Strong spin-phonon coupling in infrared and Raman spectra of SrMnO3

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Infrared reflectivity spectra of cubic SrMnO₃ ceramics reveal 18 % stiffening of the lowest-frequency phonon below the antiferromagnetic phase transition occurring at Tₙ = 233 K. Such a large temperature change of the polar phonon frequency is extraordinary and we attribute it to an exceptionally strong spin-phonon coupling in this material. This is consistent with our prediction from first principles calculations. Moreover, polar phonons become Raman active below Tₙ, although their activation is forbidden by symmetry in Pm3m space group. This gives evidence that the cubic Pm3m symmetry is locally broken below Tₙ due to a strong magnetoelectric coupling. Multiphonon and multimagnon scattering is also observed in Raman spectra. Microwave and THz permittivity is strongly influenced by hopping electronic conductivity, which is caused by small non-stoichiometry of the sample. Thermoelectric measurements show room-temperature concentration of free carriers nₑ=3.6 10²⁰ cm⁻³ and the sample composition Sr²⁺Mn₀.₉₈⁴⁺Mn₀.₀₂³⁺O₂.₉₉²⁻. The conductivity exhibits very unusual temperature behavior: THz conductivity increases on cooling, while the static conductivity markedly decreases on cooling. We attribute this to different conductivity of the ceramic grains and grain boundaries.