Imaging the structure of the domain wall between symmetries interconnected by a discontinuous transition

We have been able to observe with single particle resolution the interface between two structural symmetries that cannot be interconnected by a continuous transition. By means of an engineered 2D potential that pins the extremity of vortex strings an square symmetry was imposed at the surface of a 3D vortex solid. Using the Bitter decoration technique and on account of the continuous vortex symmetry we visualize how the induced structure transforms along the vortex direction before changing into the expected hexagonal structure at a finite distance from the surface. First order phase transitions 1,2 are often associated with a discontinuous change of topo-logical symmetry. The analysis and detection at microscopic scale of mechanisms that allow the nucleation and propagation of new symmetries is a formidable task of difficult experimental resolution. An ensemble of superconducting vortices, having a continuous symmetry along the average vortex orientation and discrete structural symmetry in planes perpendicular to it, is a unique toy model to investigate first and second order phase transitions 3–9. The vortex liquid in superconducting Bi 2 Sr 2 CaCu 2 O 8 crystals transforms through a first order thermodynamic transition into a solid (Bragg Glass 10) with hexagonal quasi-long range order. As a result of the almost ideal behavior of the vortex structure large vortex single crystals are usually obtained after the liquid-solid transition. The topology of the solid state with individual vortex resolution can be detected by magnetic decoration, observing the clumps generated by small Fe particles deposited at the vortex positions. The scope of this work is to develop a strategy to investigate how a change of structural symmetry induced at the surface of a solid of elastic strings propagates along its length. This is achieved imposing the vortex liquid to solidify in a three dimensional square lattice at one surface of a Bi 2 Sr 2 CaCu 2 O 8 crystal while the natural hexagonal vortex structure is observed at the opposite surface. In this way, we were able to study the vortex structure within the interface between two single crystals of vortices with symmetries that cannot be interconnected by a continuous phase transition. By means of electron beam lithography Fe dots of 300 nm in diameter and 60 nm height were deposited on the top surface of Bi 2 Sr 2 CaCu 2 O 8 single crystals of approximate dimensions 0.5 x 0.5 x 0.03 mm 3. The …