Phase behavior of type-II superconductors with quenched point pinning disorder: A phenomenological proposal

A general phenomenology for phase behavior in the mixed phase of type-II superconductors with weak point pinning disorder is outlined. We propose that the ``Bragg glass'' phase generically transforms via two separate thermodynamic phase transitions into a disordered liquid on increasing the temperature. The first transition is into a glassy phase, topologically disordered at the largest length scales; current evidence suggests that it lacks the long-ranged phase correlations expected of a ``vortex glass.'' This phase has a significant degree of short-ranged translational order, unlike the disordered liquid, but no quasi-long-range order, in contrast to the Bragg glass. This glassy phase, which we call a ``multidomain glass,'' is confined to a narrow sliver at intermediate fields, but broadens out both for much larger and much smaller field values. Estimates for translational correlation lengths in the multidomain glass indicate that they can be far larger than the interline spacing for weak disorder, suggesting a plausible mechanism by which signals of a two-step transition can be obscured. Calculations of the Bragg glass\char21{}multidomain glass and the multidomain glass\char21{}disordered liquid phase boundaries are presented and compared to experimental data. We argue that these proposals provide a unified picture of the available experimental data on both high-${T}_{c}$ and low-${T}_{c}$ materials, simulations, and current theoretical understanding.