On the influence of wall microdeformations on Bloch line visibility in bubble garnets (invited)

It is proposed that the main source of contrast of vertical Bloch lines (VBLs) in bubble garnets, when viewed by means of anisotropic dark field optical imaging, is a microdeformation of the wall plane at line location. The motive force for wall tilting is the compensation of the monopolar σ charges borne by VBLs. The tilted wall section acts as a phase object and allows for the formation of line images with well-defined symmetries and independent of the incident-beam polarization. All these features agree fully with experimental evidence. An approximate model of the wall deformation is presented and shown to be consistent for a given thickness and material parameters with the results of an ab initio numerical computation of the 3D structure of a VBL. The implications of the microdeformation model in terms of line observability are analyzed and compared with experimental data.