Spin Control by Electric Field
- Introduction
In semiconductor spintronics, electron spin rather than charge is the key
property. We are studying on several spin-related devices using spin-orbit
interaction. We have experimentally confirmed that the spin-orbit interaction
in a semiconductor two-dimensional electron gas channel can be controlled
by a gate voltage. This is the first step towards the creation of functional
spin devices such as a spin field effect transistor, a spin filter, and
a spin interference device.
- Gate control of spin-orbit interaction
It is well known that an external magnetic field lifts spin degeneracy. Even without an external magnetic field, an electric field perpendicular to the 2DEG yields an effective magnetic field for moving electrons. The possibility of gate control of the spin-orbit interaction was discussed in the early 1990s. However, there has been no clear experimental evidence of such controllability until we experimentally verified gate-voltage control of spin-orbit interaction in InGaAs/InAlAs heterostructures. The spin-orbit interaction parameter has since been obtained from the beating pattern in the Shubnikov-de Haas (SdH) oscillations. The dominant mechanism governing the change in the spin-orbit interaction parameter is the Rashba spin-orbit interaction. A weak anti-localization analysis has also shown that the spin-orbit interaction can be controlled by the gate voltage.
Reference
"Gate control of spin-orbit interaction in an inverted InGaAs/InAlAs heterostructure"
J. Nitta, T. Akazaki, H. Takayanagi, and T. Enoki
Phys. Rev. Lett., 78, 1335 (1997).
"Rashba spin-orbit coupling probed by the weak antilocalization analysis
in InAlAs/InGaAs/InAlAs quantum wells as a function of quantum well asymmetry"
T. Koga, J. Nitta, T. Akazaki, and H. Takayanagi
Phys. Rev. Lett., 89, 046801 (2002).
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Fig. 1(a)
Gate voltage dependence of SdH oscillations. Beating patterns in the SdH
oscillations appear due to a spin-orbit interaction. By comparing the oscillations
with the numerical simulation based on Rashba spin-orbit interaction, we
can obtain spin-orbit interaction parameter. |
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Fig. 1(b)
Spin-orbit interaction parameter a is plotted as a function of carrier concentration Ns, which is related to gate voltage Vg. Spin-orbit interaction can be controlled by a gate voltage.
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- Spin-interference device
The spin wavefunctions for spin-1/2 systems change sign after a rotation
of 2π. This peculiar quantum-effect has been experimentally
confirmed in a neutron interferometer. A spin interference device is proposed
to realize a similar kind of experiment in solid state physics.
The phases acquired by the spin wave functions during a cyclic evolution are calculated in an Aharonov-Bohm (AB) ring in the presence of the Rashba spin-orbit interaction. Figures show the operating principle and an SEM image of a fabricated spin-interference device. The gate controls the spin-orbit interaction and covers the whole AB ring area. The origin of the spin interference is the spin precession angle difference between the left and right branches. The phases acquired in the left and right branches are not the same: they have opposite signs because the precession orientation is opposite. It is interesting that the obtained spin dynamical phase is very similar to that of the spin-FET. In both cases, the origin of the phase difference is related to the spin precession.
Reference
"Spin interference device"
J. Nitta, F. E. Meijer, and H. Takayanagi
Appl. Phys. Lett. 75, 695 (1999).
"On the significance of fine structure in the frequency spectrum of
Aharonov-Bohm conductance oscillations"
F. E. Meijer, A. F. Morpurgo, T. M. Klapwijk, T. Koga@and J. Nitta
Phys. Rev. B 69, 035308 (2004)
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| Fig. 2(a): Schematic of the spin-interference device. The electron spin
feels an effective magnetic field perpendicular to its velocity direction;
therefore, the precession angles of the left and right branches are different.
A ring made of an InGaAs/InAlAs heterostructure is covered with a gate
electrode, which controls the spin-orbit interaction. |
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| Fig. 2 (b): SEM image of a fabricated spin-interference device. |
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