Tunable Exchange Interaction and Kondo Effect in QDs
- Introduction
When magnetic impurities are diluted in metal, the conduction electrons
are scattered by local spins, leading to the Kondo effect at low temperatures.
When a QD confines electrons which have nonzero total spin, it behaves
like a local magnetic impurity which scatters the conduction electrons
around it. The Kondo effect in QDs has been widely observed in QD devices.
We are interested in the device application of QDs as "artificial
impurities" in semiconducting materials.
- Tunable RKKY Interaction between QDs
We theoretically discuss the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction
between semiconductor quantum dots (QDs). When each QD having a local spin
is coupled to the conduction electrons in semiconductors, an indirect exchange
interaction, i.e., the RKKY interaction, is induced between two local spins.
The RKKY interaction between QDs, which is mediated by the Fermi sea in
semiconductors, is modulated by changing the Fermi energy, and the magnitude
or even the sign of the exchange interaction can be tuned, which leads
to a tunable magnetic transition in QD devices. We estimate the magnitude
of the RKKY interaction in QDs as a function of the electron density and
the inter-dot distance.
Reference
H. Tamura, K. Shiraishi, and H. Takayanagi, Jpn. J. Appl. Phys. 43, L691 (2004). [PDF]
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| Fig. 1: Schematic self-organized quantum dot array grown on 3DEG separated
by a barrier layer. Local spins in QDs scatter conduction electrons and
induce ferromagnetism at high temperatures. |
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| Fig. 2: The magnitude of the RKKY interaction between spins in QDs coupled to three dimensional GaAs as a function of the conduction-electron density and the inter-dot distance. |
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- Tunable Kondo screening in quantum wire coupled with double side-dots
We consider electron transport along a single-mode channel which is in
contact, via tunnel junctions in its walls, with two quantum dots (Fig.
1). Electron tunneling to and from the dots contributes to the electron
backscattering, and thus modifies the channel conductance. If the dots
carry spin, the channel conductance becomes temperature-dependent due to
the Kondo effect. The two-dot device geometry allows for a formation of
S=1 localized spin due to the indirect exchange interaction, called Ruderman-Kittel-Kasuya-Yosida
(RKKY) interaction. This device offers a possibility to study the crossover
between fully screened and under-screened Kondo impurity. We investigate
the manifestation of such crossover in the channel conductance (Fig. 2).
Reference
H. Tamura and L. I. Glazman (to be published)
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| Fig. 1: Schematic quantum wire coupled with two quantum dots. Two quantum
dots (QD-1 and -2) coupled to a quantum wire. Vnk are the coupling constants between dots n=1, 2 and the left- or right-moving
waves k=L,R. |
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| Fig. 2: Schematic dependence of the conductance as a function of temperatures T and magnetic fields B. |
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