Rare-earth intermetallic compounds exhibit rich phenomena induced by the interplay between localized f orbitals and conduction electrons. However, since the energy scale of the crystal-electric-field splitting is only a few millielectronvolts, the nature of the mobile electrons accompanied by collective crystal-electric-field excitations has not been unveiled. Here, we examine the low-energy electronic structures of CeSb through the anomalous magnetostructural transitions below the Neˊel temperature, ∼17 K, termed the 'devil's staircase', using laser angle-resolved photoemission, Raman and neutron scattering spectroscopies. We report another type of electron-boson coupling between mobile electrons and quadrupole crystal-electric-field excitations of the 4f orbitals, which renormalizes the Sb 5p band prominently, yielding a kink at a very low energy (∼7 meV). This coupling strength is strong and exhibits anomalous step-like enhancement during the devil's staircase transition, unveiling a new type of quasiparticle, named the 'multipole polaron', comprising a mobile electron dressed with a cloud of the quadrupole crystal-electric-field polarization.
@article{arxiv.2105.11772,
title = {Multipole polaron in the devil's staircase of CeSb},
author = {Y. Arai and Kenta Kuroda and T. Nomoto and Z. H. Tin and S. Sakuragi and C. Bareille and S. Akebi and K. Kurokawa and Y. Kinoshita and W. -L. Zhang and S. Shin and M. Tokunaga and H. Kitazawa and Y. Haga and H. S. Suzuki and S. Miyasaka and S. Tajima and K. Iwasa and R. Arita and Takeshi Kondo},
journal= {arXiv preprint arXiv:2105.11772},
year = {2022}
}