193 research outputs found

    X-ray scattering studies of charge stripes in transition-metal oxides

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    This thesis describes the development of single crystal x-ray scattering applied to the study of charge stripes in some exotic transition-metal oxides. Charge stripes in the cuprates and manganites are strongly associated with high T(_c) superconductivity and colossal magnetoresistance, and can be characterised by the satellite reflections located around related Bragg peaks below the charge ordering transition temperature ( T(_co). The intensities of these extremely weak satellite reflections are between l0(^-4) and 10(^-6) less than those of Bragg reflections, thus very high brilliant rotating-anode x-ray and synchrotron radiation sources are required to study them. The structure factors and correlation lengths of the charge stripes can be directly obtained by measurements of the integrated intensities and peak widths of the charge ordering satellite reflections as a function of temperature. The charge stripes in single crystals of Bi(_0.24)Ca(_o.76)MnO(_3), Nd(_0.5)Sr(_0.5)MnO(_3) and La(_5/3)Sr(_1/3)NiO(_4) have been comprehensively characterised using both laboratory and synchrotron radiation x-ray scattering. The measurements on Bi(_0.24)Ca(_0.76)MnO(_3) directly demonstrate a strong relationship between the charge stripe ordering and the first-order structural phase transition, which is common in many perovskite manganites. Direct evidence for the existence of perpendicular charge-ordered domains in manganites was obtained for the first time by x-ray scattering in Nd(_0.5)Sr(_0.5)MnO(_3), and this result demonstrates that charge stripes have complex structures in the mesoscopic length scale. Scattering was also observed at the positions in reciprocal space associated with spin ordering into stripes. These showed evidence of different critical behaviour than the corresponding charge stripe satellites. The quasi two-dimensional feature of the charge stripes in La(_5/3)Sr(_1/3)NiO(_4) was successfully demonstrated by the critical exponents of the charge stripe melting and by the measurements of their m-plane and out-of-plane correlation lengths. Experimental evidence for the existence of electronic liquid crystal phases in La(_5/3)Sr(_1/3)NiO(_4) was obtained, in agreement with recent theoretical prediction

    Weak magnetism and the Mott-state of vanadium in superconducting Sr2VO3FeAs

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    We report neutron scattering data and DFT calculations of the stoichiometric iron-arsenide superconductor Sr2VO3FeAs. Rietveld refinements of neutron powder patterns confirm the ideal composition without oxygen deficiencies. Experiments with polarized neutrons prove weak magnetic ordering in the V-sublattice of Sr2VO3FeAs at ~ 45 K with a probable propagation vector q = (1/8,1/8,0). The ordered moment of ~ 0.1 muB is too small to remove the V-3d bands from the Fermi level by magnetic exchange splitting, and much smaller than predicted from a recent LDA+U study. By using DFT calculations with a GGA+EECE functional we recover the typical quasi-nested Fermi-surface even without magnetic moment. From this we suggest that the V-atoms are in a Mott-state where the electronic correlations are dominated by on-site Coulomb-repulsion which shifts the V-3d states away from the Fermi energy. Our results are consistent with photoemission data and clearly reveal that Sr2VO3FeAs is a typical iron-arsenide superconductor with quasi-nested hole- and electron-like Fermi surface sheets, and constitutes no new paradigm. We suggest that intrinsic electron-doping through V3+/V4+ mixed valence is responsible for the absence of SDW ordering.Comment: 5 pages, 5 figure

    Approaching the Ground State of Frustrated A-site Spinels: A Combined Magnetization and Polarized Neutron Scattering Study

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    We re-investigate the magnetically frustrated, {\it diamond-lattice-antiferromagnet} spinels FeAl2_2O4_4 and MnAl2_2O4_4 using magnetization measurements and diffuse scattering of polarized neutrons. In FeAl2_2O4_4, macroscopic measurements evidence a "cusp" in zero field-cooled susceptibility around 13~K. Dynamic magnetic susceptibility and {\it memory effect} experiments provide results that do not conform with a canonical spin-glass scenario in this material. Through polarized neutron scattering studies, absence of long-range magnetic order down to 4~K is confirmed in FeAl2_2O4_4. By modeling the powder averaged differential magnetic neutron scattering cross-section, we estimate that the spin-spin correlations in this compound extend up to the third nearest-neighbour shell. The estimated value of the Land\'{e} gg factor points towards orbital contributions from Fe2+^{2+}. This is also supported by a Curie-Weiss analysis of the magnetic susceptibility. MnAl2_2O4_4, on the contrary, undergoes a magnetic phase transition into a long-range ordered state below ≈\approx 40~K, which is confirmed by macroscopic measurements and polarized neutron diffraction. However, the polarized neutron studies reveal the existence of prominent spin-fluctuations co-existing with long-range antiferromagnetic order. The magnetic diffuse intensity suggests a similar short range order as in FeAl2_2O4_4. Results of the present work supports the importance of spin-spin correlations in understanding magnetic response of frustrated magnets like AA-site spinels which have predominant short-range spin correlations reminiscent of the "spin liquid" state.Comment: 10 pages, 10 figures, double-column, accepted in Phys. Rev. B, 201

    Non-stoichiometry and the magnetic structure of Sr2CrO3FeAs

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    The iron arsenide Sr2CrO3FeAs with the tetragonal Sr2GaO3CuS-type structure was synthesized and its crystal structure re-determined by neutron powder diffraction. In contrast to previous X-ray crystallographic studies, a mixed occupancy of chromium and iron was found within the FeAs4/4 layer (93+/-1% Fe : 7+/-1% Cr). We suggest that the partial Cr-doping at the Fe site is the reason for the absence of a spin-density wave anomaly and superconductivity in this compound. Additional experiments via neutron polarization analysis revealed short-range spin correlations below ~100 K and long-range antiferromagnetic ordering below T_N = 36 K with a magnetic propagation vector of q = (1/2, 1/2, 0). The Cr3+ ions form a collinear magnetic structure of the C-type in the magnetic space group C_Pmma' (a' = a-b, b'=a+b, c'=c), where Cr3+-ions occupy the 4g (0, 1/4, z) Wyckoff position. The magnetic moments are aligned along the orthorhombic a'-axis. At 3.5 K, an ordered magnetic moment of 2.75+/-0.05 mu_B for the Cr3+-sublattice was refined.Comment: 6 pages, 6 figure

    Suppression of superconductivity by V-doping and possible magnetic order in Sr2VO3FeAs

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    Superconductivity at 33 K in Sr2VO3FeAs is completely suppressed by small amounts of V-doping in Sr2VO3[Fe0.93(+/-0.01)V0.07(+/-0.01)]As. The crystal structures and exact stoichiometries are determined by combined neutron- and x-ray powder diffraction. Sr2VO3FeAs is shown to be very sensitive to Fe/V mixing, which interferes with or even suppresses superconductivity. This inhomogeneity may be intrinsic and explains scattered reports regarding Tc and reduced superconducting phase fractions in Sr2VO3FeAs. Neutron diffraction data collected at 4 K indicates incommensurate mag- netic ordering of the V-sublattice with a propagation vector q = (0,0,0.306). This suggests strongly correlated vanadium, which does not contribute significantly to the Fermi surface of Sr2VO3FeAs.Comment: text revised, magnetic q-vector added, one reference added 4 pages, 4 figure

    Evaluation of microalgae production coupled with wastewater treatment

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    <p>In the present study, the feasibility of microalgae production coupled with wastewater treatment was assessed. Continuous cultivation of <i>Chlorella sorokiniana</i> with wastewater was tested in lab-scale flat-panel photobioreactors. Nitrogen and phosphorus removals were found to be inversely proportional to the four dilution rates, while chemical oxygen demand removal was found to be 50% at all the tested conditions. The biomass obtained at the highest dilution rate was characterized for its content of lipids, proteins and pigments. The average yields of fatty acid methyl esters (FAMEs), protein, lutein, chlorophylls and β-carotene was 62.4, 388.2, 1.03, 11.82 and 0.44 mg per gram dry biomass, respectively. Economic analysis revealed that potentially more than 70% of revenue was from the production of pigments, that is, chlorophyllin (59.6%), lutein (8.9%) and β-carotene (5.0%) while reduction in discharging costs of the treated wastewaters could account for 19.6% of the revenue. Due to the low market price of biodiesel, the revenue from the above was found to be the least profitable (1.4%). Even when combining all these different revenues, this cultivation strategy was found with the current prices to be uneconomical. Power consumption for artificial light was responsible for the 94.5% of the production costs.</p

    Uniaxial and hydrostatic pressure effects in alpha-RuCl3 single crystals via thermal-expansion measurements

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    We present high-resolution thermal-expansion and specific-heat measurements of single crystalline alpha-RuCl3. An extremely hysteretic structural transition expanding over 100 K is observed by thermal- expansion along both crystallographic axes, which we attribute to a change of stacking sequence of the RuCl3 layers. Three magnetic transitions are observed, which we link to the different stacking sequences. Using our data and thermodynamic relations, we derive the uniaxial and hydrostatic pressure derivatives of all three magnetic transitions. Our results demonstrate that magnetic order should be totally suppressed by very moderate pressures of 0.3 GPa to 0.9 GPa. Finally, we discuss why our results differ from recent hydrostatic pressure measurements and suggest a possible route to reaching the spin-liquid state in alpha-RuCl3.Comment: 6 pages, 3 figure

    Magnetic anisotropy in hole-doped superconducting Ba 0.67K 0.33Fe 2As2 probed by polarized inelastic neutron scattering

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    We use polarized inelastic neutron scattering (INS) to study spin excitations of optimally hole-doped superconductor Ba0.67_{0.67}K0.33_{0.33}Fe2_2As2_{2} (Tc=38T_c=38 K). In the normal state, the imaginary part of the dynamic susceptibility, χ′′(Q,ω)\chi^{\prime\prime}(Q,\omega), shows magnetic anisotropy for energies below ∼\sim7 meV with c-axis polarized spin excitations larger than that of the in-plane component. Upon entering into the superconducting state, previous unpolarized INS experiments have shown that spin gaps at ∼\sim5 and 0.75 meV open at wave vectors Q=(0.5,0.5,0)Q=(0.5,0.5,0) and (0.5,0.5,1)(0.5,0.5,1), respectively, with a broad neutron spin resonance at Er=15E_r=15 meV. Our neutron polarization analysis reveals that the large difference in spin gaps is purely due to different spin gaps in the c-axis and in-plane polarized spin excitations, resulting resonance with different energy widths for the c-axis and in-plane spin excitations. The observation of spin anisotropy in both opitmally electron and hole-doped BaFe2_2As2_2 is due to their proximity to the AF ordered BaFe2_2As2_2 where spin anisotropy exists below TNT_N.Comment: 5 pages, 4 figure

    Intermultiplet transitions and magnetic long-range order in Sm-based pyrochlores

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    We present bulk and neutron scattering measurements performed on the isotopically enriched 154Sm2Ti2O7^{154}\mathrm{Sm_2Ti_2O_7} and 154Sm2Sn2O7^{154}\mathrm{Sm_2Sn_2O_7} samples. Both compounds display sharp heat capacity anomalies, at 350 mK and 440 mK, respectively. Inelastic neutron scattering measurements are employed to determine the crystalline electric field (CEF) level scheme, which includes transitions between the ground-state and first excited JJ multiplets of the Sm3+\mathrm{Sm}^{3+} ion. To further validate those results, the single-ion magnetic susceptibility of the compounds is calculated and compared with the experimental DC-susceptibility measured in low applied magnetic fields. It is demonstrated that the inclusion of intermultiplet transitions in the CEF analysis is fundamental to the understanding of the intermediate and, more importantly, low temperature magnetic behaviour of the Sm-based pyrochlores. Finally, the heat capacity anomaly is shown to correspond to the onset of an all-in-all-out long-range order in the stannate sample, while in the titanate a dipolar long-range order can be only indirectly inferred.Comment: 13 pages, 10 Figure
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