arXiv subjects

Martin Sundermann

Publications and source records attributed to Martin Sundermann.

23 records · Page 2Linked to original sources

arXiv2019

Direct imaging of orbitals in quantum materials

The spectacular physical properties of quantum materials based on transition metal, rare earth, and actinide elements continue to challenge our comprehension of solid state physics and chemistry. The electronic states of these materials are dominated by the $d$ and $f$ wave functions intertwined with the strong band formation of the solid. In order to estimate which wave functions contribute to the ground state formation, we have had to rely, until now, on theoretical calculations combined with spectroscopy. Here we show that $s$-core-level non-resonant inelastic x-ray scattering ($s$-NIXS) can directly image the active orbital in real space, without the necessity of any modeling. The power and accuracy of this new technique is shown using the text-book example, x$^2$-y$^2$/3$z^2$-r$^2$ orbital of the Ni$^{2+}$ ion in NiO single crystal.

cond-mat.str-el↗

arXiv2018

U 5$f$ crystal-field ground state of UO$_2$ probed by directional dichroism in nonresonant inelastic x-ray scattering

Nonresonant inelastic x-ray scattering (NIXS) has been performed on single crystals of UO$_2$ to study the direction dependence of higher-order-multipole scattering from the uranium $O_{4,5}$ edges (90--110 eV). By comparing the experimental results with theoretical calculations the symmetry of the ground state is confirmed directly as the crystal-field (CF) $Γ_5$ triplet state within the $J$ = 4 manifold. The results also demonstrate that the directional dichroism of the NIXS spectra is sensitive to the CF strength and establish NIXS as a tool for probing CF interactions quantitatively.

cond-mat.str-el↗

arXiv2018

Tuning the hybridization and magnetic ground state of electron and hole doped CeOs$_2$Al$_{10}$: an x-ray spectroscopy study

Here we present linear and circular polarized soft x-ray absorption spectroscopy (XAS) data at the Ce $M_{4,5}$ edges of the electron (Ir) and hole-doped (Re) Kondo semiconductor CeOs$_2$Al$_{10}$. Both substitutions have a strong impact on the unusual high N$\acute{e}$el temperature, $T_N$=28.5\,K, and also the direction of the ordered moment in case of Ir. The substitution dependence of the linear dichroism is weak thus validating the crystal-field description of CeOs$_2$Al$_{10}$ being representative for the Re and Ir substituted compounds. The impact of electron- and hole-doping on the hybridization between conduction and 4$f$ electrons is related to the amount of $f^0$ in the ground state and reduction of x-ray magnetic circular dichroism. A relationship of $cf$-hybridization strength and enhanced $T_N$ is discussed. The direction and doping dependence of the circular dichroism is in agreement with strong Kondo screening along the crystallographic $a$ direction.

cond-mat.str-el↗

arXiv2018

4$f$ crystal field ground state of the strongly correlated topological insulator SmB$_6$

We investigated the crystal-electric field ground state of the 4$f$ manifold in the strongly correlated topological insulator SmB$_6$ using core level non-resonant inelastic x-ray scattering (NIXS). The directional dependence of the scattering function that arises from higher multipole transitions establishes unambiguously that the $Γ_8$ quartet state of the Sm $f^5$ $J$=$5/2$ configuration governs the ground-state symmetry and hence the topological properties of SmB$_6$. Our findings contradict the results of density functional calculations reported so far.

cond-mat.str-el↗

arXiv2016

Direct bulk sensitive probe of 5f symmetry in URu2Si2

The second-order phase transition into a hidden order phase in URu$_2$Si$_2$ goes along with an order parameter which is still a mystery, despite 30 years of research. However, it is understood that the symmetry of the order parameter must be related to the symmetry of the low lying local electronic $f$-states. Here we present results of a novel spectroscopy, namely core-level non-resonant inelastic x-ray scattering (NIXS). This method allows for the measurement of local high-multipole excitations and it is bulk sensitive. The observed anisotropy of the scattering function unambiguously shows that the 5$f$ ground state wave function is composed mainly, but essentially not purely, of the $Γ_1$ with majority $J_z$ =$|4\rangle$ + $|-4\rangle$ and/or $Γ_2$ singlet states.

cond-mat.str-el↗