1. Department of Mechanical and Materials Engineering, Portland State University, Portland, Oregon 97201, USA
2. ForWind, Institute of Physics, University of Oldenburg, Carl-von-Ossietzky-Str. 9-11, 26129 Oldenburg, Germany
3. Univ. Lyon, ENS de Lyon, Univ. Claude Bernard, CNRS, Laboratoire de Physique, 46 allée d'Italie F-69342 Lyon, France