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

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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.