Charge Ordering Due to Antiferromagnetic Correlation in Quarter-Filled Manganites

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Charge Ordering Due to Antiferromagnetic Correlation in Quarter-Filled Manganites

In the present paper, we study the zero-temperature phase diagram of the doped perovskite manganites at filling $x=0.5$ by the real-space Hartree-Fock approximation method. Our purpose is to resolve a controversial issue arising recently on the origin of the charge ordered phases in these systems. We find that the antiferromagnetic superexchange interaction between the localized spins plays the central role in producing the concerned phases. Our results confirm some speculations on this issue.     

相态对冰水混合云微观特性反演的影响研究

当云层的温度在－40℃~0℃之间时, 云层中会存在冰和水两种相态的云滴, 其散射特性与纯水云以及纯冰云特性有较大差异, 因此遥感反演混合相云层的微观和宏观物理特性具有重要的意义。本文采用冰水双层球模型模拟了冰水混合云中的云滴, 利用Mie理论计算了纯水、纯冰和冰水颗粒的单次散射特性, 分析了单次散射相函数, 不对称因子, 单次散射反照率等随着有效半径、相态、内外半径比等的变化特性。利用离散纵标法(DISORT)计算了水云和冰云对0.75 μm、2.16 μm和3.3 μm的双向反射函数, 讨论了利用纯水滴和纯冰滴反演冰水混合云滴的误差。分析结果表明, 利用0.75 μm和2.16 μm的太阳光反演冰水混合云的光学厚度和有效半径时, 光学厚度误差较大, 有效半径误差较小; 结合0.75 μm和3.3 μm的太阳光反演冰水混合云的光学厚度和有效半径时, 光学厚度误差较小, 有效半径误差较大, 其会高估其有效半径; 另外结合0.75 μm和3.3 μm这两个波长的反射函数反演冰水云的冰水混合比更为有效。     

Pressure-Induced Charge Delocalization in Manganites Studied by Means of Infrared Spectroscopy

Infrared absorption spectra of the La 0.75 Ca 0.25 MnO 3 manganite have been collected over the spectral range 600-1100 cm m 1 along isobaric paths at P =0,2,4, and 7 GPa in the 240-320 K temperature range. The pressure dependence of the insulator-to-metal transition temperature T IM has been determined from the spectra analysis and the first PT phase diagram of a manganite in the high-pressure regime is presented and discussed.     

Electron- and Hole-Doping Effects in Manganites Studied by X-Ray Absorption Spectroscopy

We investigate the electronic structures of hole-doped, La_0.7Ca_0.3MnO₃, and electron-doped, La_0.7Ce_0.3MnO₃, manganites by x-ray absorption near edge structure (XANES) spectroscopy at the O and Mn K-edges. While, the O K-edge XANES results indicate that Ca and Ce doping induce holes in O 2p derived states, the Mn K-edge XANES do not give any evidence for creation of the Mn⁴⁺ (or Mn²⁺) ions by Ca (or Ce) dopants. Such results further questions the validity of double exchange mechanism in understanding the anomalous properties of manganites.     

测自来水的电阻率

研究性学习是新课程教学改革的核心，它既是一种教学活动形式，也是一种学习活动方式。研究性主题不同于一般教学中的课外活动，而是强调学生动手动脑的实践过程。在这一过程中，在教师的指导下，学生从自然现象、社会、生活中遇到的问题出发，提出问题，并应用已有的知识设法解决问题，从中主动获取新知识，达到培养学生提出和解决问题能力的目的。本着这一教学思想我们组织开展了一次研究性学习课，将学习课全程总结浓缩成文，仅以此供同仁商榷。     

Resistivity of lanthanum and cerium

Conclusions The temperature dependence of the resistivity of cerium and lanthanum has been measured. The resistivity has been shown to decrease with increasing purity. The observed hysteresis in the temperature dependence of the resistivity of lanthanum is due to the martensite nature of the conversions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 107–109, June, 1970.     

Resistivity of inhomogeneous quantum wires

We study the effect of electron-electron interactions on the transport in an inhomogeneous quantum wire. We show that contrary to the well-known Luttinger liquid result, nonuniform interactions contribute substantially to the resistance of the wire. In the regime of weakly interacting electrons and moderately low temperatures we find a linear in T resistivity induced by the interactions. We then use the bosonization technique to generalize this result to the case of arbitrarily strong interactions.     

NMR of Colossal Magnetoresistive Manganites and Expanded Lattice Intermetallics at High Pressure

A survey is given of NMR studies of colossal magnetoresistive manganese perovskites (RE,Ca,Sr,Ba)MnO₃ (RE = rare earth) and the interstitially modified permanent magnet materials, RE₂Fe₁₇A _x (A = N,C,H) at ambient pressure and at applied pressures of up to 10 kbar. The different pressure behavior of the Mn hyperfine field found in the metallic and in the insulating manganite compounds is discussed and related to the micro- and macroscopic properties of these magnetically inhomogeneous materials. In the RE₂Fe₁₇A _x compounds a different pressure behavior of the hyperfine field at the RE sites with different number and type of interstitial atom neighbors is discussed in terms of local compressibility. The influence of the interstitial modification and the applied pressure on the RE hyperfine field and quadrupole splitting is analyzed and the impact of the chemical pressure and covalent effects on these quantities is compared. A comparison of the behavior of the magnetic state of the 4f electron shell with pressure and with interstitial modification is made and the relation to the magnetocrystalline anisotropy is discussed.     

Relationship of Polaron Exchange with Ferromagnetic and Insulator--Metal Transitions in Doped Manganites

We report the experiment results and data analyses based on a polaron exchange model for La0.7Ca0.3MnO3 and Pr0.7（Sr1-xCax）0.3MnO3 epitaxial thin films. In the polaron exchange model with an energy balance condition, critical temperature of Tc for stable ferromagnetic （FM） ordering should depend on △E as kBTc = E0 exp（-△E/kBTc）, where A E denotes the potential barrier for the exchange polarons to overcome. Using the small polaron hopping model, the resistivity peak temperature Tp is a function of the hopping energy Ehop. The dependence of Tp on Ehop is similar to the dependence of Tc on AE, which reveals that the polaron exchange relates to FM and insulator-metal transitions. The result indicates that the polaron exchange model is a simple way for describing the FM ordering, and is very helpful for understanding of complex doped manganites.     

Manganites at low temperatures and light doping: band approach and percolation

A tight-band model is employed for thee _2g orbitals in manganites. It is shown that a large intra-atomic Hund couplingJ _H and the resulting double-exchange mechanism lead to antiferromagnetic ordering along one of the cubic axes, stabilized by the cooperative Jahn-Teller effect, which further decreases the band energy, of the electrons. As a result, LaMnO₃ is a band insulator built of 2D ferromagnetic layers. The critical concentration (x _c ?0.16) for the onset of ferromagnetic and metallic behavior at low temperatures in La_1?x Sr _x MnO₃ and the phase transition are treated in a percolation approach.     

Resistivity exponent of two-dimensional lattice animals

We calculate the average resistanceR(L) of lattice animals spanningL×L cells on the square lattice using exact and Monte Carlo methods. The dynamical resistivity exponent, defined asR(L) L , is found to be =1.36±0.07. This contradicts the Alexander-Orbach conjecture, which predicts 0.8. Our value for differs from earlier measurements of this quantity by other methods yielding =1.17±0.05 and 1.22±0.08 by Havlin et al.On leave from the Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing, China.     

Orbital Ordering Effect on Magnetic Resonance Spectra in Charge-Ordered and Frustrated Manganites

Physics of the Solid State - A theoretical study of the interrelation of the crystal structure, charge, orbital, and magnetic subsystems in R1 – xAxMnO3 charge-ordered...     

Effect of Lattice Distortion on Charge Order in Manganites at Doping x = 0.5

In the present paper, we continue our investigation on the antiferromagnetic origin of the charge order observed in the half-doped manganese. By introducing a Su-Schrieffer-Heeger (SSH) type of perturbation interaction to the double-exchange Hamiltonian, we calculate again its ground-state phase diagram at filling x=0.5 by the unrestricted real-space Hartree-Fock approximation method. We find that, as the SSH electron-phonon interaction increases, the charge order parameter decreases to zero rapidly but the CE-type antiferromagnetic order becomes more stable. In other words, the charge order is much more fragile than the CE-type or the Neel-type antiferromagnetic orders under the electron-phonon perturbation. These results support the proposed theory in the recent publications that the charge order in these systems is induced by the antiferromagnetic correlations.