Manuscript received March 8, 2001; revised May 24, 2001. This work was supported by the research programme of the Stichting voor Fundamenteel On-derzoek der Materie (FOM) under the Nederlandse Stichting voor Wetenschap-pelijk Onderzoek (NWO). C. Hooijer is with the Stichting voor Fundamenteel Onderzoek der Materie (FOM), Utrecht, The Netherlands. G. Li is with the Department of Physics, Huazhong Normal University, Wuhan, China. K. Allaart and D. Lenstra are with the Division of Physics and Astronomy, Vrije Universiteit, 1081 HV Amsterdam, The Netherlands (e-mail: allaart@nat.vu.nl). Publisher Item Identifier S 0018-9197(01)07368-7.