NATURE OF MAGNETIC EXCITATIONS IN THE HIGH-FIELD PHASE OF α-RuCl₃

Sergei Zvyagin, HLD Dresden.

Spin systems with honeycomb structures are attracting a great deal of attention, in particular in connection with the famous Kitaev-Heisenberg model. This model predicts a variety of magnetic phases, 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 has been proposed as one of the prime candidates to test this model. One striking peculiarity of the spin dynamics in α-RuCl₃ 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 particular, it was unclear whether the observed excitations correspond to bound magnons or to bound Majorana spinons and, for the latter case, whether the spinons are confined or simply bound. In cooperation with our partners from Oak Ridge, USA, we performed comprehensive electron spin resonance (ESR) studies of highquality in-plane oriented single crystals of α-RuCl₃ at HLD, 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 evidenc that the low-temperature high-field ESR response is dominated by single- and two-particle processes with magnons as elementary excitations.

Figure: (a) Schematic energy diagram for α-RuCl₃ in an arbitrary magnetic field above the critical field Bc. The modes C and F are single-magnon excitations, while the modes 2C and 2F correspond to two-magnon excitations. The mode E corresponds to an excitation of a two-magnon bound state. (b) Frequency-field dependences of selected ESR modes and the color contour plot of the high-field Raman scattering intensity (latter data are from D. Wulferding et al., Nat. Commun. 30, 1603 (2020); work done at LNCMI-Grenoble).

Nature of magnetic excitations in the highfield phase of α-RuCl₃, A. N. Ponomaryov, L. Zviagina, J. Wosnitza, P. Lampen-Kelley, A. Banerjee, J.-Q. Yan, C. A. Bridges, D. G. Mandrus, S. E. Nagler, and S. A. Zvyagin, Phys. Rev. Lett. 125, 037202 (2020).

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.037202