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

Temperature- and field-dependent surface resistance measurements were conducted using Electron Spin Resonance (ESR) techniques on single-crystal $\mathrm{Ba_{0.6}K_{0.4}Fe_{2}As_{2}}$ samples (\textit{rf} frequency = 20.3 GHz). At a fixed temperature, field scans were performed at various angles of $H_0$ with respect to $H_{rf}$. To our knowledge, this is the first report of such studies on this material. For temperatures exceeding $T_C$, there was no evidence of iron ESR. 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.