Plastic vortex creep and dimensional crossovers in the highly anisotropic superconductor HgBa$_2$CuO$_{4+x}$

Abstract

In type-II superconductors exposed to magnetic fields between upper and lower critical values, Hc1 and Hc2, penetrating magnetic flux forms a lattice of vortices whose motion can induce dissipation. Consequently, the magnetization M of superconductors is typically progressively weakened with increasing magnetic field B ∝ nv (for vortex density nv). However, some materials exhibit a nonmonotonic M (B), presenting a maximum in M at what is known as the second magnetization peak. This phenomenon appears in most classes of superconductors, including low Tc materials, iron-based, and cuprates, complicating pinpointing its origin and garnering intense interest. Here, we report on vortex dynamics in optimally doped and overdoped HgBa2CuO4+x crystals, with a focus on a regime in which plastic deformations of the vortex lattice govern magnetic properties. Specifically, we find that both crystals exhibit conspicuous second magnetization peaks and, from measurements of the field-and temperature-dependent vortex creep rates, identify and characterize phase boundaries between elastic and plastic vortex dynamics, as well as multiple previously unreported transitions within the plastic flow regime. We find that the second magnetization peak coincides with the elastic-to-plastic crossover for a very small range of high fields, and a sharp crossover within the plastic flow regime for a wider range of lower fields. We find evidence that this transition in the plastic flow regime is due to a dimensional crossover, specifically a transition from 3D to 2D plastic dynamics.

Second magnetization peak, anomalous field penetration, and Josephson vortices in KCa$$_2$$Fe$$_4$$As$$_4$$F$$_2$$ bilayer pnictide superconductor

“…On the other hand, the explanation for the disappearance of the SMP is based on a plastic pinning replacing collective pinning as temperature increases 44 . The SMP has been previously observed and studied in the high-T c cuprates mostly for H c-axis [45][46][47][48][49] , as well as in the conventional low-T c superconductors 50 . It is worth mentioning that most of the vortex dynamics studies on the SMP found in the literature were conducted in less anisotropic iron-pnictides systems, with few studies on systems with moderated anisotropy (112-family and 1111-family) (with γ∼8) 29,51,52 , which also shows a SMP for H c-axis.…”