Electromagnon dispersion probed by inelastic X-ray scattering in LiCrO2

Abstract

Inelastic X-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids. In non-resonant scattering condition, the cross-section is strongly dominated by lattice vibrations (phonons). However, it is possible to probe additional degrees of freedom such as magnetic fluctuations that are strongly coupled to the phonons. The IXS spectrum of the coupled system contains not only the phonon dispersion but also the so far undetected magnetic correlation function. Here we report the observation of strong magnon–phonon coupling in LiCrO2 that enables the measurement of magnetic correlations throughout the Brillouin zone via IXS. We find electromagnon excitations and electric dipole active two-magnon excitations in the magnetically ordered phase and heavily damped electromagnons in the paramagnetic phase of LiCrO2. We predict that several (frustrated) magnets with dominant direct exchange and non-collinear magnetism show surprisingly large IXS cross-section for magnons and multi-magnon processes.

Hierarchy of Exchange Interactions in the Triangular-Lattice Spin Liquid YbMgGaO4

“…Indeed, classical Monte Carlo simulations for the instantaneous spin structure factor SðQÞ reveal that either large J AEAE 1 or relatively small J 2 ∼ 0.2J 1 yields correlations peaked at the M point of the triangular-lattice Brillouin zone [61]. Maximal correlations at the M point have been reported in all neutron scattering investigations to date [61][62][63]65]. In this regard, we note that Monte Carlo simulations with a large J AEAE 1 yield a strong modulation in SðQÞ from the first to the second Brillouin zone [61].…”

“…Experiments using other techniques, such as unpolarized [61] and polarized [65] neutron diffraction in zero field and low temperatures, may help to constrain the values further. Indeed, classical Monte Carlo simulations for the instantaneous spin structure factor SðQÞ reveal that either large J AEAE 1 or relatively small J 2 ∼ 0.2J 1 yields correlations peaked at the M point of the triangular-lattice Brillouin zone [61].…”

“…However, this projection has not always been fully included in theoretical calculations of the scattering pattern, which may help to account for the somewhat larger values of J AEAE 1 proposed in Refs. [43,65].…”

“…The second set [43], model B in Table I, includes only nearestneighbor interactions, with the J 2 ¼ 0 constraint argued as a consequence of the localized nature of Yb 3þ 4f electrons [43,65]. Both models used the g factors determined from low-frequency X-band ESR.…”

“…In fact, the nature of the dominant exchange interactions in YbMgGaO 4 is also controversial. While the overall planar anisotropy is clear from susceptibility measurements [55,62], both antiferromagnetic next-nearest-neighbor terms (J 2 ) [61] and nearest-neighbor anisotropic offdiagonal exchanges (so-called J AEAE 1 and J zAE 1 ) [43,65] have been proposed as extensions from the XXZ model. Comprehensively determining the exchange interactions in YbMgGaO 4 is of fundamental importance in deciphering the nature of its ground state.…”