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Comment on 'Density dependence of electron-spin polarization and relaxation in intrinsic GaAs at room temperature'
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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 42 (2009) 135111 (6pp) doi:10.1088/0022-3727/42/13/135111
Density dependence of electron-spin
polarization and relaxation in intrinsic
GaAs at room temperature
L H Teng, K Chen,JHWen,WZLinandTSLai
1
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering,
Zhongshan (Sen Yat-Sen) University, Guangzhou, Guangdong 510275, People’s Republic of China
E-mail: stslts@mail.sysu.edu.cn
Received 6 December 2008, in final form 5 May 2009
Published 19 June 2009
Online at
stacks.iop.org/JPhysD/42/135111
Abstract
Time-resolved circularly polarized pump–probe spectroscopy is used to study the
carrier-density dependence of the electron-spin polarization and spin relaxation dynamics in
bulk intrinsic GaAs near the bottom of the conduction band. The experimental result shows
that the initial degree of the electron-spin polarization is less than 0.5, and both the initial
degree of spin polarization and the spin relaxation time decrease with increasing carrier
densities. The simulation calculation shows that the band-gap renormalization effect has a
significant influence on the initial degree of spin polarization, but it is not the physical origin of
the decrease in the electron-spin polarization. Contrarily, the initial degree of spin polarization
can be greatly enhanced by the band-gap renormalization effect for carrier densities above
3.5 × 10
17
cm
−3
. In intrinsic GaAs, both the D’yakonov–Perel’ and the Bir–Aronov–Pikus
mechanisms play an important role. The Bir–Aronov–Pikus mechanism becomes stronger
with the increase in the carrier density, and becomes dominant at high carrier density.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Considerable interest has arisen in studies on the spin degree of
freedom in semiconductors in view of its potential applications
in spintronics and quantum computation [1, 2]. In particular,
achieving high spin polarization and long spin relaxation time
in spin systems is desirable for a number of applications,
such as the efficient injection [3] and transport [4, 5]of
spin-polarized electrons, manipulation and storage of spin
orientation [6] and switching light intensity and polarization
of a semiconductor laser [7]. It is usually thought that a
degree of electron-spin polarization of 0.5 in the conduction
band can be created with a circularly polarized light in bulk
GaAs [8, 9]. However, a recent experiment [8] showed that
only a degree of spin polarization of 0.36 can be achieved,
and the discrepancy between the measured values of 0.36
and 0.5 was explained to be likely induced by the spin-
dependent band-gap renormalization (SD-BGR) effect, but it
1
Author to whom any correspondence should be addressed.
has not been further supported experimentally or theoretically.
However, we point out that the physical explanation on the
occurrence of the degree of spin polarization of 0.36 near the
bottom of the conduction band mentioned above is incorrect,
as discussed later in this paper. That the degree of spin
polarization of 0.5 in the conduction band of GaAs could
be created with a circularly polarized light should mean an
averaged value of the degree of spin polarization over the
whole conduction band. In fact, the degree of spin polarization
in the conduction band is electron-excess-energy (kinetic
energy in the conduction band) dependent and increases with
electron excess energy [10]. The degree of spin polarization
is lower than 0.5 near the bottom of the conduction band and
higher than 0.5 near the high energy tail of the conduction
band. On the other hand, extensive investigations on spin
relaxation have been done experimentally and theoretically,
revealing that spin relaxation can be affected by various factors,
such as temperature [11–13], initial spin polarization [14],
carrier and impurity density [15–17], magnetic field [18, 19]
and drift electric field [20, 21]. Among these factors, the
0022-3727/09/135111+06$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK
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