Investigation of directional THz spin currents in topological surface states

拓扑表面态定向太赫兹自旋电流的研究

• 批准号: 238002712
• 负责人: Professor Dr. Christian Heiliger
• 金额: --
• 依托单位: Institut für Experimentalphysik
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2013
• 资助国家: 德国
• 起止时间: 2012-12-31 至 2017-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/238002712?language=en
• 关键词: Investigation; directional; THz; spin; currents

Topological insulators (TIs) exhibit topologically protected spin-polarized surface states that offer a high application potential for spin electronic devices, preferably working at frequencies reaching the elusive terahertz (THz) window. In our project, we have studied the coupling between surface/bulk electrons, spins and phonons by ultrafast excitation and probing of model 3D TIs such as Bi2Se3. We have predominantly used optical laser pulses (~1.5 eV photon energy) of suitable polarization to selectively excite surface electrons. To probe the resulting spin, charge and transport dynamics, we have developed novel measurement schemes including magneto-optic spin and terahertz (THz) conductivity and current probes. We were able to reveal important features of the ultrafast relaxation of optically spin-polarized electrons as well as novel types of ultrafast surface photocurrents.While the first funding period was focused on optical excitation and single-material samples, we want move to (i) substantially more tailored and selective excitation and (ii) hybrid TI structures in the second period. Goal (i) is achieved by using lower pump photon energies (~1 to 400 meV) in the THz to mid-infrared spectral range. In 3D model TIs such as Bi2Se3, these energies are perfectly suited to directly and selectively drive interband transitions between Dirac states. As these surface states are located in the TI bulk band gap, we expect substantially larger spin-polarization lifetimes, surface current amplitudes and exclusive insights into the intraband surface state dynamics. The dynamics of spins, charges and transport are monitored by the probes developed in the first funding period. Experiments will be accompanied by ab initio electronic-structure and transport theory. Additional control of and insight into the very early ultrafast spin dynamics is achieved by laser and THz pulses of tunable and stabilized carrier-envelope phase. Concerning goal (ii), we will study ferromagnet/TI hybrid structures in terms of the ultrafast dynamics of their excited states. We will reveal the influence of the topological surface state on dynamics of the ferromagnetic order (e.g via spin-transfer torque and optical spin torque) and the reverse effect, the ultrafast spin injection from the ferromagnet into the TI surface and bulk. Using our newly developed spin, conductivity and transport probes, we will study the dynamics of these processes in real time. We expect to gain new insights into the impact of the ferromagnet onto the TI dynamics as well as an estimate of the TI spin Hall angle. Importantly, first applications will emerge: the generation of spin torque in the adjacent ferromagnet induced by TI spin currents and spin-to-charge conversion in the TI surface/bulk states, thereby directly leading to new and efficient emitters of broadband THz radiation.

拓扑绝缘子（TIS）具有受拓扑保护的自旋偏振表面状态，可为自旋电子设备提供高应用潜力，最好以达到难以捉摸的Terahertz（THZ）窗口的频率工作。在我们的项目中，我们通过超快激发和诸如BI2SE3的模型探测表面/散装电子，旋转和声子之间的耦合。我们主要使用了合适的极化的光学激光脉冲（〜1.5 eV光子能），以选择性地激发表面电子。为了探测所得的自旋，电荷和运输动力学，我们开发了新型的测量方案，包括磁光自旋和Terahertz（THZ）电导率和电流探针。我们能够揭示光学自旋偏振电子的超快松弛以及新型超快表面光电的重要特征。虽然第一个资金期间集中在光学激发和单物质样本上，但我们希望移至（i）在第二阶段，量身定制和选择性的激发以及（ii）混合Ti结构。通过在THZ到中红外光谱范围内使用较低的泵光子能量（〜1至400 MEV）来实现目标（i）。在诸如BI2SE3之类的3D模型中，这些能量非常适合直接和选择性地驱动狄拉克状态之间的带间过渡。由于这些表面状态位于Ti块带间隙中，我们期望对自旋极化的寿命，表面电流振幅和对内映射表面状态动力学的独特见解。第一个融资期间开发的探针监测了旋转，费用和运输的动力。实验将伴随于从头算电子结构和运输理论。通过可调和稳定的载波 - envelope相的激光和THZ脉冲实现了对早期超快自旋动力学的进一步控制和见解。关于目标（ii），我们将根据其激发态的超快动态来研究铁磁/Ti混合结构。我们将揭示拓扑表面状态对铁磁顺序动力学的影响（例如，通过自旋转移扭矩和光学自旋扭矩）和反向效应，从铁磁体进入TI表面和大体的超快自旋注入。使用我们新开发的自旋，电导率和运输探针，我们将实时研究这些过程的动力学。我们期望对铁磁铁对Ti动力学的影响以及Ti自旋大厅角度的估计获得新的见解。重要的是，将出现第一应用：由Ti自旋电流引起的相邻铁磁体的产生和Ti表面/散装状态中的自旋转换转换，从而直接导致宽带THZ辐射的新发射器。