title: Quantum critical Kondo quasiparticles probed by ESR in β-YbAlB₄.
abstract: 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.
authors.affiliation: Instituto de Física Gleb Wataghin, UNICAMP, Campinas-SP, 13083-970, Brazil.
authors.forename: L M
authors.initials: LM
authors.lastname: Holanda
authors.forename: J M
authors.initials: JM
authors.lastname: Vargas
authors.forename: W
authors.initials: W
authors.lastname: Iwamoto
authors.forename: C
authors.initials: C
authors.lastname: Rettori
authors.forename: S
authors.initials: S
authors.lastname: Nakatsuji
authors.forename: K
authors.initials: K
authors.lastname: Kuga
authors.forename: Z
authors.initials: Z
authors.lastname: Fisk
authors.forename: S B
authors.initials: SB
authors.lastname: Oseroff
authors.forename: P G
authors.initials: PG
authors.lastname: Pagliuso