1. Data Storage Systems Center, Carnegie Mellon University 1 , Pittsburgh, Pennsylvania 15213, USA
2. Electrical and Computer Engineering, Carnegie Mellon University 2 , Pittsburgh, Pennsylvania 15213, USA
3. Corning Research and Development Corporation 3 , Corning, New York 14831-0001, USA
4. Materials Science and Engineering Department, Carnegie Mellon University 4 , Pittsburgh, Pennsylvania 15213, USA