Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan.
Nanoscale. 2018 Nov 15;10(44):20559-20564. doi: 10.1039/c8nr05677c.
Gate-controllable spin-orbit coupling is often one requisite for spintronic devices. For practical spin field-effect transistors, another essential requirement is ballistic spin transport, where the spin precession length is shorter than the mean free path such that the gate-controlled spin precession is not randomized by disorder. In this letter, we report the observation of a gate-induced crossover from weak localization to weak anti-localization in the magneto-resistance of a high-mobility two-dimensional hole gas in a strained germanium quantum well. From the magneto-resistance, we extract the phase-coherence time, spin-orbit precession time, spin-orbit energy splitting, and cubic Rashba coefficient over a wide density range. The mobility and the mean free path increase with increasing hole density, while the spin precession length decreases due to increasingly stronger spin-orbit coupling. As the density becomes larger than ∼6 × 1011 cm-2, the spin precession length becomes shorter than the mean free path, and the system enters the ballistic spin transport regime. We also report here the numerical methods and code developed for calculating the magneto-resistance in the ballistic regime, where the commonly used HLN and ILP models for analyzing weak localization and anti-localization are not valid. These results pave the way toward silicon-compatible spintronic devices.
栅控自旋轨道耦合通常是自旋电子器件的一个必要条件。对于实际的自旋场效应晶体管，另一个必要条件是弹道自旋输运，其中自旋进动长度短于平均自由程，使得栅控自旋进动不会被无序随机化。在这封信中，我们报告了在应变锗量子阱中高迁移率二维空穴气的磁阻中观察到的从弱局域到弱反局域的栅控交叉。从磁阻中，我们在很宽的密度范围内提取了相位相干时间、自旋轨道进动时间、自旋轨道能分裂和立方拉什巴系数。迁移率和平均自由程随着空穴密度的增加而增加，而由于自旋轨道耦合越来越强，自旋进动长度减小。当密度大于约 6×1011cm-2时，自旋进动长度变得短于平均自由程，系统进入弹道自旋输运状态。我们还在这里报告了用于计算弹道磁阻的数值方法和代码，其中常用的 HLN 和 ILP 模型用于分析弱局域和反局域是无效的。这些结果为硅兼容的自旋电子器件铺平了道路。
