Inhomogeneous ground state and the coexistence of two length scales near phase transitions in real solids

Real crystals almost unavoidably contain a finite density of dislocations. We show that this generic type of long–range correlated disorder leads to a breakdown of the conventional scenario of critical behavior and standard renormalization group techniques based on the existence of a simple, homogeneous ground state. This breakdown is due to the appearance of an inhomogeneous ground state that changes the character of the phase transition to that of a percolative phenomenon. This scenario leads to a natural explanation for the appearance of two length scales in recent high resolution small-angle scattering experiments near magnetic and structural phase transitions.