Novel low-energy collective excitation at metal surfaces V. M. Silkin, A. García-Lekue, J. M. Pitarke, E. V. Chulkov, E. Zaremba and P. M. Echenique Europhys. Lett., 66 2 (2004) 260-264 Published online: 01 April 2004 DOI: 10.1209/epl/i2003-10184-1