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 m 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 m 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.