Causality-induced pulse steepening and shock-like waves in superluminal media J. M. Hickmann, M. A. R. C. Alencar, W. F. Silva and S. T. Souza EPL, 97 4 (2012) 44009 Published online: 20 February 2012 DOI: 10.1209/0295-5075/97/44009