CONICET | Buscador de Institutos y Recursos Humanos

Order-disorder transitions between glassy phases are quite common innature and yet a detailed description of the structural changesinvolved at microscopic scales remains elusive. This issue isexperimentally challenging since scales are typically tiny,constituentsmove rapidly, and few of them, in most cases, take part in the structuraltransformation. Vortex matter in type-II superconductors is a playground wheresome of these difficulties can be tackled by adequately choosing the hostsuperconducting sample. Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8 + \delta}$ is aparadigmatic type-II superconductor presenting a glass-to-glassfirst-order transition between the Bragg and the vortex glass phaseson increasing vortex density(magnetic field). The structural properties of the quasicrystallineBragg glass have been extensively studied, but the structure of themore disordered vortex glass phase hasremained elusive up to now. Here we image, with single-vortex resolution,the structural changes occurring at this order-disorder transitionand present large field-of-viewsnapshots of the vortex glass phase.By combining real-space surface magnetic decorationand reciprocal-space bulk small-angle neutronscattering imaging techniques we found that this phase presentslarge crystallites with a proliferation of bound and unboundedge dislocations at the surface. Within the crystallites, the exponentiallydecaying orientational order and thefast algebraic growth of the positional displacement correlator are atodds with a hexatic phase. In addittion, in the vortex glass the radial and azimuthal in-plane correlationlengths are depleted at the surface as well as in the volume of the sample.Still, no dramatic change in thecorrelation length along the direction of vortices is observed within our improvedexperimental resolution, ruling out the possibility of the vortex glass being aphase with layered vortices internally decoupled along the thickness of thesample.