Hole doping-induced evolution of self-organized bulk vortex structure in the high temperature YBa$_2$Cu$_3$O$_{7-\delta}$ superconductor

We present systematic studies of the persistent current relaxation of 24 different oxygen contents of YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) as a function of oxygen defect concentration $\delta$. These studies allowed us to map out the ($\delta$,$\mu$) phase diagram of the vortex matter, where the exponent $\mu$ of the vortex structure was obtained using the scaling properties of the energy barrier against vortex motion. The reduction of the hole-doping level (an increase of $\delta$) of the material leads to a transformation of the vortex lattice into a glass and subsequently into a liquid phase. These vortex phases self-organize and produce relaxation plateaus in regions between step-like changes in the dependence of relaxation kinetics on hole doping, revealing the existence of a previously unknown correlation between the vortex structure and the hole-doping level in a cuprate superconductor, such as YBCO.