Angle and frequency dependent low field microwave absorption in electronically doped Ca 122 pnictides: Comparison of high Tc = 42 K phase in Pr, Nd, Ce and La doping

The motivation of this study is to investigate the properties of a unique interfacial superconducting phase in electron-doped Ca 122 pnictides by the Low Field Microwave Absorption (LFMA) technique. Samples are exposed to microwave radiation with frequency $\nu_{MW}$ between 1 and 24 GHz, and also to a low strength magnetic field which modulates at $\nu_{mag} = 100$ kHz. Due to their single crystalline nature, the pnictides can be oriented relative to the MW polarization and magnetic field direction. Studying this orientation dependence reveals filament-like micro-interfaces between highly doped regions and poorly doped regions. These interfaces may be responsible for a high $T_c \sim 42$ K superconducting phase. We demonstrate that this higher $T_c$ can be clearly distinguished from the low temperature bulk SC phase by the angle-dependent LFMA method. Additionally, variation of the MW frequency yields changes in the spectra which are in agreement with theoretical predictions.