Campbell G.K., A.D. Ludlow, S. Blatt, J.W. Thomsen, M.J. Martin, M.H.G. de Miranda, T. Zelevinsky, M.M. Boyd, J. Ye, S.A. Diddams, and T.P. Heavner, Metrologia45, 539-548(2008).
Hackman C., J. Levine, T.E. Parker, D. Piester, and J. Becker, Ieee Transactions On Ultrasonics, Ferroelectrics And Frequency Control53, 1570-1583(2006).