  Electron Spin Resonance (ESR) can microscopically probe both conduction
electrons (ce) and local moment (LM) spin systems in different materials. A ce
spin resonance (CESR) is observed in metallic systems based on light elements
or with enhanced Pauli susceptibility. LM ESR is frequently seen in compounds
with paramagnetic ions and localized d or f electrons. Here we report a
remarkable and unprecedented ESR signal in the heavy fermion (HF)
superconductor beta-YbAlB4[1] which behaves as a CESR at high temperatures and
acquires characteristics of the Yb3+ LM ESR at low temperature. This dual
behavior in same ESR spectra strikes as an in situ unique observation of the
Kondo quasiparticles giving rise to a new ESR response called Kondo coupled
resonant mode (KCRM). The proximity to a quantum critical point (QCP) may favor
the observation of a KCRM and its dual character in beta-YbAlB4 may unveil the
4f-electrons nature at the QCP.
