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We study electronic transport through coupled quantum dots via the RKKY interaction[1]. At zero temperature and magnetic field, we observe a quantum phase transition between the Kondo screened state and a local spin singlet as the RKKY interaction is tuned. We discuss the realization of the quantum-critical non-Fermi liquid state, originally discovered within the two-impurity Kondo model, in double quantum-dot systems[2]. Contrary to the common belief, the corresponding fixed point is robust against particle-hole and various other asymmetries, and is only unstable to charge transfer between the two dots. We propose an experimental set-up where such charge transfer processes are suppressed, allowing a controlled approach to the quantum critical state. We show that the transport and scaling properties of our system in the vicinity of the critical point are identical to those of the two-channel Kondo fixed point. We also study transport properties of the system at finite temperature and magnetic field[3]. Above the critical RKKY coupling the Kondo peak is split at zero temperature and magnetic field. However, we find that both finite temperature and magnetic field restore the Kondo resonance. Our results agree well with recent transport experiments on gold grain quantum dots in the presence of magnetic impurities[4].
[1] N. J. Craig et al, Science 304, 565 (2004). [2] Gergely Zarand, Chung-Hou Chung, Pascal Simon and Matthias Vojta, "quantum criticality in a double quantum-dot system", Phys. Rev. Lett. 97, 166802 (2006). [3] Chung-Hou Chung and Walter Hofstetter, "Kondo effect in coupled quantum dots with RKKY interaction: Finite temperature and magnetic field effects", Phys. Rev. B 76, 045329 (2007), selected for the August 6, 2007 issue of Virtual Journal of Nanoscale Science and Technology. [4] H. B. Heersche et al, Phys. Rev. Lett. 96, 017205 (2006). |
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