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Funding: Deutsche Forschungsgemeinschaft (DFG) through the SFB 1143 and the Wurzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter–ct.qmat (EXC 2147, Project No. 390858490), as well as the support of the HLD at HZDR, a member of the European Magnetic Field Laboratory (EMFL). We gratefully acknowledge support from the European Research Council (through the QUESTDO project, 714193), the Leverhulme Trust, and the Royal Society. We thank the Elettra synchrotron for access to the APE-HE beamline under proposal number 20195300. The research leading to this result has been supported by the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. Part of this work has been performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MUR Italy Progetti Internazionali) project (www.trieste.NFFA.eu).

We report on the electronic band structure, structural, magnetic, and thermal properties of Ce2Rh3Sn5. Ce LIII-edge XAS spectra give direct evidence for an intermediate valence behavior. Thermodynamic measurements reveal magnetic transitions at TN1≈2.9 K and TN2≈2.4 K. Electrical resistivity shows behavior typical for the Kondo lattices. The coexistence of magnetic order and valence fluctuations in a Kondo lattice system we attribute to a peculiar crystal structure in which Ce ions occupy two distinct lattice sites. Analysis of the structural features of Ce2Rh3Sn5, together with results of electronic band structure calculations and thermodynamic and spectroscopic data indicate that at low temperatures only Ce ions from the Ce1 sublattice adopt a stable trivalent electronic configuration and show local magnetic moments that give rise to the magnetic ordering. By contrast, our study suggests that Ce2 ions exhibit a nonmagnetic Kondo-singlet ground state. Furthermore, the valence of Ce2 ions estimated from the Ce LIII-edge XAS spectra varies between +3.18 at 6 K and +3.08 at room temperature. Thus our joined experimental and theoretical investigations classify Ce2Rh3Sn5 as a multivalent charge-ordered system.

We have investigated the magnetic ground state of CuFeAs and CuFeSb by means of $^{57}\mathrm{Fe}$-M\"ossbauer spectroscopy, muon spin rotation/relaxation $(\ensuremath{\mu}\mathrm{SR})$, neutron diffraction, and electronic structure calculations. Both materials share the 111-LiFeAs crystal structure and are closely related to the class of iron-based superconductors. In both materials there is a considerable occupancy of the Cu site by Fe, which leads to ferromagnetic moments, which are magnetically strongly coupled to the regular Fe site magnetism. Our study shows that CuFeAs is close to an antiferromagnetic instability, whereas a ferromagnetic ground state is observed in CuFeSb, supporting theoretical models of anion height driven magnetism.

Antiferromagnetic semiconductors are new alternative materials for spintronic applications and spin valves. In this work, we report a detailed investigation of two antiferromagnetic semiconductors $A$MnAs ($A=\mathrm{Li}$, LaO), which are isostructural to the well-known LiFeAs and LaOFeAs superconductors. Here we present a comparison between the structural, magnetic, and electronic properties of LiMnAs, LaOMnAs, and related materials. Interestingly, both LiMnAs and LaOMnAs show a variation in resistivity with more than five orders of magnitude, making them particularly suitable for use in future electronic devices. Neutron and x-ray diffraction measurements on LiMnAs show a magnetic phase transition corresponding to the N\'eel temperature of 373.8 K, and a structural transition from the tetragonal to the cubic phase at 768 K. These experimental results are supported by density functional theory calculations.

A valence instability of Eu in ${\text{EuPd}}_{3}{\text{B}}_{x}$ $(0\ensuremath{\le}x\ensuremath{\le}1)$ was discussed controversially in the past. In a joint theoretical and experimental study we investigate a large series of ${\text{EuPd}}_{3}{\text{B}}_{x}$ and ${\text{GdPd}}_{3}{\text{B}}_{x}$ compounds. Characterization by x-ray diffraction, metallography, energy-, and wavelength-dispersive x-ray spectroscopy as well as chemical analysis determine an existence range of ${\text{EuPd}}_{3}{\text{B}}_{x}$ up to $x\ensuremath{\le}0.53$ and $x\ensuremath{\le}0.42$ for the ${\text{GdPd}}_{3}{\text{B}}_{x}$ compounds, respectively. Our density-functional-based electronic structure calculation predict a valence change in ${\text{EuPd}}_{3}{\text{B}}_{x}$ above ${x}_{c}^{\text{DFT}}=0.19\ifmmode\pm\else\textpm\fi{}0.02$ from a nonmagnetic ${\text{Eu}}^{3+}$ state into a magnetic ${\text{Eu}}^{2+}$ state which is reflected in a discontinuity of the lattice parameter. In contrast, the related Gd compounds with a stable ${\text{Gd}}^{3+}$ state exhibit an almost linear behavior of the lattice parameter following Vegard's law. Consistent with the calculations, the x-ray diffraction data show a kink in the lattice parameter for ${\text{EuPd}}_{3}{\text{B}}_{x}$ at ${x}_{c}^{\text{XRD}}=0.22\ifmmode\pm\else\textpm\fi{}0.02$. X-ray absorption spectroscopy measurements assign this kink to a transition into a heterogeneous mixed valence state for Eu with a critical B content ${x}_{c}^{\text{XAS}}=0.22\ifmmode\pm\else\textpm\fi{}0.03$. The observed change in the mean Eu valence from ${\text{Eu}}^{3+}$ $(x\ensuremath{\le}0.2)$ toward ${\text{Eu}}^{2.5+}$ $(x=0.5)$ is supported by magnetic susceptibility and specific-heat data.

We have investigated the magnetic susceptibility, the electrical resistivity, the specific heat, the thermoelectric power, the Hall coefficient, and the thermal conductivity of the ${\text{Al}}_{13}{\text{Fe}}_{4}$ and ${\text{Al}}_{13}{(\text{Fe},\text{Ni})}_{4}$ monoclinic approximants to the decagonal quasicrystal. While the ${\text{Al}}_{13}{\text{Fe}}_{4}$ crystals are structurally well ordered, the ternary derivative ${\text{Al}}_{13}{(\text{Fe},\text{Ni})}_{4}$ contains substitutional disorder and is considered as a disordered version of the ${\text{Al}}_{13}{\text{Fe}}_{4}$. The crystallographic-direction-dependent measurements were performed along the ${a}^{\ensuremath{\ast}}$, $b$, and $c$ directions of the monoclinic unit cell, where the $({a}^{\ensuremath{\ast}},c)$ atomic planes are stacked along the $b$ direction. The electronic transport and the magnetic properties exhibit significant anisotropy. The stacking $b$ direction is the most conducting direction for the electricity and heat. The effect of substitutional disorder in ${\text{Al}}_{13}{(\text{Fe},\text{Ni})}_{4}$ is manifested in the large residual resistivity $\ensuremath{\rho}(T\ensuremath{\rightarrow}0)$ and significantly reduced thermal conductivity of this compound, as compared to the ordered ${\text{Al}}_{13}{\text{Fe}}_{4}$. Specific-heat measurements reveal that the electronic density of states at the Fermi level of both compounds is high. The anisotropic Hall coefficient ${R}_{H}$ reflects complex structure of the anisotropic Fermi surface that contains electronlike and holelike contributions. Depending on the combination of directions of the current and the magnetic field, electronlike $({R}_{H}l0)$ or holelike $({R}_{H}g0)$ contributions may dominate, or the two contributions compensate each other $({R}_{H}\ensuremath{\approx}0)$. Similar complicated anisotropic behavior was observed also in the thermopower. The anisotropic Fermi surface was calculated ab initio using the atomic parameters of the refined ${\text{Al}}_{13}{\text{Fe}}_{4}$ structural model that is also presented in this work.

The magnetism in ${\text{SrFe}}_{2}{\text{As}}_{2}$ can be suppressed by electron doping through a small substitution of Fe by Co or Ni, giving way to superconductivity. We demonstrate that a massive substitution of Fe by isovalent ruthenium similarly suppresses the magnetic ordering in ${\text{SrFe}}_{2\ensuremath{-}x}{\text{Ru}}_{x}{\text{As}}_{2}$ and leads to bulk superconductivity for $0.6\ensuremath{\le}x\ensuremath{\le}0.8$. Magnetization, electrical resistivity, and specific-heat data show ${T}_{c}$ up to $\ensuremath{\approx}20\text{ }\text{K}$. Detailed structural investigations reveal a strong decrease in the lattice parameter ratio $c/a$ with increasing $x$. Density functional theory band-structure calculations are in line with the observation that the magnetic order in ${\text{SrFe}}_{2\ensuremath{-}x}{\text{Ru}}_{x}{\text{As}}_{2}$ is only destabilized for large $x$.

High hydrostatic pressure destroys the strong ferromagnetism in $\mathrm{Y}{\mathrm{Co}}_{5}$. The transformation proceeds in a stepwise fashion, as a first-order phase transition $({P}_{\mathrm{crit}}=18\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, and is accompanied by an isomorphic (without change of symmetry) lattice collapse and a topological change of the Fermi surface. Accurate full-potential density-functional calculations enable us to ascribe these phenomena to a quasi-one-dimensional $3d$-like band, whose top under ambient pressure is situated $\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the Fermi level. Lattice compression drives the top of the nondispersive band---and the associated peak in the majority density of states---upward in energy, until they reach the Fermi level and the system becomes unstable. A similar transition at a somewhat higher pressure $(23\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ is predicted for the isomorphous compound $\mathrm{La}{\mathrm{Co}}_{5}$.

The rare-earth aluminum boride $\mathrm{Tm}\mathrm{Al}{\mathrm{B}}_{4}$ was investigated in detail. $\mathrm{Tm}\mathrm{Al}{\mathrm{B}}_{4}$ has previously been observed to exhibit an antiferromagnetic transition at $5.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In this work, an additional transition in the in-plane magnetic susceptibility ${\ensuremath{\chi}}_{\ensuremath{\perp}c}$ was observed at ${T}_{N1}\ensuremath{\approx}40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ indicated to have short-range order or low-dimensional character. More than 100 compounds are known to take the $\mathrm{Y}\mathrm{Cr}{\mathrm{B}}_{4}$ type of crystal structure. However, $\mathrm{Tm}\mathrm{Al}{\mathrm{B}}_{4}$ is only the second system in which magnetic transitions have been observed, following the $R\mathrm{Ru}{\mathrm{B}}_{4}$ compounds ($R=\text{rare}$-earth metal). Experimental and calculated results reveal a metal-like behavior for $\mathrm{Tm}\mathrm{Al}{\mathrm{B}}_{4}$. Advanced refinement of the crystal structure from single-crystal x-ray diffraction data revealed that the $\mathrm{Tm}\mathrm{Al}{\mathrm{B}}_{4}$ crystals measured are basically of the $\mathrm{Y}\mathrm{Cr}{\mathrm{B}}_{4}$ crystal structure type $(Pbma)$ but with interesting anomalies. These anomalies were indicated to arise from intrinsic building defects in the basal plane due to the presence of fragments of the closely related $\mathrm{Th}\mathrm{Mo}{\mathrm{B}}_{4}$-type $(Cmmm)$ structure. Additional features in $\mathrm{Tm}\mathrm{Al}{\mathrm{B}}_{4}$ were observed at low temperatures in the specific heat $C$ and in ${\ensuremath{\chi}}_{\ensuremath{\perp}c}$ at ${T}_{1}$,${T}_{2}l{T}_{N1}$. They are indicated to pertain to the basal plane, which points to intrinsic variations occurring in the in-plane magnetic structure and arrangement. It is proposed that the origin of this behavior is due to the presence of these intrinsic building defects in the crystal structure of $\mathrm{Tm}\mathrm{Al}{\mathrm{B}}_{4}$, which lead to different environments for the thulium atoms.

The spin polarization of ferromagnetic alkali-metal iron antimonides $\mathrm{K}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and $\mathrm{Na}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ is studied by point-contact Andreev reflection using superconducting Nb and Pb tips. From these measurements an intrinsic transport spin polarization ${P}_{t}$ of 67% and 60% for the K and Na compound, respectively, is inferred, which establishes these materials as a class of highly spin-polarized ferromagnets. The results are in accord with band structure calculations within the local spin density approximation that predict nearly 100% spin polarization in the density of states. We discuss the impact of calculated Fermi velocities and spin fluctuations on ${P}_{t}$.

A comparative study of the filled skutterudite compounds $\mathrm{Yb}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and $\mathrm{Ca}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ is presented. Crystal structure investigations and measurements of magnetic susceptibility, specific heat and electrical resistivity in magnetic field, and x-ray absorption spectroscopy have been performed. Almost identical structural, magnetic, and electronic properties of both compounds are observed. Electronic structure calculations support this similarity. It is concluded that ytterbium in ${\mathrm{Yb}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ is stable divalent and the magnetic moments in both materials are solely due to itinerant-electron paramagnetism of the $\mathrm{Fe}\ensuremath{-}\mathrm{Sb}$ polyanion. The calcium and ytterbium iron-antimony skutterudites are nearly ferromagnetic metals and their properties are mainly governed by spin fluctuations.

The synthesis, chemical, structural, and magnetic properties of alkali-metal compounds with filled-skutterudite structure, $\mathrm{Na}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and $\mathrm{K}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$, are described. X-ray and neutron diffraction and elemental analysis established the crystal structure without defects and disorder on the cation site. The temperature and pressure dependence of the cubic unit cell of $\mathrm{Na}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and the displacement parameter of Na are investigated. The electronic structure is calculated by density functional methods (LMTO, FPLO). Quantum chemical calculations (electron localization function) reveal the covalent character of both $\mathrm{Fe}\penalty1000-\hskip0pt\mathrm{Sb}$ and $\mathrm{Sb}\penalty1000-\hskip0pt\mathrm{Sb}$ interactions. Electronic structure calculations within the local density approximation exhibit a band ferromagnetic ground state and predict a half-metallic behavior. In contrast to isostructural alkaline-earth compounds ($\mathrm{Ca}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and $\mathrm{Ba}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$), the alkali-metal skutterudites are itinerant electron ferromagnets with small magnetic moments ($\ensuremath{\approx}0.25{\ensuremath{\mu}}_{\mathrm{B}}∕\mathrm{Fe}$ atom) and ${T}_{\mathrm{C}}\ensuremath{\approx}85\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Yet the paramagnetic moments of all four compounds are between $1.5{\ensuremath{\mu}}_{\mathrm{B}}$ and $1.7{\ensuremath{\mu}}_{\mathrm{B}}$ per Fe atom, indicating similar Stoner factors. Temperature-dependent $^{57}\mathrm{Fe}$ and $^{121}\mathrm{Sb}$ M\"ossbauer spectroscopies confirm the ferromagnetic state in the sodium compound with very small hyperfine fields at the iron and antimony sites.

Funding: We acknowledge support from the European Research Council (grant no. ERC-714193 QUESTDO), the Royal Society and the Max Planck Society. V.S. and O.J.C acknowledge EPSRC for PhD studentship support through grant numbers EP/L015110/1 and EP/K503162/1, respectively. D. M. acknowledges support from the International Max Planck Research School for Chemistry and Physics of Quantum Materials (IMPRS-CPQM). We report a combined experimental and theoretical study of the surface and bulk electronic structure of aluminium diboride, a non-superconducting sister compound of the superconductor MgB2. We perform angle-resolved photoemission measurements with variable photon energy, and compare them with density functional theory calculations, in order to disentangle the surface and bulk contributions to the measured spectra. Aluminium diboride is known to be aluminium deficient, Al1−δB2, which would be expected to lead to a hole doping as compared to the nominally stoichimoetric compound. Nonetheless, we find that the bulk σ states, which mediate superconductivity in MgB2, remain more than 600meV below the Fermi level. However, we also observe σ states originating from the boron terminated surface, with an order of magnitude smaller binding energy of 70meV, and demonstrate how surface hole-doping can bring these across the Fermi level. Publisher PDF