Exotic magnetism and electron-correlation phenomena at the surface and in the bulk of rare-earth based materials

该提案涉及稀土（RE）过渡金属化合物中不寻常的磁性以及电子相关现象。将特别注意的表面区域，比较其属性的大部分系统。这些材料揭示了由电子和磁自由度的微妙相互作用引起的各种各样的不寻常性质，从磁有序到超导性到近藤和重费米子（HF）行为，甚至到费米液体图像的崩溃。值得注意的是，这种现象通常只讨论了这些系统的大部分;然而，我们最近的结果明确表明，它们可能与表面的情况截然不同。上述不寻常的物质状态产生于局域4f电子与价带的扩展布洛赫态的微妙相互作用，这敏感地取决于后者的对称性和能量位置，因此，对化合物的化学组成和结构性质。价态可以通过合金化在本体中变化，并且通过在表面上沉积吸附层在（子）表面层中变化，从而允许电子和磁性的受控调谐。该提案的目的是在电子光谱研究稀土基金属间化合物（RE =稀土）的相变附近的磁有序到无序状态。我们专注于在四层ThCr 2Si 2结构中结晶的RET 2 M2化合物，该化合物可以以大的高质量单晶样品的形式制备，该样品可以在超高真空条件下容易地切割和处理，从而可以制备化学和结构明确的表面。该项目包括：（一）这些晶体的生长及其结构和磁性特性的表征;（二）通过紫外和软X射线范围内的自旋和角度分辨光电子能谱以及共振非弹性X射线散射对其进行光谱研究;（三）近表面区域的体能带结构计算和平板计算，通过理论建模来实现。对于重费米子系统，最近开发的周期性安德森模型（PAM）的方法将被使用。专题是（i）与磁性以及f-d电子杂化效应有关的（亚）表面和体现象的正确鉴别和综合表征，（ii）表面磁性和k依赖的f-d电子杂化现象及其对费米表面的影响的温度依赖性研究，（iv）作为吸附层沉积的函数的表面处的电子和磁性的修改，以及（v）光谱观察到的现象与系统的宏观低温性质的相关性。

The proposal addresses unusual magnetic properties as well as electron-correlation phenomena in rare-earth (RE) transition-metal compounds. Particular attention will be paid to the surface region, comparing its properties to those of the bulk of the systems. These materials reveal an astonishing variety of unusual properties caused by the delicate interplay of electronic and magnetic degrees of freedom, ranging from magnetic order via superconductivity to Kondo and heavy-fermion (HF) behavior and even to a break-down of the Fermi-liquid picture. Remarkably, such phenomena are usually only discussed for the bulk of these systems; however, our recent results unambiguously demonstrate that they may be dramatically different to those at the surface. The aforementioned unusual states of matter arise from delicate interactions of the localized 4f electrons with the extended Bloch-states of the valence bands, which depend sensitively on the symmetry properties and energy positions of the latter and, thus, on the chemical composition and structural properties of the compounds. The valence states can be varied in the bulk by means of alloying, and in (sub-)surface layers by deposition of adlayers on the surface, allowing for a controlled tuning of the electronic and magnetic properties. The proposal aims at an electron spectroscopic investigation of RE-based intermetallic compounds (RE = rare-earth) in the vicinity of phase transitions from magnetically ordered to non-ordered states. We focus on RET2M2 compounds crystallizing in the tetragonal layered ThCr2Si2 structure, which can be prepared in form of large, high-quality single-crystalline samples, which can be easily cleaved and handled under ultra-high vacuum conditions allowing to prepare chemically and structurally well-defined surfaces. The project comprises (i) the growth of these crystals and the characterization of their structural and magnetic properties, (ii) their spectroscopic investigation by means of spin- and angle-resolved photoelectron spectroscopy (ARPES) in the UV and soft X-ray range as well as resonant inelastic X-ray scattering (RIXS), and (iii) band structure calculations for the bulk and slab-calculations for the near surface region, complimented by theoretical modeling. For heavy-fermion systems, a recently developed approach to the periodic Anderson model (PAM) will be used. Special topics are (i) the proper discrimination and comprehensive characterization of (sub-)surface and bulk phenomena related to magnetism as well as to f-d electron-hybridization effects, (ii) temperature-dependent studies of surface magnetism and k-dependent f-d electron-hybridization phenomena and their effect on the Fermi surfaces, (iv) modifications of the electronic and magnetic properties at the surface as a function of adlayer deposition, and (v) the correlation of the spectroscopically observed phenomena with the macroscopic low-temperature properties of the systems.