CONICET | Buscador de Institutos y Recursos Humanos

Order-disorder transitions between glassy phases are quite common in nature and yet adetailed description of the structural changes entailed at microscopic scales remainselusive. This issue is experimentally challenging since scales are typically tiny, constituentsmove rapidly, and few of them, in most cases, take part in the structural transformation.Vortex matter in type-II superconductors is a playground where some of these difficultiescan be tackled by conveniently choosing the host superconducting sample.Bi Sr CaCu O is a paradigmatic type-II superconductor presenting a glassy-to-glassyfirst-order transition between the Bragg and the vortex glass phases on increasing vortexdensity (magnetic field). The structural properties of the quasicrystalline Bragg glass havebeen extensively studied, but in the case of the vortex glass phase this information hasremained elusive up to now. Here we image with single-vortex resolution the structuralchanges occurring at this order-disorder transition and reveal large field-of-view snapshotsof the vortex glass phase. By combining real-space surface magnetic decoration andreciprocal-space bulk small-angle neutron scattering imaging techniques we found thatthis phase presents large crystallites with a proliferation of bounded and unbounded edgedislocations at the surface. Within the crystallites, the exponentially decaying orientationalorder and the fast algebraic growth of the positional displacement correlator are at oddswith a hexatic phase. In addittion, in the vortex glass the radial and azimuthal in-planecorrelation lengths are depleted at the surface as well as at the volume of the sample.Nevertheless, no dramatic change in the correlation length along the direction of vortices isobserved within our improved experimental resolution, ruling out the possibility of thevortex glass being a glassy phase with layered vortices internally decoupled along thethickness of the sample.

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