Flux free growth of large FeSe1/2Te1/2 superconducting single crystals by an easy high temperature melt and slow cooling method

Abstract

We report successful growth of flux free large single crystals of superconducting FeSe1/2Te1/2 with typical dimensions of up to few cm. The AC and DC magnetic measurements revealed the superconducting transition temperature (Tc) value of around 11.5K and the isothermal MH showed typical type-II superconducting behavior. The lower critical field (Hc1) being estimated by measuring the low field isothermal magnetization in superconducting regime is found to be above 200Oe at 0K. The temperature dependent electrical resistivity ρ(T ) showed the Tc (onset) to be 14K and the Tc(ρ = 0) at 11.5K. The electrical resistivity under various magnetic fields i.e., ρ(T)H for H//ab and H//c demonstrated the difference in the width of Tc with applied field of 14Tesla to be nearly 2K, confirming the anisotropic nature of superconductivity. The upper critical and irreversibility fields at absolute zero temperature i.e., Hc2(0) and Hirr(0) being determined by the conventional one-band Werthamer–Helfand–Hohenberg (WHH) equation for the criteria of normal state resistivity (ρn) falling to 90% (onset), and 10% (offset) is 76.9Tesla, and 37.45Tesla respectively, for H//c and 135.4Tesla, and 71.41Tesla respectively, for H//ab. The coherence length at the zero temperature is estimated to be above 20Å ´ by using the Ginsburg-Landau theory. The activation energy for the FeSe1/2Te1/2 in both directions H//c and H//ab is determined by using Thermally Activation Flux Flow (TAFF) model.