The Peak Effect anomaly in low and high T$_{C}$ superconductors: stable and metaestables vortex lattice configurations

Competing interactions in the vortex lattice (VL) of type II superconductors give rise to an order-disorder transition known as Peak Effect (PE) anomaly. The strong metastability related to the PE masks the stationary VL configurations (VLCs), both in low and in high T$_{C}$ superconductors. By means of linear \textit{ac} susceptibility experiments, that avoid VL reorganization, we explore quasi-static stable and metastable states, applying different shaking protocols before measurements. In low T$_{C }$ \textit{NbSe}$_{2}$ [1] crystals we identify T,H regions where stationary configurations are maximally ordered (Bragg Glass), fully disordered or where ordered and disordered stable phases coexist. In contrast, in high T$_{C}$ \textit{YBa}$_{2}$\textit{Cu}$_{3}O_{7-y}$ crystals, metaestability seems to dominate and different metaestable VLCs are accessed depending on the previous dynamic history, with no access to the lowest energy configuration. [1] G. Pasquini \textit{et al}. Phys. Rev. Lett. \textbf{100} 247003 (2008)