Kondo Effect in a Semiconductor Quantum Dot Controlled by Spin Accumulation
Toshiyuki Kobayashi1, Shoei Tsuruta1,2, Satoshi Sasaki1, Toshimasa Fujisawa1,
Yasuhiro Tokura3, and Tatsushi Akazaki1,2
1Physical Science Laboratory, 2Tokyo University of Science, 3Optical Science Laboratory
The spin of a localized electron embedded in a semiconductor quantum dot interacts with the spins of surrounding conduction electrons to form a spin-singlet coherent quantum many-body state. The state is known as the Kondo effect, which changes the state by temperature, electric field, and magnetic field. Since both the localized and conduction electrons play important roles in forming the Kondo state, the Kondo effect should be suppressed if the conduction electrons are spin-polarized. However, any experimental investigation of the Kondo effect with spin-accumulated conduction electrons had been a challenge owing to the difficulty of generating a 100 % spin-polarized state. We have succeeded in measuring the continuous modulation of the Kondo effect by accumulating only spin-up electrons near a quantum dot using a spin filter of a quantum wire under a high magnetic field whose spin selectivity is more than 90 % .
The Kondo effect accompanies the formation of the Kondo density of states (KDOS) at the chemical potential µ of a lead, because the conduction electrons at the Fermi energy interact resonantly with the localized electron (Fig. 1). Since the KDOS moves with µ, changing the µ of just the spin-up electrons by injecting only spin-up electrons from a quantum wire to a quantum dot shifts only the spin-up KDOS and modulates the spin-splitting of the KDOS. The position of the KDOS can be measured as a peak in the differential conductance gD of a quantum dot (Fig. 2).
 T. Kobayashi et al., Phys. Rev. Lett. 104 (2010) 036804.
Fig. 1. (a) Scanning electron micrograph of the
device and the measurement setup. (b)-(d)
Chemical potentials µ and KDOS ρ when
the conductance gE of a quantum wire is
fixed at 0, e2/h, and 2e2/h and VE is
Fig. 2. Conductance peak attributed to the KDOS
shifts with VE. When the conductance gE of a
quantum wire is e2/h, the µ of a spin-up electron
shifts with VE, which is measured as a shift in
the conductance peak.
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