pattern generator was built by interconnecting Hodgkin-Huxley neurons with mutually inhibitory links mimicking gap junctions. By systematically varying the timing of current stimuli applied to individual neurons, we generate the phase lag maps of neuronal oscillators and study their dependence on the network connectivity. We identify up to six attractors consisting of triphasic sequences of unevenly spaced pulses propagating clockwise and anticlockwise. While confirming theoretical predictions, our experiments reveal more complex oscillatory patterns shaped by the ratio of the pulse width to the oscillation period. Our work contributes to validating the command neuron hypothesis.

Modulation of respiratory sinus arrhythmia in rats with central pattern generator hardware

We report on the modulation of respiratory sinus arrhythmia in rats with central pattern generator (CPG) hardware made of silicon neurons. The neurons are made to compete through mutually inhibitory synapses to provide timed electrical oscillations that stimulate the peripheral end of vagus nerve at specific points of the respiratory cycle: the inspiratory phase (φ(1)), the early expiratory phase (φ(2)) and the late expiratory phase (φ(3)). In this way the CPG hardware mimics the neuron populations in the brainstem which through connections with cardiac vagal motoneurones control respiratory sinus arrhythmia (RSA). Here, we time the output of the CPG hardware from the phrenic nerve activity recorded from rats while monitoring heart rate changes evoked by vagal nerve stimulation (derived from ECG) controlled by the CPG. This neuroelectric stimulation has the effect of reducing the heart rate and increasing the arterial pressure. The artificially induced RSA strongly depends on the timing of pulses within the breathing cycle. It is strongest when the vagus nerve is stimulated during the inspiratory phase (φ(1)) or the early expiratory phase (φ(2)) in which case the heart rate slows by 50% of the normal rate. Heart rate modulation is less when the same exact stimulus is applied during the late expiratory phase (φ(3)). These trials show that neurostimulation by CPG hardware can augment respiratory sinus arrhythmia. The CPG hardware technology opens a new line of therapeutic possibilities for prosthetic devices that restore RSA in patients where respiratory-cardiac coupling has been lost.