Electron spin resonance (ESR) can probe 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 can be 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 superconductor β-YbAlB₄ [S. Nakatsuji et al., Nature Phys. 4, 603 (2008)] which behaves as a CESR at high temperatures and acquires characteristics of the Yb³⁺ LM ESR at low temperature. This dual behavior strikes as an in situ unique observation of the Kondo quasiparticles in a quantum critical regime. The proximity to a quantum critical point may favor the appearance of this dual character of the ESR signal in β-YbAlB₄.

Electron Spin Resonance (ESR) experiments in the filled Cei 1-xYbxFe4P12 (x ≲ 0.0023) skutterudite compounds of Th cubic symmetry (Im3) reveal the coexistence of two distinct Yb3+ sites with large difference in their site occupation, hyperfine interaction and temperature dependence of the ESR parameters. These results suggest a scenario where the oversized (Fe 2P3)4-cages of the Ce1-xYb xFe4P12 compounds are occupied with small number of on-center-Yb3+ ions and a distribution of highly occupied off-center-Yb3+ions, rattling at ∼1 GHz and above ∼20 GHz, respectively. © 2010 IOP Publishing Ltd.

Electron-spin-resonance (ESR) measurements performed on the filled skutterudite system Ce1-x Ybx Fe4 P12 (x≲0.003) unequivocally reveal the coexistence of two Yb3+ resonances associated with sites of considerably different occupations and temperature behaviors. Detailed analysis of the ESR data suggests a scenario where the fraction of oversized (Fe2 P3) 4 cages that host Yb ions are filled with a low occupation of on-center Yb3+ sites and a highly occupied T -dependent distribution of off-center Yb3+ sites. Analysis of the Y 171 b3+ (I=1/2) isotope hyperfine splittings reveal that these two sites are associated with a low (∼1 GHz) and a high (≲ 15 GHz) rattling frequency, respectively. Our findings introduce Yb3+ in Th symmetry systems and use the Yb3+ ESR as a sensitive microscopic probe to investigate the Yb3+ ions dynamics. © 2009 The American Physical Society.