Optical probe of masmatic and orbital states in magnetoresistive oxides

在宽的温度范围内（10 K <T<800 K），研究了经历轨道和自旋有序化的锰氧化物单晶的非孪晶电导谱的偏振依赖性。我们发现，高度各向异性的电子结构实现由于定向顺序，在赝立方锰氧化物的轨道和各向异性的自旋结构。LaMnO_3是巨磁电阻锰氧化物的母体化合物，是具有d电子轨道有序基态的典型化合物之一。在LaMnO_3中，最低激发态在2 eV附近表现出明显的各向异性，反映了轨道有序性，但受A型自旋有序性的影响最小。这表明！LaMnO_3的电荷带隙具有较强的p-d电荷转移特性。在<1-x>具有双交换铁磁片（x=0.55）和链（x=0.7）的Nd_ Sr_xMnO_3中，电导率谱相对于平行和垂直于铁磁片或链的光I偏振具有高度各向异性。极化谱的温度变化I表明，所观察到的各向异性电荷动力学不仅来自于长程自旋序，而且来自于轨道序。在轨道有序的基态，在120 meV和170 meV之间出现清晰的拉曼结构，这与被称为“轨道波”的轨道自由度的集体激发有关。“高于尼尔温度时，拉曼谱带的强度迅速下降。这表明，在没有静态自旋序的情况下，轨道波的相干性被抑制。拉曼峰的能量随温度的升高而降低，接近轨道有序无序温度1 T_∞。Jahn-Teller畸变的抑制以及T_∞附近轨道激发的能隙可能是导致能量移动的原因

The polarization dependence of the optical conductivity spectra have been investigated over a wide temperature range (10K<T<800 K) for a detwinned single crystal of manganites that undergoes the orbital and spin ordering. We revealed that the highly anisotropic electronic structures are realized due to the directional order, of the orbital and anisotropic spin structures in the pseudo-cubic manganites. LaMnO_3, a parent compound of the colossal magnetoresistive manganites, is one of the typical compounds with the d-electron orbital ordered ground state. In LaMnO_3, the lowest-lying excitation around 2 eV shows a clear anisptropy reflecting the orbital ordering, but being least affected by the A-type spin ordering. This indicate! the strong p-d charge-transfer character for the charge gap of LaMnO_3. In Nd_<1-x>Sr_xMnO_3 with double-exchange ferromagnetic sheets (x=0.55) and chains (x=0.7), the optical conductivity spectra are highly anisotropic with respect to light I polarizations parallel and perpendicular to the ferromagnetic sheets or chains. Temperature variation I of the polarized spectra indicates that the observed anisotropic charge dynamics arises not only from the long-range spin order but also from the orbital order.Raman scattering was investigated for a single crystal of LaMnO_3; In the orbitally ordered ground state, a clear Raman structure between 120 meV and 170 meV appears, which is relevant to the collective excitation of the orbital degree of freedom called "orbital wave." The' intensity of the Raman bands rapidly decreases above the Neel temperature. This indicates that the coherence of the orbital wave is suppressed in the absence of "he static spin order. The peak energy of the Raman bands is decreased with increasing temperature toward the orbital order-disorder temperature 1 T_∞. The suppression of the Jahn-Teller distortion as well as the energy gap of the orbital excitation near T_∞ may be responsible for the observed energy shift