In strongly correlated electron systems the intricate interplay between electronic correlation effects and the tendency to form bands leads to a wealth of fascinating physical phenomena. The theoretical description of such systems is extremely complex and cannot be tackled exactly, so that ingenious modelling based on approximations must be utilized. Experiments are of utmost importance in this regard, since they provide a way to test and verify models, or to help devise better ones.
This Dissertation deals with strongly correlated uranium intermetallic systems. The interesting phases they can adopt include heavy fermion behaviour, unconventional superconductivity, hidden and multipolar order, and exotic induced magnetism. Here the hybridization between the 5f states and the conduction electrons drives the physics. The description of the 5f states is therefore of utmost importance. However, since there is no clear hierarchy of interactions like Coulomb repulsion, spin-orbit coupling, hopping and crystal-field, the modelling is difficult. This is in strong contrast to the more spatially localized 4f states of, e.g., cerium compounds. It is far from clear how to quantitatively describe the electronic structure of uranium intermetallics and whether, for example, an itinerant band approach or an impurity-type model, taking local degrees of freedom explicitly into account, would be a better starting point. In intermetallics, the situation is aggravated by the fact that the modelling lacks important pieces of information. This is not least due to the fact that understanding the formal valence, the filling of the 5f shell, and the relevant symmetries of the $5f$ electrons are experimentally demanding tasks. This Dissertation, therefore, aims at developing new methods and Ans\'atze in this direction.
We use x-ray spectroscopy to investigate the electronic structure, and in particular element-specific Inelastic X-ray Scattering (IXS); resonant (RIXS) at the U M(5) edge and non-resonant (NIXS) at the U O(4,5) edge. Both methods are innovative. For the first time, valence band RIXS measurements with sufficient resolution (150 meV) can be carried out at the U M(5) edge to measure ff excitations in intermetallic uranium compounds. Their existence, if present, provides information about the formal valence or main atomic configuration that determines the symmetry. The orientation dependence of the mutipolar excitations in NIXS (with restrictions also in RIXS), in turn, provides information about the orbital occupation. Atomic full-multiplet calculations are indispensable here.
In addition, photoelectron spectroscopy (PES) is applied, both in the soft as well as in the hard x-ray regime (HAXPES), to investigate the hybridization and localization of the 5f electrons. The energy dependence of the cross-sections allows to determine the orbital contributions in the valence band, so that parameters like, e.g., the double-counting correction for the LDA+DMFT calculations, performed by Prof. A. Hariki from the Osaka Metropolitan University, can be determined from tuning the calculations to the experimental data. This combination of PES and DFT+DMFT provides a reliable new quantitative insight into the number of electrons in the 5f shell and their degree of delocalization.
We consider UGa(2) and UB(2), respectively, as benchmark localized and itinerant systems and investigate them with IXS and PES. UGa(2) is a high-moment ferromagnet, with U-U distances above the Hill limit, while UB(2) is paramagnetic and clearly below the Hill limit. We observe sharp multiplet excitations of the 5f2 configuration in the IXS spectra of UGa(2), but none in the spectra of UB(2). The comparison of the spectra with full-multiplet calculations shows that in UGa(2) the U 5f2 configuration dominates and, from the orientation dependence (RIXS and NIXS), the crystal-field ground state can be determined. We show that the magnetism of this compound is of the induced type.
The cross-section based analysis of the valence band PES data with the LDA+DMFT approach shows that the filling of the 5f shell is similar in both compounds, but that the distribution among different configurations is considerably wider in UB(2). Also the time-dependent charge correlation functions of UGa(2) and UB(2) show a larger itinerancy in the latter compound.
The peculiarity and novelty about this combined study is that a reliable quantitative description of the electronic structure is achieved. This allows, for the first time, an accurate estimation of the 5f occupation and a quantitative description of the U\ 4f core-level PES spectra. This study paves the way to a systematic classification of uranium intermetallics.
We further investigate the substitution series URu(2-x)Fe(x)Si(2) with PES. The systematic study of isostructural and/or isoelectronic series of compounds is crucial in unveiling the origin of their physical properties. URu(2)Si(2) exhibits hidden-order as well as superconductivity, and becomes antiferromagnetic upon Fe doping. Fe substitution seemingly involves the application of chemical pressure to the system. We measure the U 4f PES core-level of the URu(2-x)Fe(x)Si(2) substitution series and observe a non-monotonic shift of spectral weight. We argue that, besides chemical pressure, the Fe density of states at the Fermi level also plays a central role and we propose and extended Doniach diagram where the two effects compete. We also measure NIXS, confirming that the ground state symmetry is a singlet or quasi-doublet of the 5f2 configuration. The magnetic properties must then be understood, as in UGa(2), in terms of induced magnetism.
We extend our study of the UT(2)Si(2) compounds to the case where T = Os, Ir, Pt and Au, i.e. 5d transition metals. Although the T= 3d and 4d transition metal systems have been extensively investigated, the T = 5d compounds lack systematic studies. The comparison of the NIXS spectra with multiplet calculations shows that also here the 5f2 dominates, only for T= Au it is not so clear. The absence of a strong directional dependence impairs the identification of the ground state symmetry. Valence band hard x-ray PES allows to probe the transition metal 5d states directly. U 4f core-level hard x-ray PES gives a qualitative indication of the filling of the 5f shell across the series.
We then focus on hexagonal UNi(2)Al(3) and apply NIXS. Like isoelectronic and isostructural UPd(2)Al(3), it is a prototypical U heavy-fermion compound, showing antiferromagnetic order and unconventional superconductivity. We observe a strong directional dependence of the NIXS spectra at low temperatures. The possible ground-state symmetries of the 5f2 configuration that fit the NIXS data are in contradiction to previous proposals from fits of the static magnetic susceptibility. We put forward a new crystal-field model that describes the high temperature magnetic susceptibility and the NIXS data at low temperatures, and that explains the magnetism.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:90853 |
| Date | 15 April 2024 |
| Creators | Marino, Andrea |
| Contributors | Tjeng, Liu Hao, Geck, Jochen, Hansmann, Philipp, Technische Universität Dresden, Max-Planck-Institut für Chemische Physik Fester Stoffe |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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