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arXiv 1402.2165cond-mat.mtrl-sci
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SrMnO3红外和拉曼光谱中的强自旋-声子耦合

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

S. Kamba, V. Goian, V. Skoromets, J. Hejtmanek, V. Bovtun, M. Kempa, F. Borodavka, P. Vanek, A. A. Belik, J. H. Lee, O. Pacherova, K. M. Rabe

摘要:

立方相SrMnO $_{3}$陶瓷的红外反射率谱表明,在T $_{N}$ = 233 K时,在反铁磁相变以下,最低频率声子的硬度为18%。极性声子频率的如此大的温度变化是非凡的,我们将其归因于这种材料中异常强的自旋-声子耦合。这与我们从第一原理计算的预测是一致的。此外,极性声子变得拉曼活性低于T $_{N}$,虽然他们的激活是禁止的对称性在$Pm\bar{3}m$空间群。这证明了立方$Pm\bar{3}m$对称性在T $_{N}$以下由于强磁电耦合而局部断裂。拉曼光谱中还观察到多声子和多磁振子散射。微波和太赫兹介电常数强烈影响跳跃电子电导,这是由小的非化学计量比的样品所造成的。热电测量显示自由载流子$n_{e}=$ 3.6 10 $^{20}$ cm $^{-3}$和样品组合物Sr $^{2+}$ Mn $_{0.98}^{4+}$ Mn $_{0.02}^{3+}$ O $_{2.99}^{2-}$的室温浓度。电导率表现出非常不寻常的温度行为:冷却时太赫兹电导率增加,而冷却时静态电导率显着降低。我们把这归因于陶瓷晶粒和晶界的不同导电性。

英文摘要:

Infrared reflectivity spectra of cubic SrMnO$_{3}$ ceramics reveal 18 % stiffening of the lowest-frequency phonon below the antiferromagnetic phase transition occurring at T$_{N}$ = 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 T$_{N}$, although their activation is forbidden by symmetry in $Pm\bar{3}m$ space group. This gives evidence that the cubic $Pm\bar{3}m$ symmetry is locally broken below T$_{N}$ 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 $n_{e}=$3.6 10$^{20}$ cm$^{-3}$ and the sample composition Sr$^{2+}$Mn$_{0.98}^{4+}$Mn$_{0.02}^{3+}$O$_{2.99}^{2-}$. 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.

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