Ab initio study of electrons and phonons in multiferroic BiFeO3

多铁性材料表现出铁磁性(或反铁磁性)、铁电性(或反铁电性)和铁弹性两种或两种以上的性质。这种材料在外加磁场时会产生电极化，在外加电场时会产生磁化作用。钙钛矿型BiFeO_3是唯一一种在室温下表现出多铁性的材料。BiFeO_3是一种具有R3c空间群的菱形扭曲钙钛矿结构的单晶，具有铁电性，在643K以下具有反铁磁行为。这种磁电材料在技术上有应用，如霍尔探针、超高频自旋波振荡器，也可用于信息存储、新兴的自旋电子学和传感器领域。最近，人们对BiFeO_3的结构、电子和磁性进行了一些研究工作。在它的单晶三方相中，它只显示出很小的铁电偏振值。然而，具有(001)取向的伪四方结构的外延生长的薄膜显示出更高的偏振数量级。用偏振拉曼散射方法研究了四方相的某些声子模。然而，对于声子的动力学性质，例如模式分配和全声子色散关系，还没有出现系统的研究。由于固体的各种物理性质取决于它们的晶格动力学性质，因此对这两个结构相(三方相和四方相)的声子进行系统的研究是很重要的。由于局域Fe3d电子态的存在，进一步研究局域态之间的自旋极化和库仑排斥等电子效应引起的声子模的变化是很有必要的。本研究的目的是对多铁晶体BiFO_3中的电子结构和声子模进行从头算理论和计算研究。我们的工作有望帮助实验者分析他们的拉曼散射测量结果。

Multiferroic materials exhibit two or more properties of ferromagnetism (or antiferromagnetism), ferroelectricity (or antiferroelectricity) and ferroelasticity. Such materials can develop electric polarisation when subjected to an external magnetic field, and can develop magnetisation when subjected to an external electric field. The perovskite BiFeO3 is known to be the only material that exhibits multiferroism at room temperature. In single crystal form itis rhombohedrally distorted perovskite with space group R3c, and is ferroelectric and exhibits antiferromagnetic behaviour up to 643 K. Such 'magneto-electric' materials have technological applications as Hall probes, UHF spin-wave oscillators, and can also be employed in potential applications in information storage, in the emerging field of spintronics, and sensors.Several works have recently been done on the study of the structural, electronic and magnetic properties of BiFeO3. In its single crystal rhombohedral phase it shows only small values of ferroelectric polarisation. However, epitaxially grown films with (001)-oriented pseudotetragonal structure shows an order of magnitude higher polarisation. Polarlized Raman scattering method has been employed to examine some phonon modes for the tetragonal phase. However, no systematic studied have emerged on dynamical properties, e.g. mode assignment and full phonon dispersion relations. As a wide range ofphysical properties of solids depend on their lattice dynamical properties, it is important to undertake a systematic study of phonons for the two structural phases (rhombohedral and tetragonal). Due to the presence of localised Fe 3d electronic states, it is further desirable to investigate changes in phonon modes due to electronic effects such as spin polarisation and strong Coulomb repulsion between the localised states.The proposed research is aimed at carrying out ab-inito theoretical and computational studies of the electronic structure and phonon modes in the multiferroic crystal BiFO3. Our work is expected to help experimentalists analyse their Raman scattering measurements.