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Register a new userHiggs Mode and Magnon Interactions in 2D Quantum Antiferromagnets from Raman Scattering
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Simon Adrian Weidinger
Publication date
2015
fields
Physics
and research's language is
English
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We present a theory for Raman scattering on 2D quantum antiferromagnets. The microscopic Fleury-Loudon Hamiltonian is expressed in terms of an effective $O(3)$ - model. Well within the Neel ordered phase, the Raman spectrum contains a two-magnon and a two-Higgs contribution, which are calculated diagramatically. The vertex functions for both the Higgs and magnon contributions are determined from a numerical solution of the corresponding Bethe-Salpeter equation. Due to the momentum dependence of the Raman vertex in the relevant $B_{1g}+E_{2g}$ symmetry, the contribution from the Higgs mode is strongly suppressed. Except for intermediate values of the Higgs mass, it does not show up as separate peak in the spectrum but gives rise to a broad continuum above the dominant contribution from two-magnon excitations. The latter give rise to a broad, asymmetric peak at $omegasimeq 2.44, J$, which is a result of magnon-magnon interactions mediated by the Higgs mode. The full Raman spectrum is determined completely by the antiferromagnetic exchange coupling $J$ and a dimensionless Higgs mass. Experimental Raman spectra of undoped cuprates turn out to be in very good agreement with the theory only with inclusion of the Higgs contribution. They thus provide a clear signature of the presence of a Higgs mode in spin one-half 2D quantum antiferromagnets.
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Synthetic antiferromagnet, comprised of two ferromagnetic layers separated by a non-magnetic layer, possesses two uniform precession resonance modes: in-phase acoustic mode and out-of-phase optic mode. In this work, we theoretically and numerically demonstrated the strong coupling between acoustic and optic magnon modes. The strong coupling is attributed to the symmetry breaking of the system, which can be realized by tilting the bias field or constructing an asymmetrical synthetic antiferromagnet. It is found that the coupling strength can be highly adjusted by tuning the tilting angle of bias field, the magnitude of antiferromagnetic interlayer exchange coupling, and the thicknesses of ferromagnetic layers. Furthermore, the coupling between acoustic and optic magnon modes can even reach the ultrastrong coupling regime. Our findings show high promise for investigating quantum phenomenon with a magnonic platform.
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