Paul H. Tobash

Mixed Cations and Structural Complexity in (Eu1-xCax)4In3Ge4 and (Eu1-xCax)3In2Ge3-The First Two Members of the Homologous Series A2[n+m]In2n+mGe2[n+m] (n, m = 1, 2, ...∞; A = Ca, Sr, Ba, Eu, or Yb)

Reported are the synthesis and the structural characterization of two members of a new homologous series of polar intermetallic compounds, which exist only with mixed alkaline-earth and rare-earth metal cations. Crystals of (Eu(1-x)Ca(x))(4)In(3)Ge(4) (0.35(1) <or= x <or= 0.70(1)) and (Eu(1-x)Ca(x))(3)In(2)Ge(3) (0.78(1) <or= x <or= 0.90(1)) have been grown using a molten In metal flux and structurally characterized by single-crystal X-ray diffraction. (Eu(1-x)Ca(x))(4)In(3)Ge(4) adopts the monoclinic Mg(5)Si(6)-type structure (space group C2/m, Z = 2, Pearson symbol mS22) with lattice parameters a = 16.874(1)-17.024(2) A, b = 4.496(3)-4.556(1) A, c = 7.473(4)-7.540(1) A, and beta = 107.306(10)-105.631(3) degrees . (Eu(1-x)Ca(x))(3)In(2)Ge(3) crystallizes with a novel orthorhombic structure (space group Pnma, Z = 4, Pearson symbol oP32) with lattice parameters in the ranges a = 7.382(2)-7.4010(9) A, b = 4.452(1)-4.4640(6) A, and c = 23.684(6)-23.734(3) A, depending on the Eu/Ca ratio. The polyanionic substructures in both cases are related and are based on InGe(4) edge-shared tetrahedra, Ge(2) dimers, and bridging In atoms in a nearly square-planar environment. The (Eu(1-x)Ca(x))(4)In(3)Ge(4) structure can be viewed as a 1:1 intergrowth of Mo(2)FeB(2)-like and TiNiSi-like fragments, whereas (Eu(1-x)Ca(x))(3)In(2)Ge(3) can be rationalized as a 2:1 intergrowth of the same structural motifs. Both phases exhibit fairly wide homogeneity ranges and exist only with mixed cations. The experimental results have been complemented by linear muffin-tin orbital tight-binding band structure calculations, as well as an analysis of the observed cationic site preferences.

Magnesium Substitutions in Rare-Earth Metal Germanides with the Orthorhombic Gd5Si4-type Structure. Synthesis, Crystal Chemistry, and Magnetic Properties of RE5−xMgxGe4 (RE = Gd−Tm, Lu, and Y)

A series of magnesium-substituted rare-earth metal germanides with a general formula RE(5-x)Mg(x)Ge(4) (x approximately = 1.0-2.3; RE = Gd-Tm, Lu, Y) have been synthesized by high-temperature reactions and structurally characterized by single-crystal X-ray diffraction. These compounds crystallize with the common Gd(5)Si(4) type structure in the orthorhombic space group Pnma (No. 62; Z = 4; Pearsons code oP36) and do not appear to undergo temperature-induced crystallographic phase transitions down to 120 K. Replacing rare-earth metal atoms with Mg, up to nearly 45% at., reduces the valence electron count and is clearly expressed in the subtle changes of the Ge-Ge and metal-metal bonding. Magnetization measurements as a function of the temperature and the applied field reveal complex magnetic structures at cryogenic temperatures and Curie-Weiss paramagnetic behavior at higher temperatures. The observed local moment magnetism is consistent with RE(3+) ground states in all cases. In the magnetically ordered phases, the magnetization cannot reach saturation in fields up to 50 kOe. The structural trends across the series and the variations of the magnetic properties as a function of the Mg content are also discussed.

Anomalous femtosecond quasiparticle dynamics of hidden order state in URu(2)Si(2)

At T(0) = 17.5 K an exotic phase emerges from a heavy fermion state in URu(2)Si(2). The nature of this hidden order (HO) phase has so far evaded explanation. Formation of an unknown quasiparticle (QP) structure is believed to be responsible for the massive removal of entropy at the HO transition, however, experiments and ab initio calculations have been unable to reveal the essential character of the QP. Here we use femtosecond pump-probe time-and angle-resolved photoemission spectroscopy (tr-ARPES) to elucidate the ultrafast dynamics of the QP. We show how the Fermi surface is renormalized by shifting states away from the Fermi level at specific locations, characterized by vector q( ) = 0.56 +/- 0.08 angstrom(-1). Measurements of the temperature-time response reveal that, upon entering the HO, the QP lifetime in those locations increases from 42 fs to few hundred fs. The formation of the long-lived QPs is identified here as a principal actor of the HO.

Synthesis, Structural Characterization, and Physical Properties of the Early Rare-Earth Metal Digermanides REGe2–x (x ≈ 1/4) [RE = La–Nd, Sm]. A Case Study of Commensurately and Incommensurately Modulated Structures

Rare-earth metal germanides with the general formula RE(4)Ge(7) (RE = La, Ce, Pr, Nd, Sm) have been synthesized using the In-flux technique. Their structures have been established from single-crystal and powder X-ray diffraction, and the structural elucidation has been aided by electron diffraction. These compounds represent superstructures of the α-ThSi(2) structure type through the long- and/or short-range vacancy ordering. RE(4)Ge(7) (RE = Pr, Nd, Sm) appear to be commensurately modulated 4-fold superstructure of REGe(2-x) (x = 1/4), while coexistence of commensurate and incommensurate modulation is revealed in the La- and Ce-analogues. These results shed more light on the structural evolution of the REGe(2-x) phases as function of the vacancy concentration and nature of the rare-earth metal. Measurements of the magnetic susceptibilities on well-characterized single-crystals show ferromagnetic, antiferromagnetic, and even spin-glass-like behaviors. Mean-field theory is used to evaluate the correlations between structural and magnetic property data. Measurements on the electrical resistivities and the heat capacities are also presented and discussed.

Nanorod Self-Assembly in High Jc YBa2Cu3O7−x Films with Ru-Based Double Perovskites

Many second phase additions to YBa2Cu3O7−x (YBCO) films, in particular those that self-assemble into aligned nanorod and nanoparticle structures, enhance performance in self and applied fields. Of particular interest for additions are Ba-containing perovskites that are compatible with YBCO. In this report, we discuss the addition of Ba2YRuO6 to bulk and thick-film YBCO. Sub-micron, randomly oriented particles of this phase were found to form around grain boundaries and within YBCO grains in bulk sintered pellets. Within the limits of EDS, no Ru substitution into the YBCO was observed. Thick YBCO films were grown by pulsed laser deposition from a target consisting of YBa2Cu3Oy with 5 and 2.5 mole percent additions of Ba2YRuO6 and Y2O3, respectively. Films with enhanced in-field performance contained aligned, self-assembled Ba2YRuO6 nanorods and strained Y2O3 nanoparticle layers. A 0.9 µm thick film was found to have a self-field critical current density (Jc) of 5.1 MA/cm2 with minimum Jc(Θ, H=1T) of 0.75 MA/cm2. Conversely, Jc characteristics were similar to YBCO films without additions when these secondary phases formed as large, disordered phases within the film. A 2.3 µm thick film with such a distribution of secondary phases was found to have reduced self-field Jc values of 3.4 MA/cm2 at 75.5 K and Jc(min, Θ, 1T) of 0.4 MA/cm2.

Gallium–tin mixing in BaGa4−xSnx [x = 0.89 (2)] with the BaAl4 structure type

Single crystals of the barium trigallium tin, BaGa3.11 (2)Sn0.89 (2), have been obtained by a reaction of the elements in a sealed Nb tube. The title compound crystallizes with the BaAl4-type (Pearsons code tI10), and some of the Ga atoms are replaced by Sn atoms. All atoms occupy special positions: Ba (4/mmm), Ga1/Sn1 (\overline{4}m2) and Ga2/Sn2 (4mm). The refined composition has also been confirmed by means of elemental microanalysis.

The Origin and Coupling Mechanism of the Magnetoelectric Effect in TMCl2-4SC(NH2)2 (TM = Ni and Co)

Most research on multiferroics and magnetoelectric effects to date has focused on inorganic oxides. Molecule-based materials are a relatively new field in which to search for magnetoelectric multiferroics and to explore new coupling mechanisms between electric and magnetic order. We present magnetoelectric behavior in NiCl2-4SC(NH2)2 (DTN) and CoCl2-4SC(NH2)2 (DTC). These compounds form tetragonal structures where the transition metal ion (Ni or Co) is surrounded by four electrically polar thiourea molecules [SC(NH2)2]. By tracking the magnetic and electric properties of these compounds as a function of magnetic field, we gain insights into the coupling mechanism by observing that, in DTN, the electric polarization tracks the magnetic ordering, whereas in DTC it does not. For DTN, all electrically polar thiourea molecules tilt in the same direction along the c-axis, breaking spatial-inversion symmetry, whereas, for DTC, two thiourea molecules tilt up and two tilt down with respect to c-axis, perfectly canceling the net electrical polarization. Thus, the magnetoelectric coupling mechanism in DTN is likely a magnetostrictive adjustment of the thiourea molecule orientation in response to magnetic order.

Hybridization gap and dual nature of the heavy-fermion compound UPd2Al3 via quasiparticle scattering spectroscopy

We present results from point-contact spectroscopy of the antiferromagnetic heavy-fermion superconductor UPd

Cerium copper diantimonide, CeCu0.93(1)Sb2

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Ternary rare-earth alumo-silicides—single-crystal growth from Al flux, structural and physical properties

Al