INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 18 (2006) L55–L61 doi:10.1088/0953-8984/18/5/L01 LETTER TO THE EDITOR Nonlinear transport at the strong intra-dot Coulomb interaction I Sandalov 1 and R G Nazmitdinov 2,3,4 1 Department of Condensed Matter Physics, Royal Institute of Technology, Electrum 229, SE-164 40 Stockholm-Kista, Sweden 2 Departament de F´ ısica, Universitat de les Illes Balears, E-07122 Palma de Mallorca, Spain 3 Max-Planck-Institut f¨ ur Physik komplexer Systeme, D-01187 Dresden, Germany 4 Bogoliubov Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, 141980 Dubna, Russia E-mail: rashid@thsun1.jinr.ru Received 20 December 2005 Published 20 January 2006 Online at stacks.iop.org/JPhysCM/18/L55 Abstract Nonlinear transport is studied in the limit of weak and strong intra-dot Coulomb interaction. The nonequilibrium self-consistent mean-field equations for one- electron transition energies of an open dot and their spectral weights are derived at the strong Coulomb interaction. In this limit populations of states involved in tunnelling equalize upon the increase of the bias-voltage window even at low temperature. This results in a simple analytical relation between the heights of the current steps and the degeneracy of a spectrum in a two-dimensional parabolic dot in a magnetic field. (Some figures in this article are in colour only in the electronic version) Recently, in experiments with a small quantum dot (QD) in the Coulomb-blockade regime [1], a fine structure was observed in the conductance as a function of gate voltage versus source–drain voltage. It was suggested that this phenomenon is mainly due to co-tunnelling [2]. The theory of co-tunnelling [2], however, neglects specific quantum effects caused by a strong Coulomb interaction (SCI), which affect the transition energies and the tunnelling rates. On the other hand, numerous papers devoted to quantum effects in transport through QDs are focused on the analysis of the Kondo phenomenon in single-level QD models (cf [3]) in a linear-response regime, which is well understood nowadays. However, single-electron spectroscopy clearly indicates that the shell structure of small dots plays an essential role in the transport at low temperatures and weak dot–lead coupling [4]. This becomes more evident when the confining energy exceeds the charging energy [5] (hereafter, this regime is called a weak Coulomb interaction (WCI) regime). One of our goals is to present a self-consistent approach to nonlinear transport through a multilevel QD in the SCI regime. We will demonstrate that even in this 0953-8984/06/050055+07$30.00 © 2006 IOP Publishing Ltd Printed in the UK L55