Spin-Transport Dynamics of Optically Spin-Polarized
Electrons in GaAs Quantum Wires
Tetsuomi Sogawa, Hiroaki Ando, and Seigo
Ando
Physical Science Laboratory
It is interesting to investigate the possibility
of applying spin states of free carriers
in semiconductors to electronic and optical
devices. In low-dimensional nanostructures
such as quantum wires (QWRs) and quantum
dots, the spin relaxation process differs
largely from that of bulk and quantum well
structures. It was also theoretically predicted
that in an ideal single-mode QWR electron-electron
(e-e) scattering between spin-polarized electrons
is completely forbidden, leading to spontaneously
spin-polarized transport. In this study,
we investigate the transport and spin relaxation
dynamics of spin-polarized electrons in narrow
QWRs using spatial- and time-resolved photoluminescence
(PL) measurement.
We used trench-buried GaAs/AlAs QWRs with
a 12 nm x 12 nm cross section. The energy
separation between conduction subbands of
this narrow wire is large enough to neglect
the inter-subband e-e scattering process
that allows the scattering between the parallel
spin electrons. The QWR sample was excited
by 1.5 ps circularly polarized laser pulses
to create spin-polarized carriers. Figure
1 shows the temporal variations of the PL
intensity profile. In this sample, a monotonous
gradient of the quantized potential exists
over about ten ƒÊm from the pattern edge
because of the non-uniformity. Thus, by creating
carriers near the edge, we can observe the
drift motion caused by the potential gradient
along QWRs, as shown in Fig.1. Figure 2 shows
the spatial dependence of the degree of spin
polarization. The spin relaxation strongly
varies with the position. Note that electrons
near the drift front, about 10-15 ƒÊm from
the excitation position, surprisingly maintain
a relatively high spin polarization. The
experiment can be explained by considering
the dependence of the diffusivity and mobility
on the spin polarization, demonstrating the
possibility of spin-dependent e-e scattering
that affects the transport properties in
the one-dimensional structures.
[1] T. Sogawa, H. Ando, and S. Ando, Phys.
Rev. B 58 (1998) 1565.
[2] T. Sogawa, H. Ando, and S. Ando, Phys.
Rev. B 61 (2000) 5535.
Fig. 1. Normalized spatial profiles of PL
intensity along the wire direction.
Fig. 2. Spatial variation of the degree of
spin polarization.
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