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Theoretical Physics Institute, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455, USA Department of Physics, University of Warwick, Coventry, CV4 7AL, UK Department of Applied Physics and ERATO Mesoscopic Correlation Project, Delft University of Technology, P. O. Box 5046, 2600 GA Delft, the Netherlands

Abstract. We review the peculiarities of transport through a quantum dot caused by the spin transition in its ground state. Such transitions can be induced by a magnetic field. Tunneling of electrons between the dot and leads mixes the states belonging to the ground state manifold of the dot. Unlike the conventional Kondo effect, this mixing, which occurs only at the singlet-triplet transition point, involves both the orbital and spin degrees of freedom of the electrons. We present theoretical and experimental results that demonstrate the enhancement of the conductance through the dot at the transition point.

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Introduction

Quantum dot devices provide a well–controlled object for studying quantum many-body physics. In many respects, such a device resembles an atom imbedded into a Fermi sea of itinerant electrons. These electrons are provided by the leads attached to the dot. The orbital mixing in the case of quantum dot corresponds to the electron tunneling through the junctions connecting the dot with leads. Voltage Vg applied to a gate – an electrode coupled to the dot capacitively – allows one to control the number of electons N on the dot. Almost at any gate voltage an electron must have a finite energy in order to overcome the on-dot Coulomb repulsion and tunnel into the dot. Therefore, the conductance of the device is suppressed at low temperatures (Coulomb blockade phenomenon[1]). The exceptions are the points of charge degeneracy. At these points, two charge states of the dot have the same energy, and an electron can hop on and off the dot without paying an energy penalty. This results in a periodic peak structure in the dependence of the conductance G on Vg . Away from the peaks, in the Coulomb blockade valleys, the charge fluctuations are negligible, and the number of electrons N is integer. Every time N is tuned to an odd integer, the dot must carry a half-integer spin. In the simplest case, the spin is S = 1/2, and is due to a single electron residing on the last occupied discrete level of the dot. Thus, the quantum dot behaves as S = 1/2 magnetic impurity imbedded into a tunneling barrier between two massive conductors. It is known[2] since mid-60’s that the presence of such impurities leads to zero-bias anomalies in tunneling conductance[3], which are R. Haug and H. Schoeller (Eds.): LNP 579, pp. 3–24, 2001. c Springer-Verlag Berlin Heidelberg 2001


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M. Pustilnik et al.

adequately explained[4] in the context of the Kondo effect[5]. The advantage of the new experiments[6] is in full control over the “magnetic impurity” responsible for the effect. For example, by varying the gate voltage, N can be changed. Kondo effect results in the increased low–temperature conductance only in the odd–N