• Background

Spin transport phenomenon is an important issue in the field of spintronics. In the the semiconductor nano-structures, such as quantum wells and quantum wires, lack of inversion symmetry and presence of time reversal symmetry induce spin-splitting owing to spin orbit interaction even in the absence of magnetic field. This effect is known as Rashba effect which act as an effective magnetic field. Since the strength of spin-splitting can be controlled by the gate voltage, the semiconductor nano-structures is good candidate for spintronics device. In such system, however, spin relaxation takes place by the spin-independent scattering event.

• Spin relaxation length

For this problem, we performed the numerical simulations of spin transport in disordered quantum wires with Rashba spin-orbit interaction.

In the case of single channel, i.e., in the narrowest limit, the spin relaxation is mostly suppressed. With increasing of the wire width, the spin relaxation becomes apparent gradually. The evaluated spin relaxation length are much longer than that obtained within semi-classical theories.

See Kaneko et al., Phys. Rev. B (2008).

• Symmetry crosover in quantum wires

In the disordered system, the translational and rotational symmetries are broken. Then, presence of time reversal symmetry and spin rotational symmetry play a decisive roles on the electron transport properties. In the presence of spin-orbit interaction, the spin-rotational symmetry is broken. In this study, we calculated symmetry sensitive quantities, such as localization length and conductance fluctuation.

In the narrow quantum wires with spin-orbit interaction, the localization length and conductance fluctuation are close to those for the absence of spin-orbit interaction. With increasing of the wire width, the localization length and conductance fluctuation turn into those in strong spin-orbit interaction. We found that such change occurs when the spin relaxation length becomes shorter than the localization length.

This effect can be interpreted as follow: The localization length stands for the spatial extent of wave function. The spin memory survives when the spin relaxation length is longer then the localization length. When the spin relaxation is shorter than the localization length, on the other hand, the effect of spin-orbit interaction in the localization length is apparent.

See Kaneko et al., Phys. Rev. B (2010).