Study on the Field-Dependent Activation Energy, U(H,T), Upper Critical Field, Hc2(0), and Irreversibility Field, µ0Hirr, 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, Hc2(0), irreversibility field, µ0Hirr, and activation energy, U0(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 Jcmag values, at zero field, were calculated to be 1.1 × 106 A/cm2 and 4 × 105 A/cm2 at 4.2 K and 10 K, respectively. These results were found to be ~ 3 times higher than the pure (0% Ag-added) sample. The best Hc2(0) values were calculated to be 64.1 T, 50.9 T, and 42.7 T for 90% Tcoffset, 50% Tcmidpoint, and 10% Tconset, 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 µ0Hirr 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), Jcmag, Hc2(0), and, µ0Hirr calculations were evaluated to support the thermally activated dissipation mechanism (TAFF) and independent vortex motion in the samples.