六方压磁YMnO3中声子-顺磁耦合的太赫兹和红外光谱证据
Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3
摘要：
研究了六方晶系压磁YMnO_3单晶的太赫兹和远红外电磁响应。在低于Neel温度T_N时，在磁导率μ_a [H（ω）在六角平面内取向]谱中观察到反铁磁共振现象。这种激发在41 ~ 32 cm ~（-1）范围内随加热而减弱，在T_N以上消失。在同一频率范围内，复介电常数ε_c谱中还存在一个弱的强阻尼激发。这种激发有助于在反铁磁和顺磁相的介电谱。它的振子强度显着增加对室温加热，从而提供证据的压磁或高阶耦合到极性声子。其他重阻尼介电激发在100 cm-1附近的顺磁相位中检测到在两个的x-c和x-a谱，它们表现出类似的温度行为。这些激发出现在远低于极性声子的磁振子分支的频率范围内，可以提醒电磁振子;然而，它们的温度依赖性是完全不同的。我们用密度泛函理论计算了整个布里渊区的声子色散分支。对这些结果和先前发表的磁振子色散分支的详细分析使我们得出这样的结论，即所观察到的吸收带源于声子-声子和声子-顺磁振子差分吸收过程。后者是通过在顺磁相中的强短程面内自旋相关来实现的。
英文摘要：
Terahertz and far-infrared electric and magnetic responses of hexagonal piezomagnetic YMnO3 single crystals are investigated. Antiferromagnetic resonance is observed in the spectra of magnetic permeability mu_a [H(omega) oriented within the hexagonal plane] below the Neel temperature T_N. This excitation softens from 41 to 32 cm-1 on heating and finally disappears above T_N. An additional weak and heavily-damped excitation is seen in the spectra of complex dielectric permittivity epsilon_c within the same frequency range. This excitation contributes to the dielectric spectra in both antiferromagnetic and paramagnetic phases. Its oscillator strength significantly increases on heating towards room temperature thus providing evidence of piezomagnetic or higher-order couplings to polar phonons. Other heavily-damped dielectric excitations are detected near 100 cm-1 in the paramagnetic phase in both epsilon_c and epsilon_a spectra and they exhibit similar temperature behavior. These excitations appearing in the frequency range of magnon branches well below polar phonons could remind electromagnons; however, their temperature dependence is quite different. We have used density functional theory for calculating phonon dispersion branches in the whole Brillouin zone. A detailed analysis of these results and of previously published magnon dispersion branches brought us to the conclusion that the observed absorption bands stem from phonon-phonon and phonon- paramagnon differential absorption processes. The latter is enabled by a strong short-range in-plane spin correlations in the paramagnetic phase.
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