Magnetoelastic hybrid excitations

The interplay between magnetic moments and lattice degrees of freedom is a general condensed matter property commonly known as magnetoelastic coupling. Specifically coupling between phonons and crystal field (CF) in rare-earth compounds was extensively studied 30 years ago by Thalmeier and others, resulting in broad theory of bound states. For a very strong magnetoelastic coupling, a creation of an additional vibronic bound state was reported [1]; and if the coupling is not too strong, one can observe mixed modes of magnetic excitons and phonons, visible as anticrossing in the inelastic scattering [2]. Curiously these effects are still not sufficiently experimentally confirmed – the bound state is in detail investigated only in CeAl2 [3] and an anticrossing without applied magnetic field was observed only indirectly in PrNi5 [4].

A reason for this could be the overall suppression of the CF research with discovery of high-temperature superconductivity in 1986, remaining still important and unresolved topic. In the recent years, researchers are looking for small signs of cooperative and hybridized effects, which could mediate unconventional superconductivity. CF – phonon coupling is one of them and it is a great time to investigate it in detail, because our knowledge and methods are now „30 years better“. Nevertheless, CF – phonon coupling became already topical again in different kinds of materials like pyrochlores [5, 6], actinides [7], multiferroics [8] or heavy fermions [9, 10].

I will present historical overview of the crystal field – phonon interplay, show you tools how to treat it and then I will focus on the properties of CeAuAl3 studied using triple axis neutron spectroscopy. There exist three examples of strong crystal field – phonon interactions and the formation of novel hybrid modes. Our results suggest that strongly hybridized crystal field – phonon excitations may, in fact, be rather common in f-electron compounds.

[1] P. Thalmeier et al., Handbook on Phys. and Chem. Rare Earths vol. 14, 225 – 341 (1991).
[2] S. Sinha, Handbook on Phys. and Chem. Rare Earths vol. 1, 489– 589 (1978).
[3] M. Loewenhaupt et al., J. Phys.: Cond. Mat. 15, S519 (2003).
[4] V. Aksenov et al., Physica B+C 120, 310 (1983).
[5] T. Fennell et al., Phys. Rev. Lett. 112, 017203 (2014).
[6] E. Constable et al., Phys. Rev. B 95, 020415® (2016).

[7] N, Magnani et al., Phys. Chem. C, 120, 4799 (2016).
[8] S. A. Klimin at al., Phys. Rev. B 93, 054304 (2016).
[9] M. Klicpera et al., Phys. Rev. B 91, 224419 (2015).
[10] D. Adroja et al., Phys. Rev. Letters 108, 216402 (2012).