Plastic vortex creep and dimensional crossovers in the highly anisotropic superconductor HgBa₂CuO_4+x

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 proportional to 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. Moreover, we find evidence that this transition in the plastic flow regime is due to a dimensional crossover, specifically, a transition from three to two-dimensional plastic dynamics.