Research: Nature of Magnetic High-Field Excitations in α-RuCl3

The famous Kitaev-Heisenberg model predicts a variety of magnetic phases for honeycomb spin structures, ranging from the conventional Néel state to a quantum spin liquid, with the spin dynamics determined by spin-flip excitations fractionalized into gapped flux and gapless Majorana fermion excitations. α-RuCl₃ is one of the prime candidates to test this model. One striking peculiarity of the spin dynamics in this material is the presence of a broad excitation continuum, which was interpreted as a potential signature of fractionalized Majorana excitations. Apart from that, a very rich excitation spectrum was revealed in the field-induced, magnetically disordered phase below the continuum, whose identification has remained an open question. In cooperation with partners from Oak Ridge, USA, we performed comprehensive electron spin resonance (ESR) studies of high-quality in-plane oriented single crystals of α-RuCl₃, focusing on its high-field spin dynamics. Combining our findings with recent inelastic neutron- and Raman-scattering data, we identified most of the observed excitations (Figure). Most importantly, we obtained firm evidence that the low-temperature high-field ESR response is dominated by single- and two-particle processes with magnons as elementary excitations.

A. N. Ponomaryov, L. Zviagina, J. Wosnitza, P. Lampen-Kelley, A. Banerjee, J.-Q. Yan, C. A. Bridges, D. G. Mandrus, S. E. Nagler, S. A. Zvyagin,
Nature of Magnetic Excitations in the High-Field Phase of α−RuCl₃,
Phys. Rev. Lett. 125, 037202 (2020) (arXiv)