[1] Univ Bristol, HH Wills Phys Lab, Bristol BS8 1TL, Avon, England
[2] ENEA, I-40138 Bologna, Italy
[3] Univ Tsukuba, Inst Mat Sci, Tsukuba, Ibaraki 305, Japan
[4] Univ Osaka Prefecture, CIAS, Sakai, Osaka 593, Japan
[5] Univ Texas Arlington, Arlington, TX 76019 USA