Optical study of Pr 1-x Ca x MnO 3 (x=0.4) in a magnetic field: Variation of electronic structure with charge ordering and disordering phase transitions
Optical study of Pr 1-x Ca x MnO 3 (x=0.4) in a magnetic field: Variation of electronic structure with charge ordering and disordering phase transitions
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DOI：
10.1103/physrevb.59.7401
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发表时间：
1999-03
影响因子：
3.7
通讯作者：
Y. Okimoto;Y. Tomioka;Y. Onose;Y. Otsuka;Y. Tokura
中科院分区：
文献类型：
--
作者：
Y. Okimoto;Y. Tomioka;Y. Onose;Y. Otsuka;Y. Tokura
研究了Pr 1− xCaxMnO 3（x= 0.4）单晶的光谱及其各向异性与温度（10-290 K）和磁场（0-7 T）的关系，该单晶在Tco = 235 K时经历电荷和轨道有序跃迁.在10 K时，b轴和c轴偏振电导率谱[σ（ω）]之间存在明显的各向异性，反映了空间电荷和轨道有序的模式。电荷有序（CO）态的差距值估计为在基态下的约0.18 eV。在7 T磁场作用下，σ（ω）从各向异性的带隙形状转变为各向同性的金属带，在宽的光子能量范围（0.05- 3eV）内具有明显的光谱重量转移。当磁场从7 T开始减小时，这种金属σ（ω）在4.5T附近突然再次变成带隙状，但两种极化之间的各向异性可能由于轨道无序而减小。在7 T的恒定磁场中研究了b轴极化强度σ（ω）的温度依赖性。随着温度的降低，σ（ω）的起始能从Tco蓝移到反铁磁自旋有序温度TN，而当自旋有序温度低于TN时，σ（ω）的起始能又发生红移。这一结果表明，电荷间隙值作为CO态的序参量与自旋有序耦合。
The temperature (10–290 K) and magnetic-field (0–7 T) dependencies of optical spectra and their anisotropy have been investigated for a single crystal of Pr 1− x Ca x MnO 3 (x= 0.4), which undergoes a charge and orbital ordering transition at T co= 235 K. A clear anisotropic feature has been observed between the b-and c-axis polarized optical conductivity spectra [σ (ω)] at 10 K, reflecting a pattern of spatial charge and orbital ordering. The gap value of the charge-ordered (CO) state is estimated to be≈ 0.18 eV in the ground state. σ (ω) is drastically transformed from an anisotropic gaplike shape into an isotropic metallic band with a conspicuous spectral weight transfer over a wide photon energy region (0.05–3 eV) by application of a magnetic field of 7 T. As the magnetic field is decreased from 7 T, such metallic σ (ω) suddenly becomes gaplike again around 4.5 T, yet the anisotropy between both polarizations is decreased perhaps owing to orbital disordering. The temperature dependence of the b-axis polarized σ (ω) was investigated in a constant magnetic field of 7 T. As the temperature decreases, the onset energy of σ (ω) shows a blueshift from T co down to the antiferromagnetic spin ordering temperature T N, but shows a redshift once the spin ordering takes place below T N. This result indicates that the charge gap value as the order parameter of the CO state couples with the spin ordering.