New theoretical approaches for correlated systems in nonequilibrium M. Eckstein, A. Hackl, S. Kehrein, M. Kollar, M. Moeckel, P. Werner and F.A. Wolf Eur. Phys. J. Special Topics, 180 (2009) 217-235 Published online: 11 June 2010 DOI: 10.1140/epjst/e2010-01219-x