193 research outputs found
X-ray scattering studies of charge stripes in transition-metal oxides
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
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
We re-investigate the magnetically frustrated, {\it
diamond-lattice-antiferromagnet} spinels FeAlO and MnAlO using
magnetization measurements and diffuse scattering of polarized neutrons. In
FeAlO, 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
FeAlO. 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} factor points towards orbital contributions from Fe.
This is also supported by a Curie-Weiss analysis of the magnetic
susceptibility. MnAlO, on the contrary, undergoes a magnetic phase
transition into a long-range ordered state below 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
FeAlO. Results of the present work supports the importance of spin-spin
correlations in understanding magnetic response of frustrated magnets like
-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
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
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
<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
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
We use polarized inelastic neutron scattering (INS) to study spin excitations
of optimally hole-doped superconductor BaKFeAs
( K).
In the normal state, the imaginary part of the dynamic susceptibility,
, shows magnetic anisotropy for energies below
7 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 5 and 0.75 meV
open at wave vectors and , respectively, with a
broad neutron spin resonance at 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
BaFeAs is due to their proximity to the AF ordered BaFeAs where
spin anisotropy exists below .Comment: 5 pages, 4 figure
Intermultiplet transitions and magnetic long-range order in Sm-based pyrochlores
We present bulk and neutron scattering measurements performed on the
isotopically enriched and
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 multiplets of the 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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