Study on the Field-Dependent Activation Energy, U(H,T), Upper Critical Field, H-c2(0), and Irreversibility Field, mu H-0(irr), Properties of Superconducting Single-Crystal Ag-Added Fe(Se, Te) Alloys

Superconducting FeTe0.5Se0.5 + Ag single-crystal samples with 0, 1, 2, 3, 4, and 5% Ag concentrations were prepared successfully by the self-flux synthesis method. Structural, electrical transport, and magnetic characterizations, including semi-theoretical calculations of upper critical field, H-c2(0), irreversibility field, mu H-0(irr), and activation energy, U-0(J, T, H), were carried out. It was found that the properties of the samples improved as the Ag addition was increased up to 3%, but began to deteriorate for higher Ag concentrations. The best properties were obtained at a 3% Ag-added sample. The J(c)(mag) values, at zero field, were calculated to be 1.1 x 10(6) A/cm(2) and 4 x 10(5) A/cm(2) at 4.2 K and 10 K, respectively. These results were found to be similar to 3 times higher than the pure (0% Ag-added) sample. The best H-c2(0) values were calculated to be 64.1 T, 50.9 T, and 42.7 T for 90% T-c(offset), 50% T-c(midpoint), and 10% T-c(onset), respectively. These results were found slightly higher than the 0, 1, 2, 4, and 5% Ag-added samples. We also found a noticeable decrease in U(H,T) calculations except for the 3% addition case. The highest mu H-0(irr) value (31 T) was obtained in the 3% Ag-added sample and decreased significantly in the 4% and 5% Ag-added samples. In addition, the data obtained from the U(H,T), J(c)(mag), H-c2(0), and, mu H-0(irr) calculations were evaluated to support the thermally activated dissipation mechanism (TAFF) and independent vortex motion in the samples.