Upper Critical Field of Four-CuO2-Layer TlBa2Ca3Cu4O11+δ Single Crystals

Magnetic measurements have been performed on single crystals of TlBa 2 Ca 3 Cu 4 O 11+δ in the magnetic fields up to 5.5 T. No change in the onset temperature of superconducting transition (T c ) was observed in the magnetic fields between 0.4 and 5.5 T, while the superconducting transitions were broadened significantly in the same field. The linear temperature-dependence of the magnetization was found in a large temperature range below T c . The slope of the magnetization versus temperature \((\frac{{d(4\pi M)}}{{dT}})\) decreased significantly with increasing magnetic field. The experimental findings indicated that the upper critical field (H C2 ) in this compound was very large and traditional methods of deriving H C2 value were not applicable. Using the measured magnetization in the reversible region, the H C2 properties of TlBa 2 Ca 3 Cu 4 O 11+δ single crystals have been studied based on a new model developed by Hao et al.