Multipole polaron in the devil's staircase of CeSb

TL;DR

This paper uncovers a new quasiparticle called the 'multipole polaron' in CeSb, formed by electrons coupled with quadrupole crystal-electric-field excitations, revealed through advanced spectroscopic techniques during complex magnetic transitions.

Contribution

It introduces the concept of the 'multipole polaron' and demonstrates its formation and characteristics in CeSb, a novel electron-boson coupling mechanism involving quadrupole excitations.

Findings

Identification of electron-quadrupole coupling in CeSb

Observation of a kink at ~7 meV in the Sb 5p band

Step-like enhancement of coupling during devil's staircase transitions

Abstract

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 N$\'{e}$el temperature, $\sim$17 K, termed the 'devil's staircase', using laser angle-resolved photoemission, Raman and neutron scattering spectroscopies. We report another type of electron-boson coupling between mobile electrons and quadrupole crystal-electric-field excitations of the 4$f$ orbitals, which renormalizes the Sb 5$p$ band prominently, yielding a kink at a very low energy ($\sim$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.