  The Kondo effect describes the scattering of conduction electrons by magnetic
impurities, manifesting as an electronic resonance at the Fermi energy with a
distinctive temperature evolution. In this letter, we present a critical
evaluation of the current methodology employed to demonstrate Kondo behavior in
transport measurements, underscoring the limitations of established theoretical
frameworks and the influence of extrinsic broadening. We introduce a novel
approach for analyzing spectroscopic indicators of the Kondo effect, employing
the Hurwitz-Fano lineshape as a model for the Kondo resonance in the presence
of extrinsic broadening. Through precise scanning tunneling spectroscopy
measurements on an exemplary spin-1/2 Kondo system, phenalenyl on Au(111), we
demonstrate the efficacy of our proposed protocol in extracting accurate
intrinsic Kondo linewidths from finite-temperature measurements. The extracted
linewidths exhibit a robust fit with a recently derived expression for the
temperature-dependent intrinsic Kondo linewidth, providing compelling evidence
for the validity of the underlying theory.
