This article has been downloaded from IOPscience. Please scroll down to see the full text article. (http://iopscience.iop.org/0953-8984/11/49/320) is available Download details: IP Address: 155.158.228.28 The article was downloaded on 11/12/2007 at 10:27 Please note that terms and conditions apply. More related content HOME | SEARCH | PACS & MSC | JOURNALS | ABOUT | CONTACT US