2001 Fiscal Year Final Research Report Summary
Optical probe of masmatic and orbital states in magnetoresistive oxides
| Project/Area Number |
12440095
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
固体物性Ⅱ(磁性・金属・低温)
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| Research Institution | The University of Tokyo |
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Principal Investigator |
TOKURA Yoshinori Graduate School of Engineering, The University of Tokyo, Professor, 大学院・工学系研究科, 教授 (30143382)
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| Co-Investigator(Kenkyū-buntansha) |
TAGUCHI Yasujiro Graduate School of Engineering, The University of Tokyo, Research Associate, 大学院・工学系研究科, 助手 (70301132)
KIMURA Tsuyoshi Graduate School of Engineering, The University of Tokyo, Lecturer, 大学院・工学系研究科, 講師 (80323525)
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| Project Period (FY) |
2000 – 2001
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| Keywords | colossal magnete resistance / orbital order / colective John Teller distortion / optical conductivity / Raman scattering / orbital wave / change order |
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| Research Abstract |
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
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