First-Order Transition in the Magnetic Vortex Matter in SuperconductingMgB2Tuned by Disorder

SANS study of vortex lattice structural transition in optimally doped (Ba1−xKx)Fe2As2

“…Below 30 K, strong pinning dominates, and no SMP is observable. At higher temperatures, the central peak gives way to collective pinning by atomic-sized point defects [8] and to the increase of the magnetic moment (SMP) at the onset field B OD [12,11,4,8], the temperature dependence of which is shown in Fig. 4d.…”

“…Upon heating, the ordered vortex solid transits to the vortex liquid. However, an increase in vortex density at low temperature is also followed by a first order transition, to a second vortex solid (or glassy) phase with strongly modified dynamics [4,10,11,12,6]. The change in current-voltage characteristics that this entails leads to the so-called "second magnetization peak" (SMP) phenomenon in numerous type II superconductors.…”

“…This has resulted in the paradigm in which, on the one hand, for weakly disordered type II superconductors, a long-range ordered low temperature vortex solid state transits via a first order melting transition, to a high-temperature vortex liquid lacking long-range superconducting order [7]. On the other hand, disordered materials with strong vortex pinning exhibit a continuous "glass" transition from a low-temperature disordered vortex state to the vortex liquid [4]. The nature (and, indeed, the existence) of the "vortex glass transition" is not definitively settled [9].…”

“…4d), suggesting that it corresponds to an intrinsic "order-disorder" transition of the vortex ensemble Refs. [1,12,11,42,4,6].…”

“…The relation between vortex lattice structure, vortex dynamics, and the vortex phase diagram has been the object of intensive studies in intermetallic [1,2,3,4], cuprate [5,7,6], and, recently, Fe-based type II superconductors [8]. This has resulted in the paradigm in which, on the one hand, for weakly disordered type II superconductors, a long-range ordered low temperature vortex solid state transits via a first order melting transition, to a high-temperature vortex liquid lacking long-range superconducting order [7].…”