Ordered arrays of quantum wires through hole patterning : ab initio and empirical electronic structure calculations

We propose an approach to the fabrication of one-dimensional nanostructures, based on the design of a pattern of channels onto a semiconductor surface. The feasibility of this approach is demonstrated by means of ab initio and empirical electronic structure calculations. When the channel diameter is sufficiently larger than the interstitial space, the resulting pillars constitute an ordered array of electronically independent, though mechanically connected nanowires. In the opposite regime a tunable metamaterial results. The proposed method provides a path to the realization of uniform quantum wires-both in size and doping characteristics-while easing electrical contacting

Bidirectional resonant tunneling spin pump

We propose a mechanism for achieving bidirectional spin pumping in conventional nonmagnetic semiconductorresonant tunnelingheterostructures under zero magnetic field. The device is designed specifically to take advantage of the special spin configuration described by the Rashba effect in asymmetric quantum wells. It induces the simultaneous flow of oppositely spin-polarized current components in opposite directions through spin-dependent resonant tunneling, and can thus generate significant levels of spin current with very little net electrical current across the tunnel structure, a condition characterized by a greater-than-unity current spin polarization. We also present modeling results on temperature dependence and finite device size effects

Thermal transport in porous Si nanowires from approach-to-equilibrium molecular dynamics calculations

We study thermal transport in porous Si nanowires (SiNWs) by means of approach-to-equilibrium molecular dynamics simulations. We show that the presence of pores greatly reduces the thermal conductivity, κ, of the SiNWs as long mean free path phonons are suppressed. We address explicitly the dependence of κ on different features of the pore topology-such as the porosity and the pore diameter-and on the nanowire (NW) geometry-diameter and length. We use the results of the molecular dynamics calculations to tune an effective model, which is capable of capturing the dependence of κ on porosity and NW diameter. The model illustrates the failure of Matthiessen's rule to describe the coupling between boundary and pore scattering, which we account for by the inclusion of an additional empirical term

High-frequency behavior of the Datta-Das spin transistor

We have studied the high-frequency behavior and the ultimate limiting factor to the cutoff frequency for the Datta-Das spin transistor using deviceMonte Carlo simulations. We have found that the maximum frequency of operation is not related to intrinsic parameters to the spin of the carriers, such as the Larmor frequency or the spin lifetime, but to the transit time through the channel

Desarrollo de software para el procesado numérico en tarjetas gráficas

Debido al gran número de transistores por mm2 que hoy en día podemos encontrar en las GPU convencionales, en los últimos años éstas se vienen utilizando para propósitos generales gracias a que ofrecen un mayor rendimiento para computación paralela. Este proyecto implementa el producto sparse matrix-vector sobre OpenCL. En los primeros capítulos hacemos una revisión de la base teórica necesaria para comprender el problema. Después veremos los fundamentos de OpenCL y del hardware sobre el que se ejecutarán las librerías desarrolladas. En el siguiente capítulo seguiremos con una descripción del código de los kernels y de su flujo de datos. Finalmente, el software es evaluado basándose en comparativas con la CPU.Degut al gran nombre de transistors per mm2 que avui en dia podem trobar a les GPU convencionals, en els darrers anys es venen utilitzant per a propòsits generals degut a que ofereixen un major rendiment per a la computació paral·lela. Aquest projecte implementa el producte sparse matrix-vector a sobre OpenCL. Als primers capítols fem una revisió de la base teòrica necessària per a comprendre el problema. Després veurem els fonaments d'OpenCL y del hardware a sobre el que s'executarà les llibreries desenvolupades. Al següent capítol seguirem amb una descripció del codi dels dos kernels i el seu flux de dades. Finalment, el software és avaluat en base a comparatives amb la CPU.Due to the large number of transistors per mm2 that today we can find in conventional GPU, in recent years it have been used for general purpose since they offer higher performance for data parallel computations. This project implements the product sparse matrix-vector on OpenCL. In the first chapter we review the theoretical basis necessary to understand the problem. Then we will see the foundations of OpenCL and hardware on which to run the developed software. In the next chapter will continue with a description of the kernel code and its data flow. Finally, the software is evaluated based on comparisons with the CPU

A resonant spin lifetime transistor

We present a device concept for a spintronictransistor based on the spin relaxation properties a two-dimensional electron gas(2DEG). The device design is very similar to that of the Datta and Das spin transistor. However, our proposed device works in the diffusive regime rather than in the ballistic regime. This eases lithographical and processing requirements. The switching action is achieved through the biasing of a gate contact, which controls the lifetime of spins injected into the 2DEG from a ferromagnetic emitter, thus allowing the traveling spins to be either aligned with a ferromagnetic collector or randomizing them before collection. The device configuration can easily be turned into a memory and a readout head for magnetically stored information

Resonant interband tunneling spin filter

We propose an InAs/GaSb/AlSb-based asymmetric resonant interband tunnelingdiode as a spin filter. The interband design exploits large valence band spin-orbit interaction to provide strong spin selectivity, without suffering from fast hole spin relaxation. Spin filtering efficiency is also enhanced by the reduction of tunneling through quasibound states near the zone center, where spin spitting vanishes and spin selectivity is difficult. Our calculations show that, when coupled with an emitter or collector capable of lateral momentum selectivity, the asymmetric resonant interband tunnelingdiode can achieve significant spin filtering in conventional nonmagnetic semiconductor heterostructures under zero magnetic field

Correlation-mediated processes for electron-induced switching between Néel States of Fe antiferromagnetic chains

The controlled switching between two quasistable Néel states in adsorbed antiferromagnetic Fe chains has recently been achieved by Loth et al. [Science 335, 196 (2012)] using tunneling electrons from an STM tip. In order to rationalize their data, we evaluate the rate of tunneling electron-induced switching between the Néel states. Good agreement is found with the experiment, permitting us to identify three switching mechanisms: (i) low STM voltage direct electron-induced transitions, (ii) intermediate STM voltage switching via spin-wave-like excitation, and (iii) high STM voltage transitions mediated by domain-wall formation. Spin correlations in the antiferromagnetic chains are the switching driving force, leading to a marked chain-size dependence

Quantum size effects in hafnium-oxide resistive switching

Discrete changes of conductance of the order of G0 = 2e2/h reported during the unipolar reset transitions of Pt/HfO2/Pt structures are interpreted as the signature of atomic-size variations of the conducting filament (CF) nanostructure. Our results suggest that the reset occurs in two phases: a progressive narrowing of the CF to the limit of a quantum wire (QW) followed by the opening of a spatial gap that exponentially reduces the CF transmission. First principles calculations show that oxygen vacancy paths in HfO2 with single- to few-atom diameters behave as QWs and are capable of carrying current with G0 conductance

Quantum size effects in hafnium-oxide resistive switching

Discrete changes of conductance of the order of G0 = 2e2/h reported during the unipolar reset transitions of Pt/HfO2/Pt structures are interpreted as the signature of atomic-size variations of the conducting filament (CF) nanostructure. Our results suggest that the reset occurs in two phases: a progressive narrowing of the CF to the limit of a quantum wire (QW) followed by the opening of a spatial gap that exponentially reduces the CF transmission. First principles calculations show that oxygen vacancy paths in HfO2 with single- to few-atom diameters behave as QWs and are capable of carrying current with G0 conductance