High Upper Critical Field in Potassium Fluoride Doped LaOFeAs Superconductor

Abstract

We report a new methodology for synthesizing oxypnictide superconductors with the commonly available potassium fluoride (KF) as a source of fluorine instead of expensive LaF <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{3}$</tex> </formula> . This results in simultaneous doping of potassium at lanthanum sites and leads to a three-fold increase in the upper critical field. We report the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$T_{c}$</tex></formula> (onset) of 28.50 K by F-doping and highest upper critical field ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\sim$</tex></formula> 122 T) at ambient pressure in the family of La–based oxypnictides. To study the contribution from inter and intra-granular current density, we compare remanent magnetization measurements on La <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{1.03}{\hbox{O}}_{0.9}{\hbox{F}}_{0.2}{\hbox{FeAs}}$</tex> </formula> and La <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{0.8}{\hbox{K}}_{0.2}{\hbox{O}}_{0.8}{\hbox{F}}_{0.2}{\hbox{FeAs}}$</tex> </formula> superconductors that show only one major peak indicating substantial electromagnetic granularity. The Seebeck effect <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$(S)$</tex></formula> with respect to temperature has negative sign and exhibits unconventional temperature dependence. Analysis of resistivity transition broadening under magnetic field reveals the signatures of Arrhenius behavior and the calculated activation energy follows power-law dependence with respect to magnetic field.