调节过渡金属laser诱导的超快退磁
Tuning the laser-induced ultrafast demagnetization of transition metals
摘要：
从理论上研究了巡游铁磁体的超快退磁动力学与初始激光激发特性的关系。一个多体pd带哈密顿量被认为是考虑到杂交，库仑相互作用，自旋轨道相互作用和耦合到激光场在同一电子水平上。这样，在非绝热量子动力学和著名的巡游电子铁磁体平衡统计力学之间建立了富有成效的联系。在脉冲吸收过程中和之后的时间演化是完全确定的，通过执行数值Lanczos传播的小集群模型与适当的Ni参数。系统地改变最相关的激光参数，即，能量密度、波长、偏振和脉冲持续时间。结果表明，它们如何允许一个控制的总吸收能量，初始激发的光谱分布，以及随后的磁化动态。计算结果表明，这些参数的合理变化不影响UFD动态定性，只有一个很小的影响的时间尺度，其特征的初始退磁。相反，我们的模型预测，退磁的程度与激光激发的电子的平均数量或吸收的光子的平均数量相关。理论结果进行了讨论，通过与现有的实验进行比较。从一个基本的角度来看，超快退磁效应的鲁棒性在理论上证明，作为一种现象，反映了金属3d价电子的内在动力学。由此打开了定制超快退磁过程的功效的各种各样的集中的可能性。
英文摘要：
The ultrafast demagnetization (UFD) dynamics of itinerant ferromagnets is theoretically investigated as a function of the characteristics of the initial laser excitation. A many-body pd-band Hamiltonian is considered which takes into account hybridizations, Coulomb interactions, spin-orbit interactions and the coupling to the laser field on the same electronic level. In this way, a fruitful connection is established between the non-adiabatic quantum dynamics and the well-known equilibrium statistical mechanics of itinerant-electron ferromagnets. The time evolution during and after the pulse absorption is determined exactly by performing numerical Lanczos propagations on a small cluster model with parameters appropriate for Ni. The most relevant laser parameters, namely, the fluence, wave length, polarization and pulse duration are varied systematically. The results show how they allow one to control the total absorbed energy, the spectral distribution of the initial excitation, and the subsequent magnetization dynamics. The calculations show that reasonable changes in these parameters do not affect the UFD dynamics qualitatively and have only a minor influence on the time scale which characterizes the initial demagnetization. In contrast, our model predicts that the degree of demagnetization correlates well with the average number of electrons excited by the laser or average number of absorbed photons. The theoretical results are discussed by comparing them with available experiments. From a fundamental perspective, the robustness of the ultrafast demagnetization effect is theoretically demonstrated, as a phenomenon reflecting the intrinsic dynamics of the metallic 3d valence electrons. A wide variety of well-focused possibilities of tailoring the efficacy of the ultrafast demagnetization process is thereby opened.
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