Magnetic excitations beyond the single- and double- magnons

Hebatalla Elnaggar , Sorbonne University

Conventional wisdom suggests that one photon that carries one unit of angular momentum (1h) can change the spin angular momentum of a magnetic site with one unit (ΔM = ±1h) at most following the selection rules. This implies that a two-photon process such as 23 resonant inelastic X-ray scattering (RIXS – see Fig. 1a-c) can change the spin angular momentum of a magnetic system with a maximum of two units (ΔM = ± 2h) [1]. Herein we describe a triple-magnon excitation in the altermagnetic system, -Fe2O3, which contradicts this conventional wisdom that only 1- and 2-magnon excitations are possible in a resonant inelastic X-ray scattering experiment [2].

Figure 1: Schematic of Resonant Inelastic X-ray Scattering (RIXS). (a) The initial state of a 3d transition metal plus a photon with energy ℏin, wave-vector kin. (b) The intermediate state where a 2p electron is excited to the empty 3d states leaving a core-hole that exists for few fs. (c) The final state where a valence 3d electron fills the core-hole and a photon with energy ℏout, wave-vector kout is emitted. The energy and momentum transfer are given by ℏ(in - out) and ℏ(kin-kout), respectively. (d) Fe 2p3d RIXS measured in -Fe2O3 single crystal where we observed multi-magnons.

We observe an excitation at exactly three times the magnon energy, along with additional excitations at four and five times the magnon energy, suggesting the presence of quadruple and quintuple magnons as well (see Fig. 1d). Guided by theoretical calculations, we reveal how a two-photon scattering process can create exotic higher-rank magnons and the relevance of these quasiparticles for understanding spin non-conserving interactions where the lattice degree of freedom acts as a reservoir of angular momentum.

References:
[1]- A. Nag, et. al., Many-body physics of single and double spin-flip excitations in NiO, Phys. Rev. Lett., 124, 067202 (2020).
[2]- H. Elnaggar, et. al., Magnetic excitations beyond the single- and double-magnons, Nat. Commun. 14, 2749 (2023).

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