1. Toshiba Research Europe Limited, Cambridge Research Laboratory, 260 Cambridge Science Park, Milton Road, Cambridge, CB4 0WE, UK.
2. Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, UK.
3. High Field Magnet Laboratory, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, Netherlands.