Upper critical field of four-CuO2-layer TlBa2Ca3Cu4O11+δ single crystals

Abstract

Measurements of magnetization in the reversible region were performed as a function of temperature on TlBa2Ca3Cu4O11+δ single crystals in magnetic fields up to 5.5T with the field direction applied along the crystalline c-axis. A linear temperature dependence was observed between 70 and 116K for the reversible magnetization and the slope, (dM)/(dT), decreased with increasing field strength. Near Tc (128K), the magnetization showed a rounding. Both the field-dependent dM/dT – values in the linear region and the rounding behavior of the magnetization near Tc indicated that the upper critical field, Hc2, in the TlBa2Ca3Cu4O11+δ system might be very large. Therefore, the reversible magnetization was measured in the low field range, i.e. H≪Hc2, and the traditional method used to obtain the Hc2 value based on the Abrikosov formula near Hc2 was not applicable. By using the model of Hao et al. [Z. Hao, J.R. Clem, M.W. McElfresh, L. Civale, A.P. Malozemoff, F. Holtzberg, Phys. Rev. B 43 (1991) 2844], the reversible magnetization data were analyzed to determine the Ginzburg–Landau parameter κ and the slope of the upper critical field at Tc, (dHc2/dT)|Tc. It was found that κ=109, and Hc2(0)=160T in the TlBa2Ca3Cu4O11+δ crystals with the field parallel to the c-axis.