Alois Loidl

Publications of Alois Loidl

• Interplay between Superconductivity and Magnetism in Rb0.8Fe1.6Se2 under Pressure

  Authors: Vadim Ksenofontov, Sergey A. Medvedev, Leslie M. Schoop, Gerhard Wortmann, Taras Palasyuk, Vladimir Tsurkan, Joachim Deisenhofer, Alois Loidl, Claudia Felser

  12/2011;

  High-pressure magnetization, structural and 57Fe M\"ossbauer studies were performed on superconducting Rb0.8Fe1.6Se2.0 with Tc = 32.4 K. The superconducting transition temperature gradually decreasesHigh-pressure magnetization, structural and 57Fe M\"ossbauer studies were performed on superconducting Rb0.8Fe1.6Se2.0 with Tc = 32.4 K. The superconducting transition temperature gradually decreases on increasing pressure up to 5.0 GPa followed by a marked step-like suppression of superconductivity near 6 GPa. No structural phase transition in the Fe vacancy-ordered superstructure is observed in synchrotron XRD studies up to 15.6 GPa, while the M\"ossbauer spectra above 5 GPa reveal the appearance of a new paramagnetic phase and significant changes in the magnetic and electronic properties of the dominant antiferromagnetic phase, coinciding with the disappearance of superconductivity. These findings underline the strong correlation between antiferromagnetic order and superconductivity in phase-separated AxFe2-x/2Se2 (A = K, Rb, Cs) superconductors.

• Competition of magnetism and superconductivity in underdoped (Ba1-xKx)Fe2As2

  Authors: Marianne Rotter, Marcus Tegel, Inga Schellenberg, Falko M. Schappacher, Rainer Poettgen, Joachim Deisenhofer, Axel Guenther, Florian Schrettle, Alois Loidl, Dirk Johrendt

  01/2009;

  Polycrystalline samples of underdoped (Ba1-xKx)Fe2As2 (x<=0.4) were synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific heat and 57Fe-Moessbauer-spectroscopy. ThePolycrystalline samples of underdoped (Ba1-xKx)Fe2As2 (x<=0.4) were synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific heat and 57Fe-Moessbauer-spectroscopy. The structural phase transition from tetragonal to orthorhombic lattice symmetry shifts towards lower temperatures, becomes less pronounced at x = 0.1-0.2 and is no longer present at x = 0.3. Bulk superconductivity is observed in all samples except (Ba0.9K0.1)Fe2As2 by resistivity and magnetic susceptibility measurements. Specific heat data show a broad SDW phase transition in (Ba0.9K0.1)Fe2As2, which is hardly discernible in (Ba0.8K0.2)Fe2As2. No SDW anomaly is found in the specific heat of optimally doped (Ba0.6K0.4)Fe2As2, where C changes by 0.1 J/K at Tc = 37.3 K. 57Fe-Moessbauer-spectra show full magnetic hyperfine field splitting, indicative of antiferromagnetic ordering at 4.2 K in samples with x = 0-0.2, but zero magnetic hyperfine field in samples with x = 0.3. The spectra of (Ba0.9K0.1)Fe2As2 and (Ba0.8K0.2)Fe2As2 in the phase transition regions are temperature-dependent superpositions of magnetic and non-magnetic components, caused by inhomogeneous potassium distribution. Our results suggest the co-existence of AF magnetic ordering and superconductivity without mesoscopic phase separation in the underdoped region and show unambiguously homogeneous superconducting phases close to optimal doping. This is in contrast to recently reported results about single crystal (Ba1-xKx)Fe2As2.

• A U(V) chalcogenide: synthesis, structure, and characterization of K2Cu3US5.

  Authors: Danielle L Gray, Lisa A Backus, Hans-Albrecht Krug von Nidda, S Skanthakumar, Alois Loidl, L Soderholm, James A Ibers

  Inorganic chemistry. 09/2007; 46(17):6992-6.

  The compound K2Cu3US5 was obtained by the reaction of K2S, UCl4, CuCl, and S at 973 K. K2Cu3US5 crystallizes in a new structure type in space group Cmcm of the orthorhombic system in a cell ofThe compound K2Cu3US5 was obtained by the reaction of K2S, UCl4, CuCl, and S at 973 K. K2Cu3US5 crystallizes in a new structure type in space group Cmcm of the orthorhombic system in a cell of dimensions a = 3.9374(6) A, b = 13.813(2) A, c = 17.500(3) A, and V = 951.8(2) A3 at 153 K. The structure comprises (2)(infinity)[UCu3S52-] slabs separated by K+ cations. The slabs are built from CuS4 tetrahedra and US6 octahedra. Their connectivity differs from other known octahedral/tetrahedral packing patterns. In the temperature range 130-300 K the compound exhibits Curie-Weiss magnetic behavior with mu(eff) = 2.45(8) mu(B). This result together with both the bond distances and bond valence calculations and the absence of a Cu2+ ESR signal support the formulation of the above compound as K+2Cu+3U5+S2-5.

• Formation of Zn(1-x)MnxS nanowires within mesoporous silica of different pore sizes.

  Authors: Felix J Brieler, Petra Grundmann, Michael Fröba, Limei Chen, Peter J Klar, Wolfram Heimbrodt, Hans-Albrecht Krug von Nidda, Thomas Kurz, Alois Loidl

  Journal of the American Chemical Society. 02/2004; 126(3):797-807.

  Arrays of highly ordered Zn(1-x)MnxS quantum wires with x ranging from 0.01 to 0.3 and with lateral dimensions of 3, 6, and 9 nm were synthesized within mesoporous SiO2 host structures of the MCM-41Arrays of highly ordered Zn(1-x)MnxS quantum wires with x ranging from 0.01 to 0.3 and with lateral dimensions of 3, 6, and 9 nm were synthesized within mesoporous SiO2 host structures of the MCM-41 and SBA-15 type. The hexagonal symmetry of these arrays (space group p6m) and the high degree of order was confirmed by X-ray diffraction and transmission electron microscopy (TEM) studies. Physisorption measurements show the progressive filling of the pores of the SiO2 host structures, while TEM and Raman studies reveal the wire-like character of the incorporated Zn(1-x)MnxS nanostructures. X-ray absorption near-edge structure, extended X-ray absorption fine structure, photoluminescence excitation (PLE), and electron paramagnetic resonance studies confirm the good crystalline quality of the incorporated Zn(1-x)MnxS guest species and, in particular, that the Mn2+ ions are randomly distributed and are situated on tetrahedrally coordinated cation sites of the Zn(1-x)MnxS wires for all x up to 0.3. The amount of Mn2+ ions loosely bound to the surface of the Zn(1-x)MnxS nanowires is less than 4% of the total Mn content even for the 3 nm nanostructures up to the highest Mn content of x = 0.3. The effects of the reduction of the lateral dimensions on electronic properties of the diluted magnetic semiconductor were studied by PLE spectroscopy. Due to the quantum confinement of the excitons in the wires an increase of the direct band gap with decreasing particle size is observed.

• Interplay of superexchange and orbital degeneracy in Cr-doped LaMnO3

  Authors: Joachim Deisenhofer, Michalis Paraskevopoulos, Hans-Albrecht Krug von Nidda, Alois Loidl

  07/2002;

  We report on structural, magnetic and Electron Spin Resonance (ESR) investigations in the manganite system LaMn_{1-x}Cr_{x}O_{3} (x<=0.5). Upon Cr-doping we observe a reduction of the Jahn-TellerWe report on structural, magnetic and Electron Spin Resonance (ESR) investigations in the manganite system LaMn_{1-x}Cr_{x}O_{3} (x<=0.5). Upon Cr-doping we observe a reduction of the Jahn-Teller distortion yielding less distorted orthorhombic structures. A transition from the Jahn-Teller distorted O' to the pseudocubic O phase occurs between 0.3<x<0.4. A clear connection between this transition and the doping dependence of the magnetic and ESR properties has been observed. The effective moments determined by ESR seem reduced with respect to the spin-only value of both Mn^{3+} and Cr^{3+} ions.

• Ordered arrays of II/VI diluted magnetic semiconductor quantum wires: formation within mesoporous MCM-41 silica.

  Authors: Felix J Brieler, Michael Fröba, Limei Chen, Peter J Klar, Wolfram Heimbrodt, NiddaHans-AlbrechtKrug Von, Alois Loidl

  Chemistry (Weinheim an der Bergstrasse, Germany). 01/2002; 8(1):185-94.

  We present a novel way of synthesising highly ordered arrays of hollow Cd(1-x)Mn(x)S quantum wires with lateral dimensions of 3-4 nm separated by 1-2 nm SiO2 barriers by forming Cd(1-x)Mn(x)S (0 < orWe present a novel way of synthesising highly ordered arrays of hollow Cd(1-x)Mn(x)S quantum wires with lateral dimensions of 3-4 nm separated by 1-2 nm SiO2 barriers by forming Cd(1-x)Mn(x)S (0 < or = x < or = 1) semiconductors inside the pore system of mesoporous MCM-41 SiO2 host structures. X-ray diffraction and transmission electron microscopy (TEM) studies reveal the hexagonal symmetry of these arrays (space group p6m) and confirm the high degree of order. Physisorption measurements show the filling of the pores of the MCM-41 SiO2. The X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), electron paramagentic resonance (EPR), and Raman studies confirm the good crystalline quality of the incorporated (Cd,Mn)S guest. The effects of reducing the lateral dimensions on the magnetic and electronic properties of the diluted magnetic semiconductor were studied by photoluminescence (PL) and PL excitation spectroscopy and by SQUID and EPR measurements in the temperature range 2-400 K. Due to the quantum confinement of the excitons in the wires, an increase of about 200 meV in the direct band gap was observed. In addition, the p-d hybridisation-related bowing of the band gap as a function of Mn concentration in the wires is much stronger than in the bulk. This effect is related to the increase in the band gap due to quantum confinement, which shifts the p-like valence band edge closer to the 3d-related states of Mn in the valence band. Thus, the p-d hybridisation and the strength of the band gap bowing are increased. Compared to bulk (II,Mn)VI compounds, antiferromagnetic coupling between the magnetic moments of the Mn2+ ions is weaker. For the samples with high Mn concentrations (x > 0.8) this leads to a suppression of the phase transition of the Mn system from paramagnetic to antiferromagnetic. This effect can be explained by the fact that the lateral dimensions of the wires are smaller than the magnetic length scale of the antiferromagnetic ordering.