108 research outputs found
Spin Photovoltaic Effect in Quantum Wires with Rashba Interaction
We propose a mechanism for spin polarized photocurrent generation in quantum
wires. The effect is due to the combined effect of Rashba spin-orbit
interaction, external magnetic field and microwave radiation. The
time-independent interactions in the wire give rise to a spectrum asymmetry in
k-space. The microwave radiation induces transitions between spin-splitted
subbands, and, due to the peculiar energy dispersion relation, charge and spin
currents are generated at zero bias voltage. We demonstrate that the generation
of pure spin currents is possible under an appropriate choice of external
control parameters
Radiation-induced current in quantum wires with side-coupled nano-rings
Photocurrent generation is studied in a system composed of a quantum wire
with side-coupled quantum rings. The current generation results from the
interplay of the particular geometry of the system and the use of circularly
polarized radiation. We study the energy-momentum conservation for optical
transitions involving electrons moving forwards and backwards in the wire. Due
to the lack of time-reversal symmetry in the radiation, the optical transitions
depend on the direction of motion of the electrons, leading to a current at
zero bias voltage. The photocurrent increases with the number of rings within a
wide range of physical parameters. A weak non-linear dependence of the current
in the number of rings, related to quantum interference effects, is also
predicted. This geometry suggests a scalable method for the generation of
sizeable photocurrents based on nanoscale components.Comment: 7 pages, 6 figure
Experimental demonstration of associative memory with memristive neural networks
When someone mentions the name of a known person we immediately recall her face and possibly many other traits. This is because we possess the so-called associative memory - the ability to correlate different memories to the same fact or event. Associative memory is such a fundamental and encompassing human ability (and not just human) that the network of neurons in our brain must perform it quite easily. The question is then whether electronic neural networks - electronic schemes that act somewhat similarly to human brains - can be built to perform this type of function. Although the field of neural networks has developed for many years, a key element, namely the synapses between adjacent neurons, has been lacking a satisfactory electronic representation. The reason for this is that a passive circuit element able to reproduce the synapse behaviour needs to remember its past dynamical history, store a continuous set of states, and be "plastic" according to the pre-synaptic and post-synaptic neuronal activity. Here we show that all this can be accomplished by a memory-resistor (memristor for short). In particular, by using simple and inexpensive off-the-shelf components we have built a memristor emulator which realizes all required synaptic properties. Most importantly, we have demonstrated experimentally the formation of associative memory in a simple neural network consisting of three electronic neurons connected by two memristor-emulator synapses. This experimental demonstration opens up new possibilities in the understanding of neural processes using memory devices, an important step forward to reproduce complex learning, adaptive and spontaneous behaviour with electronic neural networks
Neuromorphic, Digital and Quantum Computation with Memory Circuit Elements
Memory effects are ubiquitous in nature and the class of memory circuit
elements - which includes memristors, memcapacitors and meminductors - shows
great potential to understand and simulate the associated fundamental physical
processes. Here, we show that such elements can also be used in electronic
schemes mimicking biologically-inspired computer architectures, performing
digital logic and arithmetic operations, and can expand the capabilities of
certain quantum computation schemes. In particular, we will discuss few
examples where the concept of memory elements is relevant to the realization of
associative memory in neuronal circuits, spike-timing-dependent plasticity of
synapses, digital and field-programmable quantum computing
Emulation of floating memcapacitors and meminductors using current conveyors
We suggest circuit realizations of emulators transforming memristive devices
into effective floating memcapacitive and meminductive systems. The emulator's
circuits are based on second generation current conveyors and involve either
four single-output or two dual-output current conveyors. The equations
governing the resulting memcapactive and meminductive systems are presented.Comment: Electronics Letters (in press
Accumulation of Electron Spin Polarization at Semiconductor Interfaces
In this Brief Report we study theoretically the propagation of electron spin polarization through an interface separating two n-type semiconductor regions within the two-component drift-diffusion model in an applied electric field. It is assumed that inhomogeneous spin polarization is created locally by a continuous source of spin polarization and is driven through the boundary by the electric field. The spin polarization distribution is calculated analytically. We find that for specific values of parameters describing the system, the electron spin polarization is accumulated near the interface. A simple analytical expression for the amplitude of spin accumulation as a function of the system parameters is found. The obtained results will be useful in designing new spintronic devices
Influence of Nuclear Spin Polarization on Quantum Wire Conductance
In this work, we study a possibility to measure the transverse and
longitudinal relaxation times of a collection of polarized nuclear spins
located in the region of a quantum wire via its conductance. The interplay of
an external in-plane magnetic field, spin-orbit interaction, and the changing
field of the spin-polarized nuclei cause the conductance of the quantum wire to
evolve in time. We show that it is possible to extract the transverse and
longitudinal relaxation times of the spin-polarized nuclei from the time
dependence of the conductance.Comment: Presented at the 2004 IEEE NTC Quantum Device Technology Worksho
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