Kondo-excitons and Auger processes in self-assembled quantum dots
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Kondo-excitons and Auger processes in self-assembled quantum dots A. O. Govorov1,2 , K. Karrai3 , R. J. Warburton4 , and A. V. Kalameitsev2 1 Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, USA 2 Institute of Semiconductor Physics, 630090 Novosibirsk, Russia 3 Center for NanoScience and Sektion Physik, Ludwig-Maximilians-Universit¨at, 80539 M¨ unchen, Germany 4 Department of Physics, Heriot-Watt University, Edinburgh EH14 4AS, UK ABSTRACT We describe theoretically novel excitons in self-assembled quantum dots interacting with a two-dimensional (2D) electron gas in the wetting layer. In the presence of the Fermi sea, the optical lines become strongly voltage-dependent. If the electron spin is nonzero, the width of optical lines is given by kB TK , where TK is Kondo temperature. If the spin is zero, the exciton couples with the continuum due to Auger-like processes. This leads to anticrossings in a magnetic field. Such states can be called Kondo-Anderson excitons. Some of the described phenomena are observed in recent experiments. INTRODUCTION Many-body phenomena and excitons in quantum dots (QDs) attract presently much interest. The number of electrons in QDs is voltage tunable. This makes it possible to study single electron effects. One example of many-body phenomena is the Kondo effect induced by the non-zero spin. So far, the Kondo effect was studied mostly in relation to transport properties [1]. In optics, Kondo-type effects were discussed with respect to nonlinear shakeup processes in nanostructures [2]. Here we study theoretically novel exciton states in self-assembled QDs coupled with delocalized electron states. If the electron spin of the QD is non-zero, the resulting states can be called Kondo-excitons. The Kondo temperature of these excitons is given by the small dimensions of QDs and can be as high as 10 K. The Kondo effect manifests itself as peculiar, temperature-dependent optical lines. If the exciton spin is zero, a hybridization of the final state can occur due to Auger-like processes. Another effect is the transition between different exciton ground states on increasing the Fermi energy. In the transition regime, the optical lines become strongly voltage-dependent. Some of above-mentioned effects have been observed in optical spectra of single InAs QDs [3]. Charged excitons X n− , observed in experiments [3–5], contain n + 1 electrons and one hole. In the exciton, the hole is optically excited; the electrons are supplied by tunneling from the back contact and by optical excitation (fig.1). The voltage applied between the top and back contact, Ugate , makes it possible to control the number of electrons in a QD. If Ugate is sufficiently high, electrons fill the wetting layer, a 2D quantum well. The Fermi energy of a 2D gas in the wetting layer is found from the conditions of equilibrium, ²F = (a∗0 /4d)∆Ugate , where a∗0 is the Bohr radius and ∆Ugate is the voltage measured from the point at which the wetting layer starts to load [6]. In our model, an asymmetric QD
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