Physical Science Laboratory
We theoretically proposed a method to control electron-spin
state electrically by tuning a magnetic interaction between local spins confined
in quantum dots and conduction electrons in semiconductors [1]. When quantum
dots confining electrons are coupled to conducting semiconductors, the conduction
electron can jump into and out of the quantum dot and interact with the confined
electron in it. This process induces an antiferromagnetic interaction between
the confined electron and the conduction electron. When two quantum dots are
coupled to the conducting semiconductor as shown in Figure, the conduction
electron can go back and forth between two dots. If the distance of two dots
is much smaller than the Fermi wavelength, the conduction electron can travel
without losing spin orientation and the localized spins in dots are aligned
in the same direction, which results in a ferromagnetic interaction known
as Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Since the degradation
of spin orientation depends on the electron density in the conduction region,
one can tune the spin state both in parallel and anti-parallel way by changing
gate voltages.
The key for electric control of spins is the artificial magnetic impurities
formed in the quantum dots and the tunable electron-density in semiconducting
materials. These features make it possible to realize a new type of spintronics
device.
By utilizing the RKKY interaction, one can expect to achieve ferromagnetism
in an array of semiconducting quantum dots where all the spins are aligned
in the same direction. Experimental search for detecting the RKKY interaction
between dots has already started [2], which will help to prove our theoretical
prediction.
[1] H. Tamura, K. Shiraishi, and H. Takayanagi, Japan Journal of Applied
Physics 43 (2004) L691-693.
[2] N. J. Craig, J. M. Taylor, E. A. Lester, C. M. Marcus, M. P. Hanson, and
A. C. Gossard, Science 304 (2004) 565-567.
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Figure: Two quantum dots coupled to the conduction electron system. Electron spins in two dots are aligned in the downward direction when the conduction electron with opposite (up) spin jumps back and forth between two dots. |