Conduction-Electron Resonance Linewidth in Dilute Magnetic Alloys above the Kondo Critical Field at Very Low Temperatures

The zero-temperature transverse dynamic susceptibility at frequency $\Omega$, ${{\chi }_e}^{-}\left(\Omega \right)$, is calculated diagrammatically for conduction electrons in a dilute magnetic alloy. The static magnetic field is assumed to be sufficiently large to quench the Kondo divergence. The exchange interaction is treated to second order in the coupling constant $J$. It is found that the conduction-electron magnetic-resonance response is a $\delta$ function at the exchange-"dressed" conduction-electron resonance frequency ${{\omega }_e}^{\prime }$, with a superimposed Lorentzian envelope centered at ${{\omega }_e}^{\prime }$, which is finite ("turns on") at frequencies $\Omega >{\omega }_s$, where ${\omega }_s$ is the localized spin resonance frequency. The implications of this result are discussed with reference to the magnetic-resonance bottleneck, and previous (approximate) calculations of the zero-temperature magnetic-resonance response.