Disorder driven lock-in transitions of 3D CDWs and related structures

Thermal fluctuations are known to play an important role in low-dimensional systems which may undergo incommensurate-commensurate or (ibr an accidentally commensurate wavevector) lock-in transitions. In particular, an intermediate floating phase with algebraically decaying correlations exists only in D = 2 dimensions, whereas in higher dimensions most features of the phase diagram are mean-field like. Here we will show, that the introduction of frozen-in disorder leads to strong fluctuation effects even in D < 4 dimensions. For commensurate wavevectors the lattice pinning potential dominates always over weak impurity pinning if p < p(c) = 6/pi (D = 3), where p denotes the degeneracy of the commensurate phase. For larger p a disorder driven continuous transition between a long-range ordered locked-in phase and quasi-long-range ordered phase, dominated by impurity pinning, occurs. Critical exponents of this transition, which is characterized by a zero temperature fixed point, are calculated within an expansion in 4 - D. The generalization to incommensurate wavevectors will be discussed. If the modulation in the quasi-long-range ordered phase has hexagonal symmetry, as e.g. for flux-line lattices, the algebraic decay is nonuniversal and depends on the Poisson ratio of the elastic constants. Weakly driven transport is dominated by thermally activated creep in both phases, but with different creep exponents.