量子自旋转移矩诱导的非经典磁化动力学和电子-磁化纠缠
Quantum spin-transfer torque induced nonclassical magnetization dynamics and electron-magnetization entanglement
摘要：
标准的自旋转移力矩（STT）-其中自旋极化电流驱动被视为经典矢量的磁化动力学-要求电流携带的电子自旋和铁磁层的磁化之间的非共线性。然而，最近的实验[A。Zholud等人，物理修订信函119，257201（2017）]在低温下观察自旋阀中的磁化动力学，即使当电子自旋与磁化共线时，也指出STT中被忽视的量子效应，这可能导致高度非经典的磁化状态。利用全量子多体处理方法，即以自旋极化波包形式注入的电子与各向异性量子海森堡铁磁链中的局域自旋相互作用，我们将量子STT定义为由于初始多体态不是电子+局域自旋系统的本征态而引起的局域自旋的任意时间演化.对于由注入的自旋向下的电子从局部自旋向上散射引起的时间演化，电子子系统和局部自旋子系统之间发生纠缠，导致退相干，因此，总磁化强度收缩，但没有从其初始取向旋转，这解释了实验。此外，同样的过程-纠缠，从而诱导退相干-也存在于标准的非共线几何，连同通常的磁化旋转。这是因为在量子多体图像中，STT仅由电子自旋下降的因子态引起，而共线几何与非共线几何的唯一区别在于包含该因子态的初始多体态对时间演化过程中产生的可分离多体量子态叠加的贡献的相对大小.
英文摘要：
The standard spin-transfer torque (STT)---where spin-polarized current drives dynamics of magnetization viewed as a classical vector---requires noncollinearity between electron spins carried by the current and magnetization of a ferromagnetic layer. However, recent experiments [A. Zholud et al., Phys. Rev. Lett. 119, 257201 (2017)] observing magnetization dynamics in spin valves at cryogenic temperatures, even when electron spin is collinear to magnetization, point at overlooked quantum effects in STT which can lead to highly nonclassical magnetization states. Using fully quantum many-body treatment, where an electron injected as spin-polarized wave packet interacts with local spins comprising the anisotropic quantum Heisenberg ferromagnetic chain, we define quantum STT as any time evolution of local spins due to initial many-body state not being an eigenstate of electron+local-spins system. For time evolution caused by injected spin-down electron scattering off local up-spins, entanglement between electron subsystem and local spins subsystem takes place leading to decoherence and, therefore, shrinking of the total magnetization but without rotation from its initial orientation which explains the experiments. Furthermore, the same processes---entanglement and thereby induced decoherence---are present also in standard noncollinear geometry, together with the usual magnetization rotation. This is because STT in quantum many-body picture is caused only by electron spin-down factor state, and the only difference between collinear and noncollinear geometries is in relative size of the contribution of initial many-body state containing such factor state to superpositions of separable many-body quantum states generated during time evolution.
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