Multiple peaks in the ac susceptibility of untwinned Y-Ba-Cu-O single crystals: A manifestation of the peak effect

Abstract

We have measured the ac susceptibility of several high quality twinned and detwinned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (YBCO) single crystals. We see multiple peaks in the \ensuremath{\chi}''(T) curves, and explain them as a manifestation of the peak effect in ${\mathit{J}}_{\mathit{c}}$(T) near ${\mathit{T}}_{\mathit{c}}$ and increased ${\mathit{J}}_{\mathit{c}}$ at low temperatures. We have investigated the effect of electron irradiation on these peaks, and found that it increases ${\mathit{J}}_{\mathit{c}}$ of the sample. We suggest that the narrow, high-temperature (just below ${\mathit{T}}_{\mathit{c}}$) peak in \ensuremath{\chi}''(T) is an indication that the sample is clean, and therefore exhibits a first-order melting transition of the flux-line lattice. We also propose that the peak effect in ${\mathit{J}}_{\mathit{c}}$ is a result of the softening of the lattice before it melts, in accordance with Pippard's model of this phenomenon for low-temperature superconductors. Finally, we suggest a method of determining the true irreversibility line.