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Multiplicative Noise Induces Zero Critical Frequency

Stochastic Bloch equations which model the fluorescence of two level molecules and atoms, NMR experiments and Josephson junctions are investigated to illustrate the profound effect of multiplicative noise on the critical frequency of a dynamical system. Using exact solutions and the cumulant expansion we find two main effects: (i) even very weak noise may double or triple the number of critical frequencies, which is related to an instability of the system and (ii) strong multiplicative noise may induce a non-trivial zero critical frequency thus wiping out the over-damped phase.