AC-measured shifts inT _c of high temperature superconductors: The effect of interlayer structure

Abstract

The apparentT _c of some high temperature superconductors such as BiSrCaCuO and TlBaCaCuO measured by low (10–20,000 Hz) a.c. inductance techniques is shifted downward by up to 30K in the presence of modest (400 Oe) d.c. magnetic fields. Dc magnetization (SQUID) measurements of the same samples show no such shifts in the same magnetic fields. Other materials like YBaCuO and TlPbSrCaCuO do not show such shifts ofT _c in a.c. inductance measurements in d.c. magnetic fields. While BiSrCaCuO and TlBaCaCuO are known to exhibit flux creep/flux flow phenomena resulting in low irreversibility line behaviour as measured by both a.c. and SQUID techniques, the aboveT _c shift is apparently a new phenomenon. Materials exhibiting this property contain two or more layers of metal ions between the CuO conductance layers (2212, 2223 structure). Materials with only one metal ion layer between the CuO layers (123, 1223 structure) do not show the apparentT _c shift, suggesting that the phenomenon is a result of the properties of magnetically-isolated CuO layers. Implications of the latter will be discussed.