铁-苯磁性团簇上的超快自旋操控的第一性原理研究

【作者】 杜红；

【导师】 金蔚；

【作者基本信息】 陕西师范大学 ， 凝聚态物理， 2019， 硕士

【摘要】 为了满足人们对高密度存储和高速信息处理能力日益增长的需求,寻找合适的纳米尺度磁性材料来实现超快自旋操控成了近年来研究热点之一。过渡金属和苯环形成的复合物由于其独特的物理和化学性质成为了大家公认的最具潜力的分子磁性材料。本文采用第一性原理计算方法对FeinBZn(m,n≤2)磁性团簇的构型、红外光谱和电子结构进行了研究,在此基础上探索了激光和磁场效应对自旋动力学的影响,得到了一些规律性认识。论文主要内容和结果如下:首先,用Hartree-Fock方法对四个铁-苯磁性团簇进行几何优化,得到相应能量最低的几何构型。为确保所得结构的稳定性,进一步计算了各自的红外频率以保证所得优化构型无虚频。结果表明:所得四个铁-苯磁性团簇的基态均为三重态,且均呈现半三明治或三明治构型。其与苯环相关的振动模式及对应频率与实验数据基本保持一致,其中,苯环呼吸模式和C-C-C摇摆模式发生了一定程度的红移,非平面C-H摇摆和C-H伸缩发生了一定程度的蓝移。然后,利用对称性自适应组态相互作用方法计算多体电子更精确的基态和激发态。在此基础上考虑自旋轨道耦合和加入静磁场,在激光脉冲的影响下,探索铁-苯磁性团簇上基于∧进程的超快自旋动力学。结果表明:金属Fe原子数量的增加会引入较强的3d电子关联,造成d电子态能级数目的增多;苯环分子数量的增加会增强苯环的诱导效应,造成3d-π电子间较强的相互作用,使得电荷转移态增多。团簇能级分布的不同特点造成相应的自旋动力学行为有所不同。对于在四个结构上所实现的可逆超快自旋翻转动力学,单苯环Fe团簇所需激光能量会比相应双苯环团簇激光能量低。对于在两磁中心团簇Fe2Bz和Fe2Bz2上实现的自旋转移,前者伴随着与自旋转移方向相反的电荷转移,后者表现出可逆特点。为了更好地指导激光诱导自旋动力学在实验上的实现,本论文分别研究了激光半高宽和磁场强度对所得自旋动力学的影响。研究结果表明:当激光偏离最优半高宽时,自旋动力学的保真度在附近均有一定程度的降低,总体来说,自旋翻转对激光半高宽变化的容忍度比自旋转移高,实验上较易实现。另外,在激光半高宽为最优半高宽的整数倍时,其翻转动力学保真度呈现出和二能级体系在共振拉比频率情况下相似的能级交替特点。磁场强度效应研究结果表明:施加的磁场强度只有在合适的范围内才能够实现保真度较高的自旋动力学。过小或过大都会使末态转移率过低,前者来自能态的不可区分性,后者由于Paschen-Back效应所带来的体系的自旋轨道耦合的相对弱化和能级交错。基于不同铁-苯团簇所实现的自旋动力学对磁场强度变化的敏感性不同的事实,本文提出了结合线性梯度磁场来获得器件的磁性空间分辨率,进而达到精确存储和读写的目的。本文的第一性原理研究可望为铁-苯团簇上由激光诱导的超快操纵的实现及相关自旋电子器件的设计提供理论指导和思路,进一步推动其在未来高密度磁存储及量子计算机的应用和发展。更多还原

【Abstract】 In order to meet the increasing demand for high-density storage and high-speed information processing capabilities,searching suitable nano-scale magnetic materials to achieve ultrafast spin manipulation has become one of the research hotspots in recent years.The transition-metal benzene complexes are considered to be one of the most promising molecular magnet material species due to their unique physical and chemical properties.In this paper,by using the first-principles calculation method,we study the geometric conformation,infrared spectra and electronic structures of magnetic clusters FemBzn(m,n≤2),based on which the effects of laser and magnetic field on spin dynamics are further explored.The main contents and results are as follows:Firstly,we perform the Hartree-Fock calculation to optimize the four iron-benzene magnetic clusters.To ensure the stabilities of the obtained structures,frequencies calculations are further performed.The results show that the ground states of the four clusters are all triplet with(half-)sandwich configurations.The calculated benzene-related vibrational frequencies show a decent agreement with experiment,with the modes of the benzene ring breathing and C-C-C bend being red shifted,and the plane C-H bend and C-H stretching modes being blue shifted.Then,the more accurate ground and excited many-body states of the systems are obtained by applying the symmetry-adapted cluster configuration interaction method.After including the spin-orbit coupling and adding a static magnetic field,the∧-process-based ultrafast spin dynamics on iron-benzene magnetic clusters are explored under the influence of the well tailored laser pulses.It’s found that an additional Fe atom can introduce stronger 3d electron correlations and produce more d states,while an additional benzene ring can give rise to stronger 3d-π electron interactions due to the inductive-I effects and result in more charge transfer states.The different energy distributions of the clusters will cause different spin dynamics features.For the ultrafast spin-flip scenarios,the laser energies for driving spin flip in clusters Fe1.2Bz are lower than those for the corresponding Fe1,2Bz2 ones.For the two ultrafast spin-transfer scenarios obtained in clusters Fe2Bz and Fe2Bz2,one is accompanied with charge transfer and the other is reversible.Based on these scenarios,the effects of full width of half maximum(FWHM)of the laser pulse and the magnetic field strength on them are investigated for the purpose of guiding future experimental implementations.We find that the derivation from the resonant FWHM within a moderate range always causes a decrease of the fidelity.In general,spin-flip scenarios have larger tolerance values than spin transfer with respect to the laser FWHM,indicating the fact that they are easier to be realized in experiment.In addition,when the FWHM is an integer multiple of the resonant FWHM,the fidelity of the spin flip behaves similarly to the oscillation of the initial and final states of the two-level system under the resonant Rabi frequency.The study of magnetic field effect shows that high fidelity of the spin dynamics can be achieved only within a suitable range of magnetic field strength.Too small and too large values always bring about low fidelity,the former is due to the indistinguishability of the states,and the latter is due to the Paschen-Back effect,which leads to the weakening of the spin-orbit coupling and level crossing.Based on the fact that different spin dynamics show different sensitivity to the magnetic field strength,we could apply this sensitivity to obtain the lateral resolution of a certain sample device and achieve the purpose of accurate storage and writing.The study of the laser-induced ultrafast spin dynamics in FemBzn(m,n≤2)clusters could provide theoretical guidance and valuable reference for the future experimental realization and spintronic device design,and promote the related applications and development in high-density storage and quantum calculating.更多还原