Exciton spin dynamics in diluted magnetic semiconductor quantum wells

在这个项目中，我们的目标是发展一个微观的量子动力学理论来解释稀磁半导体纳米结构中的自旋动力学，考虑到由于与磁性掺杂的交换耦合以及库仑相互作用而产生的关联，库仑相互作用特别是引起激子关联。所考虑的具有重要技术意义的系统在基于关联和量子相干动力学现象的积极使用的创新应用方面特别有前景。这尤其适用于需要相干自旋控制的信息技术中的应用。未来在这个方向上取得进展的基础是加深对自旋动力学的理解，这需要一个超越当前最先进水平的理论。作为我们的新理论的第一个应用，我们将尝试识别量子阱中激子激发的自旋动力学中的量子动力学特征。这些研究将促进对自旋弛豫的理解，并应提供关于量子相干对自旋弛豫的作用的见解。特别是，我们将探索自旋弛豫偏离可由速率方程建模的非相干过程的程度。在此背景下，我们还将分析不同激子态之间的能量再分布对自旋动力学的影响。本项目的进一步研究重点是分析与磁性掺杂的交换相互作用和光激发载流子的自旋-轨道耦合对自旋动力学的影响。在稀磁半导体中，交换作用通常占主导地位。然而，改变合适的材料参数，特别是在多组分复合半导体中，允许在很大范围内调整这些相互作用的相对强度。甚至可以实现两种联轴器具有相同强度的情况。对后一种情况下自由载流子的激发的研究引起了人们的期望，即在激子激发的情况下，当两个耦合是可比较的时，自旋系统应该出现一种定性的新的动力学行为。这可能为新的自旋控制方法铺平道路。另一个目标是探索通过使用定义明确的轨道角动量的光(扭曲光)激发激子态来控制自旋动力学的可能性。该项目的这一部分将阐明这样一个问题，即作为专题研究重点的新型光源的使用是否为外部操纵自旋开辟了新的途径，这些方法不同于传统方法，例如通过圆偏振传统激光进行自旋定向。

In this project we aim to develop a microscopic, quantum-kinetic theory for the spin dynamics in diluted magnetic semiconductor nanostructures accounting for both, correlations due to the exchange coupling with magnetic dopants as well as the Coulomb interaction, which in particular gives rise to excitonic correlations. The considered technologically important systems are especially promising in terms of innovative applications based on the active use of correlations and quantum coherent dynamical phenomena. This applies in particular to applications in information technology that require a coherent spin control. The basis for future progress in this direction is a deepened understanding of spin dynamics which requires a theory beyond the current state-of-the-art. As a first application of our new theory we shall try to identify quantum kinetic signatures in the spin dynamics of excitonic excitations in quantum wells. These studies will advance the understanding of spin relaxation and should provide insight concerning the role of quantum coherences for spin relaxation. In particular, we shall explore in how far the spin relaxation deviates from an incoherent process that can be modelled by rate equations. In this context we shall also analyze the impact of energetic redistributions between different excitonic states on the spin dynamics.A further focus of the research within this project is the analysis of the interplay between influences of the exchange interaction with magnetic dopants and the spin-orbit coupling of optically excited carriers on the spin dynamics. In diluted magnetic semiconductors usually the exchange interaction dominates. However, varying suitable material parameters in particular in multi-component composite semiconductors allows for tailoring the relative strengths of these interactions in a wide range. Even situations where both couplings are of comparable strength can be realized. Studies of excitations of free carriers in the latter regime give rise to the expectation that also in the case of excitonic excitations a qualitatively new dynamical behavior of the spin system should arise when both couplings are comparable. This could pave the way towards new approaches of spin control.Another target is to explore the possibilities to control the spin dynamics by exciting excitonic states using light with well-defined orbital angular momentum ('twisted light'). This part of the project shall shed light on the question whether the use of novel types of light sources, such as twisted light, that are a focus of topical research opens new ways for the external manipulation of spins that differ from traditional methods as, e. g., the spin orientation via circularly polarized conventional laser light.