Spintronics in Nanoscale Devices

June 21, 2013 17:18 PSP Book - 9in x 6in 01-Hedin-c01

Chapter 1

Spin-Polarized Transport in Quantum

Dots System with Rashba Spin–Orbit

Interaction

Xing-Tao An

a

and Jian-Jun Liu

b

a

School of Sciences, Hebei University of Science and Technology,

Shijiazhuang, Hebei 050018, China

b

Physics Department, Shijiazhuang University,

Shijiazhuang, Hebei 050035, China

[e-mail]

Recently, spin phenomena in mesoscopic systems have come into

the focus of attention. How to efficiently control and manipulate the

spin in semiconductor mesoscopic structures is the central issue in

spintronics. Quantum dots are among the most studied mesoscopic

systems due to their physical properties as well as their potential

applications in electronic devices. The interplay between spin inter-

ference with spin–orbit interaction or electron interaction gives rise

to a variety of unexpected spin phenomena in mesoscopic systems

and allows moreover to control and engineer the spin of the electron.

In this review, an efficient and flexible numerical algorithm, i.e.,

Keldysh nonequilibrium Green’s function approach is explained and

employed to investigate the spin-dependent transport properties in

multi-quantum dots with spin–orbit interaction and multi-leads.

Spintronics in Nanoscale Devices

Edited by Eric R. Hedin and Yong S. Joe

Copyright

c

 2013 Pan Stanford Publishing Pte. Ltd.

ISBN 978-981-4411-69-1 (Hardcover), 978-981-4411-70-7 (eBook)

www.panstanford.com

© 2014 by Taylor & Francis Group, LLC

June 21, 2013 17:18 PSP Book - 9in x 6in 01-Hedin-c01

2 Spin-Polarized Transport in Quantum Dots System

1.1 Introduction

Spintronics is one of the most attractive investigation frontiers

in condensed matter physics and material science due to its

potential application in nanodevices. The generation and detection

of spin-polarized electric currents through a mesoscopic system are

important issues and main goals for spintronics [1]. Electron spin

automatically comprises two levels that is a natural representation

of a qubit [2–4]; moreover the spin has a long decoherence time. The

quantum dot (QD) is an elementary cell of nanoelectronic devices.

The electron spin in the QD is considered as an ideal candidate for

the qubit to realize quantum computing in the future [1]. In order to

utilize the electron spin in the QD as a qubit, one first has to figure

out how to efficiently polarize and manipulate the spin in the QD,

i.e., writing a spin into the QD. For a mesoscopic QD system, it is

a natural idea to couple with ferromagnetic (FM) leads or to use

magnetic field to control and manipulate the spin [2, 3]. However,

these two methods are not feasible in current experiments. First, it is

very difficult to inject the spin from a FM lead into a semiconductor,

and for the second proposal, one has to confine a strong magnetic

field to the small region of a QD [5, 6]. Therefore, it is desirable

to realize spin-polarized transport or the spin accumulation just

by using the intrinsic property of the QD but not with the help of

complex experimental conditions.

More than 10 years ago, Datta and Das proposed a spin

transistor based on the Rashba spin–orbit interactions (RSOIs) in a

semiconductor sandwiched between two FM leads [7]. Since then,

some theoretical and experimental studies have been proposed to

improve the efficiency of spin polarization in transport systems

based on the RSOI [8–15]. The RSOI can couple the spin degree

of freedom of an electron to its orbital motion and vice versa,

thereby giving a useful handle for manipulating and controlling the

electron spin by external electric fields or gate voltages. The RSOI

is a relativistic effect at the low-speed limit, but it is believed to be

substantial in some semiconductors. During the past two decades, a

great number of studies have been drawn to improve the efficiency

of spin polarization in the transport system based on the RSOI

but not under magnetic field or coupled FM leads [1]. Recently,

© 2014 by Taylor & Francis Group, LLC