2,719 research outputs found
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
Spin-phonon coupling in antiferromagnetic chromium spinels
The temperature dependence of eigenfrequencies and intensities of the IR
active modes has been investigated for the antiferromagnetic chromium spinel
compounds CdCr2O4, ZnCr2O4, ZnCr2S4, ZnCr2Se4, and HgCr2S4 by IR spectroscopy
for temperatures from 5 K to 300 K. At the transition into the magnetically
ordered phases, and driven by spin-phonon coupling, most compounds reveal
significant splittings of the phonon modes. This is true for geometrically
frustrated CdCr2O4, and ZnCr2O4, for bond frustrated ZnCr2S4 and for ZnCr2Se4,
which also is bond frustrated, but dominated by ferromagnetic exchange. The
pattern of splitting is different for the different compounds and crucially
depends on the nature of frustration and of the resulting spin order. HgCr2S4,
which is almost ferromagnetic, exhibits no splitting of the eigenfrequencies,
but shows significant shifts due to ferromagnetic spin fluctuations.Comment: 15 pages, 6 figure
Flow-acoustic Characterisation of a Cavity-based Combustor Configuration
This study concerns the flow-acoustic characterisation of a cavity-based combustor configuration. A well-validated numerical tool has been used to simulate the unsteady, two-dimensional reacting flow. Initially, a conventional flow over a cavity with dimensions and conditions corresponding to a compact cavity combustor was studied. Cavity mass injections in the form of fuel and air injections required for trapped vortex formation were then employed and the resonance features of this configuration were studied. The results indicate that the cavity depth mode resonance mechanism is dominant at the conditions studied in this work and that the oscillation frequencies do not change with cavity air injection. This observation is important since it implies that the only important variable which can alter resonant frequencies is the cavity depth. With combustion, the pressure oscillation amplitude was observed to increases significantly due to periodic entrainment of the cavity air jet and fluctuation of fuel-air mixture composition to produce highly fluctuating heat-release rates. The underlying mechanisms of the unsteady flow in the cavity combustor identified in this study indicate the strong dependence of the acoustics on the cavity injection strategies.Defence Science Journal, 2011, 61(6), pp.523-528, DOI:http://dx.doi.org/10.14429/dsj.61.87
Optical phonons, spin correlations, and spin-phonon coupling in the frustrated pyrochlore magnets CdCr2O4 and ZnCr2O4
We report on infrared, Raman, magnetic susceptibility, and specific heat
measurements on CdCr2O4 and ZnCr2O4 single crystals. We estimate the
nearest-neighbor and next-nearest neighbor exchange constants from the magnetic
susceptibility and extract the spin-spin correlation functions obtained from
the magnetic susceptibility and the magnetic contribution to the specific heat.
By comparing with the frequency shift of the infrared optical phonons above TN
, we derive estimates for the spin-phonon coupling constants in these systems.
The observation of phonon modes which are both Raman and infrared active
suggest the loss of inversion symmetry below the Neel temperature in CdCr2O4 in
agreement with theoretical predictions by Chern and coworkers [Phys. Rev. B 74,
060405 (2006)]. In ZnCr2O4 several new modes appear below TN, but no phonon
modes could be detected which are both Raman and infrared active indicating the
conservation of inversion symmetry in the low temperature phase.Comment: 11 pages, 13 figure
Dimensional Reduction applied to QCD at three loops
Dimensional Reduction is applied to \qcd{} in order to compute various
renormalization constants in the \drbar{} scheme at higher orders in
perturbation theory. In particular, the function and the anomalous
dimension of the quark masses are derived to three-loop order. Special emphasis
is put on the proper treatment of the so-called -scalars and the
additional couplings which have to be considered.Comment: 13 pages, minor changes, references adde
Polar phonons and spin-phonon coupling in HgCr2S4 and CdCr2S4
Polar phonons of HgCr2S4 and CdCr2S4 are studied by far-infrared spectroscopy
as a function of temperature and external magnetic field. Eigenfrequencies,
damping constants, effective plasma frequencies and Lyddane-Sachs-Teller
relations, and effective charges are determined. Ferromagnetic CdCr2S4 and
antiferromagnetic HgCr2S4 behave rather similar. Both compounds are dominated
by ferromagnetic exchange and although HgCr2S4 is an antiferromagnet, no phonon
splitting can be observed at the magnetic phase transition. Temperature and
magnetic field dependence of the eigenfrequencies show no anomalies indicating
displacive polar soft mode behavior. However, significant effects are detected
in the temperature dependence of the plasma frequencies indicating changes in
the nature of the bonds and significant charge transfer. In HgCr2S4 we provide
experimental evidence that the magnetic field dependence of specific polar
modes reveal shifts exactly correlated with the magnetization showing
significant magneto-dielectric effects even at infrared frequencies.Comment: 8 pages, 8 figure
Fiji Flag Change: Social Media Responds
This In Brief seeks to discuss and assess the public social media responses from the two main social media sites that were publicised by the Fijian government, for public feedback on the flag change proposal
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