Phonons, Q -dependent Kondo spin fluctuations, and 4f phonon resonance in YbAl3

Autor: V. R. Fanelli, Filip Ronning, D. G. Mazzone, J. M. Lawrence, Lucas Lindsay, Andrew D. Christianson, Marein C. Rahn, Ayman Said, Eric D. Bauer, Sai Mu
Rok vydání: 2020
Předmět:
Zdroj: Physical Review B. 102
ISSN: 2469-9969
2469-9950
DOI: 10.1103/physrevb.102.205135
Popis: $4f$ intermediate valence (IV) compounds are canonical hosts of correlated electron physics and can contribute to our understanding of the larger class of correlated electron materials. Here we study the prototype IV compound $\mathrm{Yb}{\mathrm{Al}}_{3}$ which exhibits a nonintegral valence with a moderately heavy fermion ground state and a large Kondo temperature (${T}_{\mathrm{K}}\ensuremath{\sim}500--600\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). To better characterize the correlated physics of $\mathrm{Yb}{\mathrm{Al}}_{3}$, we have measured the phonon and the magnetic excitation spectra on single crystals of this material by time-of-flight inelastic neutron scattering and inelastic x-ray scattering. We have also performed theoretical calculations of the phonon spectra. We present three findings of these measurements. First, we observe that the measured phonon spectra can be described adequately by a calculation based on standard DFT+$U$ density functional theory. The calculated energies, however, are 10% too low compared to the measured energies. This discrepancy may reflect a hardening of the phonons due to dynamic $4f$ correlations. Second, the low-temperature spin fluctuations on the Kondo energy scale ${k}_{\mathrm{B}}{T}_{\mathrm{K}}$ have a momentum ($\mathbit{Q}$) dependence similar to that seen recently in the IV compound $\mathrm{CeP}{\mathrm{d}}_{3}$. For that system, the $\mathbit{Q}$ dependence has been attributed to particle-hole excitations in a coherent itinerant $4f$ correlated ground state. We suggest a similar origin for the momentum dependence seen in $\mathrm{Yb}{\mathrm{Al}}_{3}$. This $\mathbit{Q}$ dependence disappears as the temperature is raised towards room temperature and the $4f$ electron band states become increasingly incoherent. Such a coherent/incoherent crossover is expected to be generic for correlated electron systems. Third, a low-temperature magnetic peak observed in the neutron scattering near 30 meV shows dispersion identical to a particular optic-phonon branch. This $4f/\mathrm{phonon}$ resonance disappears for $T\ensuremath{\ge}150\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The phonon spectrum appears to be unaffected by the resonance. We discuss several possibilities for the origin of this unusual excitation, which may be unique to $\mathrm{Yb}{\mathrm{Al}}_{3}$. We suggest that the excitation may arise from the large amplitude beating of the light Al atoms against the heavy Yb atoms, resulting in a dynamic $4f/3p$ hybridization.
Databáze: OpenAIRE
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