Spin relaxation of "upstream" electrons in quantum wires: Failure of the drift diffusion model

The classical drift diffusion (DD) model of spin transport treats spin relaxation via an empirical parameter known as the ``spin diffusion length''. According to this model, the ensemble averaged spin of electrons drifting and diffusing in a solid decays exponentially with distance due to spin dephasing interactions. The characteristic length scale associated with this decay is the spin diffusion length. The DD model also predicts that this length is different for ``upstream'' electrons traveling in a decelerating electric field than for ``downstream'' electrons traveling in an accelerating field. However this picture ignores energy quantization in confined systems (e.g. quantum wires) and therefore fails to capture the non-trivial influence of subband structure on spin relaxation. Here we highlight this influence by simulating upstream spin transport in a multi-subband quantum wire, in the presence of D'yakonov-Perel' spin relaxation, using a semi-classical model that accounts for the subband structure rigorously. We find that upstream spin transport has a complex dynamics that defies the simplistic definition of a ``spin diffusion length''. In fact, spin does not decay exponentially or even monotonically with distance, and the drift diffusion picture fails to explain the qualitative behavior, let alone predict quantitative features accurately. Unrelated to spin transport, we also find that upstream electrons undergo a ``population inversion'' as a consequence of the energy dependence of the density of states in a quasi one-dimensional structure.Comment: 13 figures. To appear in Phys. Rev.

Effect of discrete impurities on electron transport in ultra-short MOSFET using 3D Monte Carlo simulation

This paper discusses the influence of the channel impurity distribution on the transport and the drive current in short-gate MOSFET. In this purpose, a careful description of electron-ion interaction suitable for the case of discrete impurities has been implemented in a 3D particle Monte Carlo simulator. This transport model is applied to the investigation of 50 nm MOSFET operation. The results show that a small change in the number of doping impurities or in the position of a single discrete impurity in the inversion layer may significantly influence the drain current. This effect is not only related to threshold voltage fluctuations but also to variations in transport properties in the inversion layer, especially at high drain voltage. The results are analyzed in terms of local fluctuations of electron velocity and current density. In a set of fifteen simulated devices the drive current Ion, determined at VGS = VDS = 0.6 V, is found to vary in a range of 23% according to the position of channel impurities.Comment: 31 pages, 13 figures, revised version: discussions and references added, to be published in IEEE Trans. Electron. Device

Schottky-barrier induced spin dephasing in spin injection

An ensemble Monte Carlo method is used to study the spin injection through a ferromagnet-semiconductor junction where a Schottky barrier is formed. It is shown that the Schottky-barrier-induced electric field which is confined in the depletion region and parallel to the injection direction, is very large. This electric field can induce an effective magnetic field due to the Rashba effect and cause strong spin dephasing.Comment: 5 pages, 5 figures. Phys. Rev B 72, 2005 (in press

Monte Carlo study of coaxially gated CNTFETs: capacitive effects and dynamic performance

Carbon Nanotube (CNT) appears as a promising candidate to shrink field-effect transistors (FET) to the nanometer scale. Extensive experimental works have been performed recently to develop the appropriate technology and to explore DC characteristics of carbon nanotube field effect transistor (CNTFET). In this work, we present results of Monte Carlo simulation of a coaxially gated CNTFET including electron-phonon scattering. Our purpose is to present the intrinsic transport properties of such material through the evaluation of electron mean-free-path. To highlight the potential of high performance level of CNTFET, we then perform a study of DC characteristics and of the impact of capacitive effects. Finally, we compare the performance of CNTFET with that of Si nanowire MOSFET.Comment: 15 pages, 14 figures, final version to be published in C. R. Acad. Sci. Pari

Silicene spintronics: Fe(111)/silicene system for efficient spin injection

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Rashba precession in quantum wires with interaction

Rashba precession of spins moving along a one-dimensional quantum channel is calculated, accounting for Coulomb interactions. The Tomonaga--Luttinger model is formulated in the presence of spin-orbit scattering and solved by Bosonization. Increasing interaction strength at decreasing carrier density is found to {\sl enhance} spin precession and the nominal Rashba parameter due to the decreasing spin velocity compared with the Fermi velocity. This result can elucidate the observed pronounced changes of the spin splitting on applied gate voltages which are estimated to influence the interface electric field in heterostructures only little.Comment: now replaced by published versio

Electron Spin Decoherence in Bulk and Quantum Well Zincblende Semiconductors

A theory for longitudinal (T1) and transverse (T2) electron spin coherence times in zincblende semiconductor quantum wells is developed based on a non-perturbative nanostructure model solved in a fourteen-band restricted basis set. Distinctly different dependences of coherence times on mobility, quantization energy, and temperature are found from previous calculations. Quantitative agreement between our calculations and measurements is found for GaAs/AlGaAs, InGaAs/InP, and GaSb/AlSb quantum wells.Comment: 11 pages, 3 figure

Anisotropic splitting of intersubband spin plasmons in quantum wells with bulk and structural inversion asymmetry

In semiconductor heterostructures, bulk and structural inversion asymmetry and spin-orbit coupling induce a k-dependent spin splitting of valence and conduction subbands, which can be viewed as being caused by momentum-dependent crystal magnetic fields. This paper studies the influence of these effective magnetic fields on the intersubband spin dynamics in an asymmetric n-type GaAs/AlGaAs quantum well. We calculate the dispersions of intersubband spin plasmons using linear response theory. The so-called D'yakonov-Perel' decoherence mechanism is inactive for collective intersubband excitations, i.e., crystal magnetic fields do not lead to decoherence of spin plasmons. Instead, we predict that the main signature of bulk and structural inversion asymmetry in intersubband spin dynamics is a three-fold, anisotropic splitting of the spin plasmon dispersion. The importance of many-body effects is pointed out, and conditions for experimental observation with inelastic light scattering are discussed.Comment: 8 pages, 6 figure