钙钛矿锰氧化物中电荷有序和相分离现象的超声研究
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摘要
具有钙钛矿结构的过渡金属氧化物是一种典型的强关联体系，该体系中的电子、自旋、轨道与晶格声子之间存在着强烈的相互作用，并因此导致了一系列奇特的物理现象，如高温超导电性、绝缘体—金属转变、巨磁电阻效应、电荷有序和相分离现象等。这些物理现象的发现不但极大地丰富了凝聚态物理的内涵，同时在磁存储、磁探测和磁传感器等领域有着潜在的应用前景。近年来，钙钛矿锰氧化物以其超大巨磁阻(CMR)效应而引起了研究者的广泛关注。人们先后用双交换模型、极化子模型和电．声子相互作用模型等来解释其物理本质，但是随着研究的不断深入，在该体系中还发现了电荷、轨道有序等现象，且相互之间存在强烈的耦合，导致体系中多相共存。这些物理现象在现有的理论框架下很难完全理解，要建立一个完整的理论模型还需要对该体系中的各种物理现象进行系统深入研究，彻底弄清电荷、自旋、轨道等之间的关联和相互作用规律。
超声声速与衰减对固体材料中的各种相变非常敏感，一直是研究各种磁性相变、结构相变、自旋涨落以及其它与电-声子耦合有关的电荷有序和Jahn-Teller效应等物理现象的一种有效方法。本论文系统地测量了Nd_(1-x)Ca_xMnO_3及其A、B位掺杂体系在各种电磁和结构相变附近的超声声速温度谱，发现了一系列的超声异常现象。结合电磁测试结果，对该体系中的各种交互作用进行了分析，给出了电-声子相互作用和自旋-声子相互作用的实验证据，并进一步证明了Jahn-Teller电-声子相互作用是产生电荷／轨道有序的重要原因之一。作为对比研究，我们还测量了层状钴氧化物YBaCo_2O_(5+δ)在低温下的超声声速和衰减的温度谱，并对该体系中存在的超声异常现象进行了深入分析。主要研究内容如下：
1．Nd_(1-x)Ca_xMnO_3中的电荷有序态与Jahn-Teller效应
高掺杂Nd_(1-x)Ca_xMnO_3的电荷有序相变温度和长程反铁磁有序温度分别处在不同的温区，因此是研究电荷有序态的物理机制以及电—声子、自旋—声子耦合作用的理想体系。我们系统地测量了Nd_(1-x)Ca_xMnO_3(x=0.4～0.8)低温下(15～300K)的超声声速温度谱，结合电磁性质的测量，研究了该体系中电荷有序态的稳定性和Jahn-Teller耦合能随Ca~(2+)掺杂量的变化关系，并探讨了Jahn-Teller晶格畸变
The perovskite type transition-metal oxides are the typical strongly correlated systems due to their strong coupling among charge, spin, orbit and lattice degree of freedom. Many related physical phenomena such as super-conducting, metal-insulator transition, charge and orbital ordering, and phase separation have been found in these compounds. The divalent alkaline-earth ions doped perovskite type manganites have recently received much attention due to their 'colossal magnetoresistance' (CMR) effect. Recent studies for manganites have revealed that the CMR effect is attributed not only to the double-exchange interaction but also to other interactions and competitions, such as charge-orbital ordering, electron-phonon coupling, and phase coexistence. However, the microscopic origin of these intriguing phenomena in manganites is still a widely studied and yet unresolved problem. So it is necessary to further investigate the correlations between the charge, spin, and orbit in these compounds.
In this dissertation, we systematically measured the ultrasonic sound velocity and attenuation properties near electronic, magnetic and structural phase transitions in Nd_(1-x)Ca_xMnO_3, Nd_(0.5)Ca_(0.5)Mn_(1-x)M_xO_3 (M=Co,Al) and La_(0.5-x)Nd_xCa_(0.5)MnO_3. A series of dramatic ultrasonic anomalies were observed in these compounds. Combined with the transport and magnetic measurements, we found that the charge ordering in Nd_(1-x)Ca_xMnO_3 is dominated by Jahn-Teller effect. In addition, we made detailed analyses of the correlated interactions in layered cobalt oxides YBaCo_2O_(5+δ) and gave experimental evidence of the electron-phonon coupling the compound. 1. Studies of the charge ordering and Jahn-Teller effect in Nd_(1-x)Ca_xMnO_3
The stability of CO state and Jahn-Teller coupling energy Ejt as a function of Ca~(2+) concentration have been investigated by measuring the ultrasonic velocity,
magnetization and resistivity for Nd_(1-x)Ca_xMnO_3(x=0.4~0.8). It was found that the increases with the increasing of Ca~(2+) concentration x, and reaches the maximum at around x=0.6, which is similar in characteristics to the Tco. The results imply that the
超声声速与衰减对固体材料中的各种相变非常敏感，一直是研究各种磁性相变、结构相变、自旋涨落以及其它与电-声子耦合有关的电荷有序和Jahn-Teller效应等物理现象的一种有效方法。本论文系统地测量了Nd_(1-x)Ca_xMnO_3及其A、B位掺杂体系在各种电磁和结构相变附近的超声声速温度谱，发现了一系列的超声异常现象。结合电磁测试结果，对该体系中的各种交互作用进行了分析，给出了电-声子相互作用和自旋-声子相互作用的实验证据，并进一步证明了Jahn-Teller电-声子相互作用是产生电荷／轨道有序的重要原因之一。作为对比研究，我们还测量了层状钴氧化物YBaCo_2O_(5+δ)在低温下的超声声速和衰减的温度谱，并对该体系中存在的超声异常现象进行了深入分析。主要研究内容如下：
1．Nd_(1-x)Ca_xMnO_3中的电荷有序态与Jahn-Teller效应
高掺杂Nd_(1-x)Ca_xMnO_3的电荷有序相变温度和长程反铁磁有序温度分别处在不同的温区，因此是研究电荷有序态的物理机制以及电—声子、自旋—声子耦合作用的理想体系。我们系统地测量了Nd_(1-x)Ca_xMnO_3(x=0.4～0.8)低温下(15～300K)的超声声速温度谱，结合电磁性质的测量，研究了该体系中电荷有序态的稳定性和Jahn-Teller耦合能随Ca~(2+)掺杂量的变化关系，并探讨了Jahn-Teller晶格畸变
The perovskite type transition-metal oxides are the typical strongly correlated systems due to their strong coupling among charge, spin, orbit and lattice degree of freedom. Many related physical phenomena such as super-conducting, metal-insulator transition, charge and orbital ordering, and phase separation have been found in these compounds. The divalent alkaline-earth ions doped perovskite type manganites have recently received much attention due to their 'colossal magnetoresistance' (CMR) effect. Recent studies for manganites have revealed that the CMR effect is attributed not only to the double-exchange interaction but also to other interactions and competitions, such as charge-orbital ordering, electron-phonon coupling, and phase coexistence. However, the microscopic origin of these intriguing phenomena in manganites is still a widely studied and yet unresolved problem. So it is necessary to further investigate the correlations between the charge, spin, and orbit in these compounds.
In this dissertation, we systematically measured the ultrasonic sound velocity and attenuation properties near electronic, magnetic and structural phase transitions in Nd_(1-x)Ca_xMnO_3, Nd_(0.5)Ca_(0.5)Mn_(1-x)M_xO_3 (M=Co,Al) and La_(0.5-x)Nd_xCa_(0.5)MnO_3. A series of dramatic ultrasonic anomalies were observed in these compounds. Combined with the transport and magnetic measurements, we found that the charge ordering in Nd_(1-x)Ca_xMnO_3 is dominated by Jahn-Teller effect. In addition, we made detailed analyses of the correlated interactions in layered cobalt oxides YBaCo_2O_(5+δ) and gave experimental evidence of the electron-phonon coupling the compound. 1. Studies of the charge ordering and Jahn-Teller effect in Nd_(1-x)Ca_xMnO_3
The stability of CO state and Jahn-Teller coupling energy Ejt as a function of Ca~(2+) concentration have been investigated by measuring the ultrasonic velocity,
magnetization and resistivity for Nd_(1-x)Ca_xMnO_3(x=0.4~0.8). It was found that the increases with the increasing of Ca~(2+) concentration x, and reaches the maximum at around x=0.6, which is similar in characteristics to the Tco. The results imply that the
引文
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