Phonon study by neutron scattering on the orbital-order of the intermediate-spin-state of Co^<3+> and the spin-state transition

LaCoO_3中的Co^&lt；3+&gt；离子在100K附近发生了从低自旋(LS，S=0)到中自旋(IS，S=1)的自旋态跃迁。本研究的目的是通过用中子散射观察到的声子来阐明Jahn-Tella活性Co^&lt；3+&gt；在IS态的轨道顺序。2005年完成了中子测量，并将研究扩展到超声波测量，以研究与自旋态转变相关的晶格反常。在中子研究中，观察到了沿[δ，δ，δ]的声学(A)模和两个横向光学(TO)模。我们发现TO模在整个与自旋-态转变相关的布里渊区都表现出显著的软化。在伪立方倒易晶格的R点处软化最为明显，其结果与拉曼光谱中观察到的E_g模一致。相反，在LA和TA模式下的声学声子都没有显示出ANOM…在超声测量中，观察到了与100K左右的自旋态转变有关的LA声速的显著减慢，这种减速被解释为自旋态布居对声压的响应。在150K以上，声速取决于声频。根据德拜模型计算的晶格驰豫速率具有活化型温度依赖性，尝试频率为187×10~(-6)S^~(-1)~(-1)，能量为11 meV。这种缓慢的晶格弛豫归因于自旋态布居对声音的“慢”响应，在该温度区，由于对轨道有序起伏速率的抑制，建立了IS态Co原子的轨道有序。综上所述，本研究首次揭示了处于IS态的Co原子的轨道涨落，以及自旋态和晶格的强耦合。较少

Co^<3+> ions in LaCoO_3 exhibit spin-state transition from the low-spin (LS, S=0) to an intermediate-spin (IS, S=1) state around 100 K. The purpose of the present study, described in the proposal, was to clarify the orbital-order of the Jahn-Tellar active Co^<3+> in the IS state through phonons observed by neutron scattering. The neutron measurement was completed in 2005, and the study was extended to the ultrasonic measurement to investigate lattice anomalies associated with the spin-state transition.In the neutron study, the acoustic (A) and two transverse optical (TO) modes along [δ,δ,δ] were observed. We found that the TO modes show remarkable softening over the whole Brillouin zone associated with the spin-state transition. The softening is most pronounced at the R-point of the pseudo-cubic reciprocal lattice, and the result at the R-point is consistent with the E_g modes observed in the Raman spectroscopy. In contrast, the acoustic phonons both in the LA and TA modes show no anom … More alous softening associated with the spin-state transition.In the ultrasonic measurements, a remarkable slowing down of the LA sound velocity was observed associated with the spin-state transition around 100 K. The slowing down is interpreted as due to the response of the spin-state population to the sound pressure. Above 150 K, the sound velocity depends on the sound frequency. The relaxation rate of the lattice evaluated based on the Debye model has an activation-type temperature dependence with the attempt frequency of 187x10^6 s^<-1> and the energy of 11 meV. This slow lattice relaxation is attributed to the "slow" response of the spin-state population to the sound in the temperature region where the orbital-order of the IS state Co atoms is established since the response is suppressed to the rate of the orbital-order fluctuation.In summary, the present study first reveals the orbital fluctuation of the Co atoms in IS state, and also the strong coupling of the spin-state and the lattice. Less