Elastic and plastic deformations of charge density waves

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Abstract
An elastic theory of CDW deformations under electric fields is presented, on the basis of a simple phase Hamiltonian. The emphasis is put on the internal rigidity of the modulation which indeed resembles an « electronic solid », and on the anisotropy of its elastic constants. CDW's continous deformations are described by a strain tensor given by the relative variations of the wavevector of the modulation. Stresses occur due to the combined effect of external electric field and pinning points. Under low stresses the globally pinned condensate retains phase continuity : the structure has no defects. Under larger stresses dislocation loops are nucleated involving local amplitude variations of the CDW. We describe the nucleation process for elementary loops under an external field and compare as a consequence thresholds for impurity depinning and boundary depinning (surfaces or interfaces). A characteristic sample length is found under which contact pinning prevails and a coherent deformation (polarization) is built