This article has been downloaded from IOPscience. Please scroll down to see the full text article. (http://iopscience.iop.org/0957-0233/19/4/045601) is available Download details: IP Address: 155.247.167.161 The article was downloaded on 22/04/2008 at 06:43 Please note that terms and conditions apply. More related content HOME | SEARCH | PACS & MSC | JOURNALS | ABOUT | CONTACT US