1. Contribution from the Debye Institute, Condensed Matter and Interfaces, Utrecht University, Post Office Box 80 000, 3508 TA Utrecht, The Netherlands, Philips Research Laboratories, Professor Holstlaan 4, 5656 AA Eindhoven, The Netherlands, and Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1 2678 JK Delft, The Netherlands