380 research outputs found
Biological wastewater treatment in aeration tanks
Development of mathematical model for prediction of output parameters of aeration
tank with account of dissolved oxygen, oxygen , sludge, substrate transfer and biological treatment. The mathematical model may be used in predicting the effectiveness of aeration tank under different regimes of work
Magnetic susceptibility of the frustrated spinels ZnCr2O4, MgCr2O4 and CdCr2O4
We analyzed the magnetic susceptibilities of several Cr spinels using two
recent models for the geometrically frustrated pyrochlore lattice, the Quantum
Tetrahedral Mean Field model and a Generalized Constant Coupling model. Both
models can describe the experimental data for ACr2 O4 (with A = Zn, Mg, and Cd)
satisfactorily, with the former yielding a somewhat better agreement with
experiment for A = Zn, Mg. The obtained exchange constants for nearest and
next-nearest neighbors are discussed.Comment: 4 pages, 1 figure, 1 table, conferenc
Evolution of optical properties of chromium spinels CdCrO, HgCrS, and ZnCrSe under high pressure
We report pressure-dependent reflection and transmission measurements on
ZnCrSe, HgCrS, and CdCrO single crystals at room
temperature over a broad spectral range 200-24000 cm. The pressure
dependence of the phonon modes and the high-frequency electronic excitations
indicates that all three compounds undergo a pressure-induced structural phase
transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for
CdCrO, HgCrS, and ZnCrSe, respectively. The
eigenfrequencies of the electronic transitions are very close to the expected
values for chromium crystal-field transitions. In the case of the chalcogenides
pressure induces a red shift of the electronic excitation which indicates a
strong hybridization of the Cr d-bands with the chalcogenide bands.Comment: Accepted for publication in Phys. Rev.
Low-temperature structural transition in FeCr_2S_4
Transmission electron microscopy studies of [110] and [111] oriented
FeCr_2S_4 single crystals at different temperatures reveal a structural
transition at low temperatures indicating a cubic-to-triclinic symmetry
reduction within crystallographic domains. The overall crystal symmetry was
found to be reduced from Fd3m to F-43m. The triclinic distortions were
suggested to result from the combined actions of tetragonal distortions due to
the Jahn-Teller active Fe^2+ ions and trigonal distortions due to a
displacement of the Cr^3+ ions in the direction.Comment: 4 pages, 8 figure
Tuning orbital-selective correlation effects in superconducting RbFeSeS
We report on terahertz time-domain spectroscopy on superconducting and
metallic iron chalcogenides RbFeSeS. The
superconducting transition is reduced from 32 K () to 22 K
(), and finally suppressed () by isoelectronic substitution of Se
with S. Dielectric constant and optical conductivity exhibit a
metal-to-insulator transition associated with an orbital-selective Mott phase.
This orbital-selective Mott transition appears at higher temperature
with increasing sulfur content, identifying sulfur substitution as an efficient
parameter to tune orbital-dependent correlation effects in iron-chalcogenide
superconductors. The reduced correlations of the charge carriers can
account for the suppression of the superconductivity and the pseudogap-like
feature between and that was observed for .Comment: 6 pages, 4 figure
Multiferroicity and colossal magneto-capacitance in Cr-thiospinels
The sulfur based Cr-spinels RCr2S4 with R = Cd and Hg exhibit the coexistence
of ferromagnetic and ferroelectric properties together with a pronounced
magnetocapacitive coupling. While in CdCr2S4 purely ferromagnetic order is
established, in HgCr2S4 a bond-frustrated magnetic ground state is realized,
which, however, easily can be driven towards a ferromagnetic configuration in
weak magnetic fields. This paper shall review our recent investigation for both
compounds. Besides the characterization of the magnetic properties, the complex
dielectric permittivity was studied by means of broadband dielectric
spectroscopy as well as measurements of polarization hysteresis and
pyro-currents. The observed colossal magneto-capacitive effect at the magnetic
transition seems to be driven by an enormous variation of the relaxation
dynamics.Comment: 10 pages, 11 figure
Multiferroic behavior in CdCr2X4 (X = S, Se)
The recently discovered multiferroic material CdCr2S4 shows a coexistence of
ferromagnetism and relaxor ferroelectricity together with a colossal
magnetocapacitive effect. The complex dielectric permittivity of this compound
and of the structurally related CdCr2Se4 was studied by means of broadband
dielectric spectroscopy using different electrode materials. The observed
magnetocapacitive coupling at the magnetic transition is driven by enormous
changes of the relaxation dynamics induced by the development of magnetic
order
Thermodynamic evidence of fractionalized excitations in {\alpha}-RuCl3
Fractionalized excitations are of considerable interest in recent
condensed-matter physics. Fractionalization of the spin degrees of freedom into
localized and itinerant Majorana fermions are predicted for the Kitaev spin
liquid, an exactly solvable model with bond-dependent interactions on a
two-dimensional honeycomb lattice. As function of temperature, theory predicts
a characteristic two-peak structure of the heat capacity as fingerprint of
these excitations. Here we report on detailed heat-capacity experiments as
function of temperature and magnetic field in high-quality single crystals of
{\alpha}-RuCl3 and undertook considerable efforts to determine the exact phonon
background. We measured single-crystalline RhCl3 as non-magnetic reference and
performed ab-initio calculations of the phonon density of states for both
compounds. These ab-initio calculations document that the intrinsic phonon
contribution to the heat capacity cannot be obtained by a simple rescaling of
the nonmagnetic reference using differences in the atomic masses. Sizable
renormalization is required even for non-magnetic RhCl3 with its minute
difference from the title compound. In {\alpha}-RuCl3 in zero magnetic field,
excess heat capacity exists at temperatures well above the onset of magnetic
order. In external magnetic fields far beyond quantum criticality, when
long-range magnetic order is fully suppressed, the excess heat capacity
exhibits the characteristic two-peak structure. In zero field, the lower peak
just appears at temperatures around the onset of magnetic order and seems to be
connected with canonical spin degrees of freedom. At higher fields, beyond the
critical field, this peak is shifted to 10 K. The high-temperature peak located
around 50 K is hardly influenced by external magnetic fields, carries the
predicted amount of entropy, R/2 ln2, and may resemble remnants of Kitaev
physics
Colossal magnetocapacitance and colossal magnetoresistance in HgCr2S4
We present a detailed study of the dielectric and charge transport properties
of the antiferromagnetic cubic spinel HgCr2S4. Similar to the findings in
ferromagnetic CdCr2S4, the dielectric constant of HgCr2S4 becomes strongly
enhanced in the region below 60 - 80 K, which can be ascribed to polar
relaxational dynamics triggered by the onset of ferromagnetic correlations. In
addition, the observation of polarization hysteresis curves indicates the
development of ferroelectric order below about 70 K. Moreover, our
investigations in external magnetic fields up to 5 T reveal the simultaneous
occurrence of magnetocapacitance and magnetoresistance of truly colossal
magnitudes in this material.Comment: 4 pages, 4 figure
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