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arXiv · 1604.04249

Longitudinal magnon, inversion breaking and magnetic instabilities in the pseudo-Kagome francisites Cu3Bi(SeO3)2O2X with X=Br, Cl

V. Gnezdilov·Yu. Pashkevich·V. Kurnosov·P. Lemmens·E. Kuznetsova·P. Berdonosov·V. Dolgikh·K. Zakharov·A. Vasiliev
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Abstract

We performed Raman studies and a dielectric characterization of the pseudo-kagome Cu3Bi(SeO3)2O2X (X = Cl, Br). These compounds share competing nearest-neighbour ferromagnetic exchange and frustrating next-nearest-neighbour antiferromagnetic exchange as well as highly noncollinear magnetic ground state. However, at low temperature they differ with respect to the existence of inversion symmetry. For both compounds there exists a pronounced interplay of polar phonon modes with quantum magnetic fluctuations. A novel Raman mode appears for temperatures below the Neel temperature with a Fano lineshape and an enormous intensity that exceeds most of the phonon lines. We discuss a possible contribution of longitudinal magnons to this signal. In contrast, one magnon scattering based on linear transvers magnons is excluded based on a symmetry analysis of spin wave representations and Raman tensor calculations. There exists evidence that in these pseudo-kagome compounds magnetic quantum fluctuations carry an electric dipole moment. Our data as well as a comparison with previous far-infrared spectra allow us to conclude that Cu3Bi(SeO3)2O2Cl changes its symmetry most likely from Pmmn to P21mn with a second order structural phase transition at T*=120 K and becomes multiferroic. Cu3Bi(SeO3)2O2Br represents an interesting counter part as it does not show this instability and stays inversion symmetric down to lowest temperatures, investigated.

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V. Gnezdilov, Yu. Pashkevich, V. Kurnosov, P. Lemmens, E. Kuznetsova, P. Berdonosov, V. Dolgikh, K. Zakharov, A. Vasiliev. 2016-04-14. Longitudinal magnon, inversion breaking and magnetic instabilities in the pseudo-Kagome francisites Cu3Bi(SeO3)2O2X with X=Br, Cl. https://doi.org/10.1103/physrevb.96.115144

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