Magnetoelastic hybrid excitations in CeAuAl ₃

Abstract

Nearly a century of research has established the Born–Oppenheimer approximation as a cornerstone of condensed-matter systems, stating that the motion of the atomic nuclei and electrons may be treated separately. Interactions beyond the Born–Oppenheimer approximation are at the heart of magneto-elastic functionalities and instabilities. We report comprehensive neutron spectroscopy and ab initio phonon calculations of the coupling between phonons, CEF-split localized 4f electron states, and conduction electrons in the paramagnetic regime ofCeAuAl3, an archetypal Kondo lattice compound. We identify two distinct magneto-elastic hybrid excitations that form even though all coupling constants are small. First, we find a CEF–phonon bound state reminiscent of the vibronic bound state (VBS) observed in other materials. However, in contrast to an abundance of optical phonons, so far believed to be essential for a VBS, the VBS inCeAuAl3arises from a comparatively low density of states of acoustic phonons. Second, we find a pronounced anticrossing of the CEF excitations with acoustic phonons at zero magnetic field not observed before. Remarkably, both magneto-elastic excitations are well developed despite considerable damping of the CEFs that arises dominantly by the conduction electrons. Taking together the weak coupling with the simultaneous existence of a distinct VBS and anticrossing in the same material in the presence of damping suggests strongly that similarly well-developed magneto-elastic hybrid excitations must be abundant in a wide range of materials. In turn, our study of the excitation spectra ofCeAuAl3identifies a tractable point of reference in the search for magneto-elastic functionalities and instabilities.

Ubiquity of amplitude-modulated magnetic ordering in the H−T phase diagram of the frustrated non-Fermi-liquid YbAgGe

“…Thus, the dynamic coupling between the crystal lattice and the magnetic degrees of freedom has to be inspected carefully. A coupling between the CE levels and the phonons may be one possibility, which has been seen in CeAuAl 3 for instance [29]. A strong coupling between CE levels and lattice vibrations would affect the elastic constants of YbAgGe, and has indeed been observed in previous reports [6].…”

Thef-electron State of the Heavy Fermion Superconductor NpPd5Al2and the Isostructural Family

“…A possible mechanism of the energy shift may be the coupling with phonon [24]. It is impressive that the Debye cutoff of the phonon frequency ∼ 20 meV is very close to the high-energy CEF levels as shown in Fig.…”

Lattice dynamics in CePd2Al2 and LaPd2Al2

“…The experimental studies in CeCuAl 3 6 , CePd 2 Al 2 5 and CeAuAl 3 7 propose the existence of a bound state, but its signature was found mainly in the magnetic excitations spectra. The study of phonon dispersion curves is still missing with some exception for CeAuAl 3 and CeCuAl 3 7 11 .…”

“…The experimental studies in CeCuAl 3 6 , CePd 2 Al 2 5 and CeAuAl 3 7 propose the existence of a bound state, but its signature was found mainly in the magnetic excitations spectra. The study of phonon dispersion curves is still missing with some exception for CeAuAl 3 and CeCuAl 3 7 11 . The study of low energy phonon dispersion curves (mainly acoustic) on CeAuAl 3 7 shows the anticrossing of a non-dispersive magnetic excitation and an acoustic phonon branch, which was not observed before.…”

“…It was shown, that this approach cannot explain the magnetic excitation spectra measured in several compounds, e.g. in pyrochlores Tb 2 Ti 2 O 7 1 , Ho 2 Ti 2 O 7 2 , intermetallics CeAl 2 3 , PrNi 2 4 , CePd 2 Al 2 5 , CeCuAl 3 6 and recently in CeAuAl 3 7 . The Ce-based intermetallics have a very prominent position with a relatively small number of expected CEF excitations.…”