Electron-Spin Control based on RKKY Interaction

Hiroyuki Tamura
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.

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.


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