Multipole polaron in the devil's staircase of CeSb

CeSb has been an intriguing candidate for probing electronic correlations because of the anomalous magneto-structural transitions below T_N~17 K, called “devil’s staircase". The established 4f order induces a tetragonal lattice contraction, dramatic changes in electron transports, and electronic reconstructions [1]. All these facts suggest the importance of the interplay between the localized 4f states and the itinerant electrons for understanding the anomalous transitions. However, revealing the underlying many-body interaction going through the devil’s staircase has remained out of reach.

Here, we examine the low-energy electronic structures of CeSb in the devil’s staircase, using laser angle-resolved photoemission spectroscopy, Raman spectroscopy, and neutron spectroscopy. We found a new type of electron–boson coupling, which leads to a unique quasiparticle state named the ‘multipole polaron’ [2]. This new coupling mediates between the itinerant electrons and the bosons constituted by the crystal-electric-field excitations of the localized 4f states.

[1] K. Kuroda, Y. Arai et al., Nat. Commun. 11, 2888 (2020). [2] Y. Arai et al., Nat. Mater. 21, 410 (2022).