Quenched disorder: demixing thermal and disorder ﬂuctuations I. Balog ∗ and K. Uzelac † Institute of Physics, P.O.Box 304, Bijeniˇ cka cesta 46, HR-10001 Zagreb, Croatia Abstract We present a ﬁnite size scaling study of the phase transition in the 2d Potts model modiﬁed by random bond disorder, in which the average is taken over quantities calculated at quasicritical temperatures of individual disorder conﬁgurations. We used the recently proposed Equilibriumlike invaded cluster algorithm, which allows to examine separately the ﬂuctuations in the thermody- namical ensemble from the ﬂuctuations induced by changing the disorder conﬁguration. We point out the crucial role of the spatial inhomogeneities on all scales for the critical behavior of the sys- tem. These inhomogeneities are formed by “dressing” of the disorder via critical correlations and are demonstrated to exist for any system size despite of the critical ﬂuctuations in thermodynami- cal ensemble. Such inhomogeneities were previously not thought to be relevant in disorder altered classical systems when critical temperature is ﬁnite. However, we conﬁrm that only by averaging at quasicritical temperatures of each disorder conﬁguration, the thermal critical exponent y τ is not obscured by the inﬂuence of the aforementioned spatial inhomogeneities. PACS numbers: 05.50.+q, 64.60.F-, 75.10.Hk, 2.70.-c ∗ Electronic address: balog@ifs.hr † Electronic address: katarina@ifs.hr 1