Exotic magnetism and electron-hybridization phenomena in rare-earth based materials

We explore unusual magnetic properties as well as electron-correlation phenomena in rare-earth (RE) transition-metal materials. This is experimental proposal and it aims at an electron spectroscopic investigation of RE-based intermetallic compounds (RE=Eu, Gd, Sm, Ho and Yb) in the vicinity of phase transitions from magnetically ordered to non-ordered states. We focus on RET₂M₂ materials (T = Rh, Ir, Co, Ni; M = Si, P) crystallizing in the layered ThCr₂Si₂ structure. They can be prepared in form of sufficiently large, high-quality single-crystalline samples, which can be easily cleaved and handled under ultra-high vacuum conditions. Precise attention is paid to the surface region, comparing its properties to those in 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 discussed for the bulk of such systems, however, our recent results unambiguously demonstrate that they may dramatically be 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 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.

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 measurements disclosing the evolution of magnetism at the surface and in the bulk and coupling of these subsystems, (iii) the k-resolved investigation of f-d electron-hybridization phenomena including their dynamics in the femtosecond range, (iv) modifications of the electronic and magnetic properties at the surface of the RET₂M₂ systems upon deposition of adlayers on top, and (v) the correlation of the spectroscopically observed phenomena with the macroscopic low-temperature properties of the systems.

The respective samples are produced by our cooperation partners in Uni Dresden and Uni Frankfurt am Main. We perform their spectroscopic investigation by means of angle-resolved photoelectron spectroscopy (ARPES) in the UV and soft X-ray range, resonant inelastic X-ray scattering (RIXS) and femtosecond time-resolved two-photon photoemission (2PPE). The theoretical description includes the band structure calculations for slabs and bulk as well as modeling. Most of the experiments are performed at the synchrotron radiation facilities: Diamond (England), SLS (Switzerland), ESRF (France) and BESSY-II (Germany).