MgB2 energy gap determination by scanning tunnelling spectroscopy by Heitmann, T, Bu, S, Kim, D, Choi, J, Giencke, J, Eom, C, Regan, K, Rogado, N, Hayward, M, He, T, Slusky, J, Khalifah, P, Haas, M, Cava, R, Larbalestier, D, Rzchowski, MS