Resistive Losses in Single-Crystal Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$

Recently, we conducted surface resistance measurements using electron spin resonance techniques on single-crystal Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ samples (\textit{rf} frequency = 20.3 GHz), testing temperature and field dependence. In the superconducting state, the samples exhibit dissipative losses which increase monotonically as a function of applied field for fixed temperature. The level of field-dependent dissipation increases as $T$ approaches $T_C$ from below, and vanishes at the transition. The dissipation is also dependent on the angle between the \textit{rf} field and the static field, consistent with flux-flow models of other high-temperature superconductors, such as YBCO.