Revisiting the Theory of Finite Size Scaling in Disordered Systems: νCan Be Less Than 2/d

Abstract

For phase transitions in disordered systems, an exact theorem provides a bound on the finite size correlation length exponent: ν_{FS}<= 2/d. It is believed that the true critical exponent νof a disorder induced phase transition satisfies the same bound. We argue that in disordered systems the standard averaging introduces a noise, and a corresponding new diverging length scale, characterized by ν_{FS}=2/d. This length scale, however, is independent of the system's own correlation length ξ. Therefore νcan be less than 2/d. We illustrate these ideas on two exact examples, with ν< 2/d. We propose a new method of disorder averaging, which achieves a remarkable noise reduction, and thus is able to capture the true exponents.