Vortex fluctuations and interlayer coupling in cuprates

The reversible magnetization (M) curves of ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_{\mathit{m}\mathrm{-}1}$${\mathrm{Cu}}_{\mathit{m}}$${\mathrm{O}}_{2\mathit{m}+2+\mathrm{\delta }}$ (m=1, 2, and 3) at different H cross at a temperature T* and M=M* as suggested by various two-dimensional (2D) vortex-fluctuation models. However, the obtained M*/T* was found to be independent of either m or the sheet density 1/s of the ${\mathrm{CuO}}_2$ blocks, where s is the size of the unit cell along the c axis, in contrast to the predictions of the models. A survey of published data shows a similar situation, i.e., M*/T* in either Bi- or Ti-based compounds is independent on s. This observation suggests that the coupling between neighboring ${\mathrm{CuO}}_2$ blocks may be non-negligible in its effects on the magnetization, and that modifications to the existing 2D fluctuation models need to be made. © 1996 The American Physical Society.