Optical probe of masmatic and orbital states in magnetoresistive oxides

在较宽的温度范围内(10K&lt；T&lt；800K)研究了经过轨道和自旋有序的重结晶锰氧化物单晶的光学电导谱的偏振特性。我们发现，在伪立方锰氧化物中，由于轨道和各向异性自旋结构的定向有序性，实现了高度各向异性的电子结构。LaMnO_3是巨磁阻型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，在2 eV附近的最低激发峰表现出明显的各向异性，反映了轨道有序，但受A型自旋有序的影响最小。这表明！LaMnO_3带隙的强p-d电荷转移特性。在具有双交换铁磁片(x=0.55)和双交换铁磁链(x=0.7)的Nd_xt；1-x&gt；Sr_xMnO_3中，光导光谱相对于平行和垂直于铁磁片或链的光I极化是高度各向异性的。极化光谱的温度变化I表明，观察到的各向异性电荷动力学不仅来自长程自旋有序，而且来自轨道有序。研究了LaMnO_3单晶的拉曼散射；在轨道有序的基态下，在120 meV到170 meV之间出现了清晰的拉曼结构，这与轨道自由度的集体激发有关，称为“轨道波”。拉曼光谱的强度在Neel温度以上迅速降低。这表明，在没有静态自旋有序的情况下，轨道波的相干性受到抑制。拉曼谱带的峰值能量随着温度的升高而向轨道有序-无序温度1 T_∞方向降低。J-T失真的抑制以及轨道激发在T_∞附近的能隙可能是观测到的能量漂移的原因

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