Peak Effect in Polycrystalline Vortex Matter

The peak effect (PE) in $J_{c}$ in weakly-disordered type-II superconductors is believed to mark the transition between a disordered vortex state and a quasi-ordered Bragg glass regime. Some strongly-disordered type-II superconductors, such as the binary alloy V-21at.\%Ti, also exhibit ``peak effect'' at temperatures close to $H_{c2}$, despite lack of atomic long-range order and presence of sample composition inhomogeneities. SANS field-cooled measurements on a V-21at.\%Ti sample show that both deep in the mixed state and close to the PE transition, there exist no long-range orientationally-ordered vortex lattices (VL's). The neutron scattering data analysis shows that the diffraction radial widths do not change significantly as a function of field, suggesting that VL states ordered on the scale of $\mu m$ exist. We conjecture that the ``peak effect'' in V-21at.\%Ti corresponds to the disordering of ordered VL Larkin domains. The V-21at.\%Ti peak effect phase diagram is mapped via ac susceptometry. This measurement reveals that the peak effect disappears below a certain field, as has been reported in other superconductors.