Local magnetic moments and their interaction in correlated systems at reduced dimensions

这个项目的主要重点是实验研究低维相关系统中表面和界面上的局部磁矩及其相互作用。X射线(磁)二向色性是对这种超薄样品中局部力矩的一种最方便和固有的元素特有的探测，因为它有限的探测深度与相关的长度尺度相匹配。利用XMCD，我们通过具体观察关联电子系统的局域部分，研究了近藤晶格和重费米子复杂的磁性。在前面的阶段中，我们已经在很大程度上建立了利用磁x射线光谱学来解开决定多体磁响应的关键参数的复杂相互作用的方法学。在任何给定的系统中，这些参数包括局域晶场对称性和由此产生的电子基态，杂质近藤温度和依赖于温度的价态，以及有效的、部分屏蔽的顺磁矩的大小和它们的相互磁耦合。在此基础上，我们现在将系统地将XMCD分析扩展到较低的温度，进入相干区域。除了顺磁重费米子态之外，还可以发生向磁性有序的转变，在这种情况下，屏蔽或未屏蔽的力矩的排序在理论上是可能的。在这里，我们期望XMCD能提供关于系统中局部力矩所遵循的特定情形的直接证据，这在顺磁和铁磁基态的情况下是最方便的。这些研究将在单晶衬底上制备的有序表面合金上进行。使用合金厚度作为附加参数，我们可以确定合金表面的修饰行为，并尝试探索二维Kondo晶格的极限。引入提供大的自旋轨道耦合的重元素可能会导致增强的近藤效应和增加的(铁)磁有序的趋势。因此，我们将把我们的研究扩展到三元重费米子化合物，以探索增强的自旋轨道相互作用和相关的表面Rashba分裂的后果。接口。与P4项目合作，我们还将使用XMCD技术来表征氧化物异质结构的磁性界面性质。我们将特别利用扫描X射线显微镜(SXM)的最新发展。Sxm的横向分辨率(通常为≤30 nm)将用于绘制表示例如磁化密度分布的光谱特征的空间变化。因此，LAO/STO界面的超导和铁磁性并存等现象可以用来检验空间相分离的发生和相关的长度尺度。SXM中的光谱学提供的另一个机会是通过横向进入断裂试件来提高界面的灵敏度。

The primary focus of this project is the experimental study of local magnetic moments and their interactions in low-dimensional, correlated systems at surfaces and interfaces. X-ray (magnetic) dichroism is a most convenient and inherently element specific probe of local moments in such ultrathin samples since its limited probing depth matches the relevant length scales. Using XMCD, we address the intricate magnetic properties of Kondo lattices and heavy fermions by specifically observing the localized part of the correlated electron system. In the preceding period, we have largely established the methodology of disentangling the complex interplay of the key parameters which determine the many body magnetic response using magnetic x-ray spectroscopy. In any given system these include local crystal field symmetry and resulting electronic ground configuration, impurity Kondo temperature and temperature dependent valence as well as the magnitudes of the effective, partially screened paramagnetic moments and their mutual magnetic coupling. On this basis we shall now systematically extend the XMCD analysis to lower temperatures, entering the coherence regime. Besides the paramagnetic heavy fermion state a transition towards magnetic order can occur, in which case ordering of either screened or unscreened moments is possible on theoretical grounds. Here, we expect XMCD to provide direct evidence about the particular scenario obeyed by the local moments in the systems that shall be investigated – most conveniently in instances of paramagnetic and ferromagnetic ground states. These investigations will be conducted on ordered surface alloys, prepared on single crystalline substrates. Using the alloy thickness as an additional parameter we can identify modified behavior right at the alloy surface and attempt to explore the limit of two-dimensional Kondo lattices. Introducing heavy elements which provide large spin orbit coupling may induce both, an enhanced Kondo effect and an increased tendency towards (ferro-) magnetic order. We shall therefore extend our studies to ternary heavy fermion compounds to explore the consequences of enhanced spin orbit interaction and associated Rashba splitting at surfaces resp. interfaces. In collaboration with project P4, we shall additionally use XMCD techniques to characterize magnetic interface properties of oxide heterostructures. We shall particularly take advantage of most recent developments in scanning x-ray microscopy (SXM). The lateral resolution in SXM (routinely ≤ 30 nm) will be used to map out spatial variations of spectral features representing, e.g., magnetization density distributions. Phenomena such as the coexistence of superconductivity and ferromagnetism, reported for the LAO/STO interface could thus be tested for the occurrence of spatial phase separation and the associated length scales. Yet another opportunity provided by spectroscopy within a SXM is to enhance the sensitivity with respect to the interface through lateral access in fractured specimens.