Magnetization reversal within Dzyaloshinskii—Moriya interaction under on-site Coulomb interaction in BiCrO3

First-principals calculations show that magnetization reversal is accompanied by the opposite sense of rotation of the neighboring oxygen octahedra along the [1 1 1] direction which is called the antiferrodistortive displacement in BiCrO3. The coupling between magnetization and antiferrodistortive distortion is mainly caused by Dzyaloshinskii- Moriya interaction which is driven by the eg-eg states antiferromagnetic interaction in Cr-3d. A critical value of on-site Coulomb interaction prohibiting the Dzyaloshinskii-Moriya interaction and thus the magnetization reversal is found to be 1.3 eV.     

Ab initio prediction on ferrotoroidic and electronic properties of olivine Li4MnFeCoNiP4O16

First-principles calculations predict that olivine Li4MnFeCoNiP4O16 has a large toroidal moment and ferrimagnetic configuration with a magnetic moment of 1.99μB per formula unit. Density functional theory plus U (DFT+U) shows an indirect band gap of 0.65 eV in this hypothetical material. The band gap is not simply related to the electronic conductivity when it is used as cathode material in rechargeable Li-ion batteries. Based on the orbital-resolved density of states for the transition-metal ions in the hypothetical material, Co, Ni and Mn are in the high-spin configuration while Fe is in the low-spin configuration, which leads to a large resulting toroidal moment deriving from the Co and Ni ions. The spin configuration of the transition-metal ions in the system breaks the space-and time-inversion symmetry and leads to the magnetoelectric property simultaneously. The ferrotoroidic domain, the fourth form of ferroic, is observed in this new material, as in the case of LiCoPO4 reported recently.     

石墨烯在钙钛矿太阳能电池中的应用

正近年来,基于有机-无机杂化钙钛矿材料的太阳能电池(简称钙钛矿太阳能电池,perovskite solar cells)因为具备优良的光电转换性能、成本低廉且易于规模化生产等特点,而在国际上备受关注。2009年CH3NH3Pb I3首次被用作染料敏化太阳能电池中的光吸收剂,获得了3.8%的光电转换效率,这掀起了钙钛矿太阳能电池的研究热潮。短短几年内,钙钛矿太阳能电池的光电效率得到迅速提升。最近美国国家能源部可再生能源实验室(NREL)认证的     

Magnetization reversal within Dzyaloshinskii Moriya interaction under on-site Coulomb interaction in BiCrO₃

First-principals calculations show that magnetization reversal is accompanied by the opposite sense of rotation of the neighboring oxygen octahedra along the [1 1 1] direction which is called the antiferrodistortive displacement in BiCrO3.The coupling between magnetization and antiferrodistortive distortion is mainly caused by Dzyaloshinskii-Moriya interaction which is driven by the eg-eg states antiferromagnetic interaction in Cr-3d.A critical value of on-site Coulomb interaction prohibiting the Dzyaloshinskii-Moriya interaction and thus the magnetization reversal is found to be 1.3eV.     

Ab initio study on magnetoelectric and electronic properties in Pb2TiVO6

First principles calculations within the projected augmented-wave (PAW) method, using the local spin density approximation plus U (LSDA+U) scheme, show that the tetragonal Pb2TiVO6 is a potential multiferroic material with antiferromagnetic (AFM) spin configuration. It has a magnetic moment of 1 μB in a one unit cell originating from the non-bonding orbital dxy in a majority spin channel and a band gap of 1.45 eV with proper U. The large BEC (Born effective charge) of Pb and Ti shows that the stereochemical activity of Pb and Ti may provide the possibility of switchable paths for the ferroelectricity in this hypothetical material. The insulating property and the lower resistivity in the recent prepared PbVO3 can be significantly improved by adopting the Ti.     

Ferroelectric Ba0.75Sr0.25TiO3 tunable charge transfer in perovskite devices

Interfacial charge recombination is a main issue causing the efficiency loss of the perovskite solar cells (PSCs). Here, ferroelectric Ba_0.75Sr_0.25TiO₃ (BST) is introduced as a polarization tunable layer to promote the interfacial charge transfer of the PSCs. The coexistence of ferroelectric polarization and charge carriers in BST is confirmed by density functional theory (DFT) calculations. Experimental characterization demonstrates the polarization reversal and the existence of domain in BST film. The BST film conductivity is tested as 2.98×10^-4 S/cm, which is comparable to the TiO₂ being used as the electron transporting layer (ETL) in PSCs. The calculations results prove that BST can be introduced into the PSCs and the interfacial charge transfer can be tuned by ferroelectric polarization. Thus, we fabricated the BST-based PSCs with a champion power conversion efficiency (PCE) of 19.05% after poling.     

宽光谱全正色散锁模掺镱光纤激光器

介绍了一种全正色散宽光谱被动锁模掺镱光纤激光器,利用非线性偏振旋转技术实现全正色散掺镱光纤激光器的被动锁模.当泵浦功率输出为500mW时,激光脉冲输出功率大于139mW,重复频率约为28.1MHz,脉冲宽度为3.8ps.为了进一步研究全正色散光纤激光器的宽光谱输出特性,在腔内熔接50m单模光纤,同时去除双折射滤波片,在泵浦功率为500mW时,观察到稳定锁模单脉冲耗散孤子,光谱范围为1 005~1 140nm,输出激光脉冲最大平均功率为90mW,重复频率为3.58MHz,脉冲宽度为519ps.     

Coupling of Magnetization and Structural Distortions in Multiferroic BiFeO3： an Ab Initio Density Functional Theory Study

The coupling between magnetism and structural distortions in BiFeO3 （BFO） is investigated using density functional theory by considering the spin-orbit effect. Computational results show that the resulting magnetization M is rotated by reversal of sense of rotation of the oxygen octahedra in the double cell. The resulting magnetization is determined by the antiferrodistortive （AFD） distortions and ferroelectric （FE） displacements. This work clarifies the previous view that magnetism is only coupled with, and determined by, FE displacements. The excellent ferroelectricity is attributed significantly to the anomaly of Born effective charge of Bi, which is caused by the stereochemically active long pair of Bi 6s.     

Ab initio study on phase transition and magnetism of BiFeO3 under pressure

In this paper the first-principles calculations within local spin density approximation (LSDA)+U show that BiFeO3 experiences a mixed phase state with P4mm structure being the intermediate phase before the pressure of phase transition is reached. The critical pressure for the insulator-metal transition (IMT) is found to be about 50 GPa. A pressure induced crossover of high-spin states and low-spin states is observed close to the IMT pressure in R3c structure. The LSDA+U calculations account well for the mechanism of the IMT and crossover of spin states predicted in recent experiment (Ref.[1]).     

Electronic structures and magnetoelectric properties of tetragonal BaFeO3: an ab initio density functional theory study

First-principles calculations have been performed to investigate the ground state electronic properties of BaFeO₃ (BFO). Local spin density approximation (LSDA) plus U (LSDA+U) treatment modified the metallic behaviour to insulated one with a band gap of 4.12eV. The spontaneous polarization was found to be 89.3\muC/cm² with Berry phase scheme in terms of the modern theory of polarization. Fe-3d e_g were split into two singlet states (d_z² and d _x²-y²}), and Fe-3d t_2g were split into one doublet states(d_xz and d_yz) and one singlet states(d_xy) after Fe and O displaced along the c axis. Meanwhile the occupation numbers of d_z², d_yz, d_xz and O_T p_z (on the top of Fe) were increased at the expense of those in xy plane. Our results showed that it was the sensitivity of hybridization to ferroelectric distortions, not just the total change of hybridization, that produced the possibility of ferroelectricity. Moreover, the increasing occupation numbers of O_T p_z and Fe d_z² favoured the 180^o coupling between Fe-3d e_g and Fe-3d t_2g, leading to ferromagnetic ordering, which has been confirmed by the increase of magnetic moment by 0.13μ_B per formula unit in the polarized direction. Hence, the magnetization can be altered by the reversal of external electric field.