1. Department of Physics, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 and Data Storage Institute, 5 Engineering Drive 1, Singapore 117608
2. Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 and Data Storage Institute, 5 Engineering Drive 1, Singapore 117608
3. Data Storage Institute, 5 Engineering Drive 1, Singapore 117608
4. Department of Physics, National University of Singapore, Singapore 119260