J_{eff} Description of the Honeycomb Mott Insulator α-RuCl_{3}.

Novel ground states might be realized in honeycomb lattices with strong spin-orbit coupling. Here we study the electronic structure of α-RuCl_{3}, in which the Ru ions are in a d^{5} configuration and form a honeycomb lattice, by angle-resolved photoemission, x-ray photoemission, and electron energy loss spectroscopy supported by density functional theory and multiplet calculations. We find that α-RuCl_{3} is a Mott insulator with significant spin-orbit coupling, whose low energy electronic structure is naturally mapped onto J_{eff} states. This makes α-RuCl_{3} a promising candidate for the realization of Kitaev physics. Relevant electronic parameters such as the Hubbard energy U, the crystal field splitting 10 Dq, and the charge transfer energy Δ are evaluated.

Room-temperature synthesis, hydrothermal recrystallization, and properties of metastable stoichiometric FeSe.

Room-temperature precipitation from aqueous solutions yields the hitherto unknown metastable stoichiometric iron selenide (ms-FeSe) with tetragonal anti-PbO type structure. Samples with improved crystallinity are obtained by diffusion-controlled precipitation or hydrothermal recrystallization. The relations of ms-FeSe to superconducting ß-FeSe(1-x) and other neighbor phases of the iron-selenium system are established by high-temperature X-ray diffraction, DSC/TG/MS (differential scanning calorimetry/thermogravimetry/mass spectroscopy), (57)Fe Mössbauer spectroscopy, magnetization measurements, and transmission electron microscopy. Above 300 °C, ms-FeSe decomposes irreversibly to ß-FeSe(1-x) and Fe(7)Se(8). The structural parameters of ms-FeSe (P4/nmm, a = 377.90(1) pm, c = 551.11(3) pm, Z = 2), obtained by Rietveld refinement, differ significantly from literature data for ß-FeSe(1-x). The Mössbauer spectrum rules out interstitial iron atoms or additional phases. Magnetization data suggest canted antiferromagnetism below T(N) = 50 K. Stoichiometric non-superconducting ms-FeSe can be regarded as the true "parent" compound for the "11" iron-chalcogenide superconductors and may serve as starting point for new chemical modifications.

Large variations in the magnetic ordering behavior of EuCu(2)As(2) with the application of external pressure and magnetic field.

The influence of external pressure on the electrical transport and magnetic properties of EuCu(2)As(2), crystallizing in a ThCr(2)Si(2)-type structure, is reported. The system is known to be an antiferromagnet below T(N) ≈ 15 K in the absence of external magnetic fields. We find that there is a gradual reduction of T(N) with the application of a magnetic field with an extrapolated value of the critical field of around 18 kOe which can drive T(N) to zero. Electrical resistivity under pressure (<11 GPa) reveals that the magnetic ordering temperature is pushed up dramatically to higher temperatures which is quite interesting if compared with the behavior in isostructural FeAs-based systems containing Eu. Above 7 GPa, the pressure-induced state appears to be ferromagnetic. The results thus reveal interesting changes in the magnetic ordering behavior of this compound with increasing pressure and magnetic fields.

Incommensurately modulated lanthanide coinage-metal diarsenides. I. CeAu(1-delta)As2 [delta = 0.015 (2)]--a new distortion variant of the HfCuSi2 type with irregularly stacked cis-trans chains of arsenic atoms.

Rare-earth metal coinage-metal diarsenides LnTAs2 (Ln = Y, La, Ce-Lu; T = Cu, Ag, Au) are known to crystallize in structures closely related to the HfCuSi2 type, which comprises a stacking sequence of puckered TAs layers and planar square As nets, separated by the Ln atoms. CeAu(1-delta)As2, with delta = 0.015 (2), shows an incommensurate positional modulation of the arsenic atoms in the planar As nets. Based on X-ray diffraction data on a twinned crystal, a structure model in the monoclinic superspace group P12(1)/m1(alpha0gamma)00 (No. 11.1) with basic unit-cell parameters of a = 5.804 (1), b = 5.814 (1), c = 10.179 (1) A and beta = 90.09 (8) degrees is presented. The components of the modulation wavevector q = alphaa* + 0b* + gammac* are alpha = 0.08 (1) and gamma = 0.39 (1). The structure comprises cis-trans chains and rectangles of As atoms and displays an intricate stacking sequence of the modulated arsenic nets. Rod groups and layer groups of the respective arrangements are identified and the arrangement of different motifs of the modulated As net is discussed.

Incommensurately modulated lanthanide coinage-metal diarsenides. II. GdCuAs2, GdAu(1-delta)As2 and TbAu(1-delta)As2--new distortion variants of the HfCuSi2 type with irregularly stacked zigzag chains of arsenic atoms.

GdCuAs2, GdAu(1-delta)As2 and TbAu(1-delta)As2 crystallize as incommensurately modulated variants of the HfCuSi2 type. Structure models have been developed in the monoclinic superspace group P12(1)/m1(alpha0gamma)00 (No. 11.1). The components of the modulation wavevectors q = alphaa* + 0b* + gammac* are alpha = 0.04 (1) and gamma = 0.48 (1) for GdCuAs2, alpha = 0.03 (1) and gamma = 0.48 (1) for GdAu(1-delta)As2 and alpha = 0.02 (1) and gamma = 0.46 (1) for TbAu(1-delta)As2. The predominant effect of the positional modulation is the distortion of a square net of arsenic atoms, which results in planar zigzag chains. Rod groups and layer groups of the respective structure motifs are identified and discussed.

Magnetic and electrical transport anomalies of RMAs(2) (R = Pr and Sm, M = Ag and Au).

The results of magnetization, heat capacity and electrical resistivity (ρ) studies of the compounds RMAs(2) (R = Pr and Sm; M = Ag, Au), crystallizing in an HfCuSi(2)-derived structure, are reported. PrAgAs(2) orders antiferromagnetically at T(N) = 5 K. The Au analog, however, does not exhibit long range magnetic order down to 1.8 K. We infer that this is due to subtle differences in their crystallographic features, particularly noting that both the Sm compounds with identical crystal structures as that of the former order magnetically nearly at the same temperature (about 17 K). It appears that, in PrAgAs(2), SmAgAs(2) and SmAuAs(2), there is an additional magnetic transition at a lower temperature, as though the similarity in the crystal structure results in similarities in magnetism as well. The ρ for PrAgAs(2) and PrAuAs(2) exhibits a negative temperature coefficient in some temperature range in the paramagnetic state. SmAuAs(2) exhibits a magnetic Brillouin-zone gap effect in ρ at T(N), while SmAgAs(2) shows a well-defined broad minimum well above T(N) around 45 K. Thus, these compounds reveal interesting magnetic and transport properties.

Enhanced electrical resistivity before Néel order in the metals RCuAs2 (R=Sm, Gd, Tb, and Dy).

We report an unusual temperature (T) dependent electrical resistivity (rho) behavior in a class of ternary intermetallic compounds of the type RCuAs2 (R=rare earths). For some rare earths (Sm, Gd, Tb, and Dy) with negligible 4f hybridization, there is a pronounced minimum in rho(T) far above respective Néel temperatures (T(N)). However, for the rare earths which are more prone to exhibit such a rho(T) minimum due to 4f-covalent mixing and the Kondo effect, this minimum is depressed. These findings, difficult to explain within the hitherto-known concepts, present an interesting scenario in magnetism.