Resistive transition curves in magnetic fields for Tl2Ba2Can-1CunOy (n=1, 2, 4) compounds: Dependence on the number of Cu-O layers.

Abstract

A systematic measurement of the superconducting resistive transition curves in magnetic fields has been performed for ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{\mathit{y}}$ (n=1, 2, 4) compounds. It is found that the broadening of the transition curves varies systematically with the number of Cu-O layers (i.e., the value n). The slope of the field-versus-temperature curves in fields parallel to the c plane increases with decreasing n, while the slope in perpendicular fields is almost independent of n. This result suggests that the upper critical fields and the coherence lengths of these compounds have a strong correlation with n. It is concluded that the coherence length along the c axis of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{\mathit{y}}$ is smaller than the thickness of the Tl-O-layer region. According to the Josephson-coupled layer model, ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{\mathit{y}}$ is expected to be a two-dimensional superconductor.