8,737 research outputs found
Field Induced Magnetic Ordering and Single-ion Anisotropy in the Quasi-1D Haldane Chain Compound SrNi2V2O8: A Single Crystal investigation
Field-induced magnetic ordering in the Haldane chain compound
SrNiVO and effect of anisotropy have been investigated using
single crystals. Static susceptibility, inelastic neutron scattering,
high-field magnetization, and low temperature heat-capacity studies confirm a
non-magnetic spin-singlet ground state and a gap between the singlet ground
state and triplet excited states. The intra-chain exchange interaction is
estimated to be 0.1 meV. Splitting of the dispersions into two
modes with minimum energies 1.57 and 2.58 meV confirms the existence of
single-ion anisotropy . The value of {\it D} is estimated to be
meV and the easy axis is found to be along the
crystallographic {\it c}-axis. Field-induced magnetic ordering has been found
with two critical fields [0.2 T and
0.5 T at 4.2 K]. Field-induced
three-dimensional magnetic ordering above the critical fields is evident from
the heat-capacity, susceptibility, and high-field magnetization study. The
Phase diagram in the {\it H-T} plane has been obtained from the high-field
magnetization. The observed results are discussed in the light of theoretical
predictions as well as earlier experimental reports on Haldane chain compounds
Heat capacity study of BaFeAs: effects of annealing
Heat-capacity, X-ray diffraction, and resistivity measurements on a
high-quality BaFeAs sample show an evolution of the
magneto-structural transition with successive annealing periods. After a 30-day
anneal the resistivity in the (ab) plane decreases by more than an order of
magnitude, to 12 cm, with a residual resistance ratio 36; the
heat-capacity anomaly at the transition sharpens, to an overall width of less
than K, and shifts from 135.4 to 140.2 K. The heat-capacity anomaly in both the
as-grown sample and after the 30-day anneal shows a hysteresis of 0.15 K,
and is unchanged in a magnetic field H = 14 T. The X-ray and
heat-capacity data combined suggest that there is a first order jump in the
structural order parameter. The entropy of the transition is reported
Forecasting Volatility of Dhaka Stock Exchange: Linear Vs Non-linear Models
Prior information about a financial market is very essential for investor to invest money on parches share from the stock market which can strengthen the economy. The study examines the relative ability of various models to forecast daily stock indexes future volatility. The forecasting models that employed from simple to relatively complex ARCH-class models. It is found that among linear models of stock indexes volatility, the moving average model ranks first using root mean square error, mean absolute percent error, Theil-U and Linex loss function criteria. We also examine five nonlinear models. These models are ARCH, GARCH, EGARCH, TGARCH and restricted GARCH models. We find that nonlinear models failed to dominate linear models utilizing different error measurement criteria and moving average model appears to be the best. Then we forecast the next two months future stock index price volatility by the best (moving average) model
Streamflow disaggregation: a nonlinear deterministic approach
International audienceThis study introduces a nonlinear deterministic approach for streamflow disaggregation. According to this approach, the streamflow transformation process from one scale to another is treated as a nonlinear deterministic process, rather than a stochastic process as generally assumed. The approach follows two important steps: (1) reconstruction of the scalar (streamflow) series in a multi-dimensional phase-space for representing the transformation dynamics; and (2) use of a local approximation (nearest neighbor) method for disaggregation. The approach is employed for streamflow disaggregation in the Mississippi River basin, USA. Data of successively doubled resolutions between daily and 16 days (i.e. daily, 2-day, 4-day, 8-day, and 16-day) are studied, and disaggregations are attempted only between successive resolutions (i.e. 2-day to daily, 4-day to 2-day, 8-day to 4-day, and 16-day to 8-day). Comparisons between the disaggregated values and the actual values reveal excellent agreements for all the cases studied, indicating the suitability of the approach for streamflow disaggregation. A further insight into the results reveals that the best results are, in general, achieved for low embedding dimensions (2 or 3) and small number of neighbors (less than 50), suggesting possible presence of nonlinear determinism in the underlying transformation process. A decrease in accuracy with increasing disaggregation scale is also observed, a possible implication of the existence of a scaling regime in streamflow
Consequences of critical interchain couplings and anisotropy on a Haldane chain
Effects of interchain couplings and anisotropy on a Haldane chain have been
investigated by single crystal inelastic neutron scattering and density
functional theory (DFT) calculations on the model compound SrNiVO.
Significant effects on low energy excitation spectra are found where the
Haldane gap (; where is the intrachain exchange
interaction) is replaced by three energy minima at different antiferromagnetic
zone centers due to the complex interchain couplings. Further, the triplet
states are split into two branches by single-ion anisotropy. Quantitative
information on the intrachain and interchain interactions as well as on the
single-ion anisotropy are obtained from the analyses of the neutron scattering
spectra by the random phase approximation (RPA) method. The presence of
multiple competing interchain interactions is found from the analysis of the
experimental spectra and is also confirmed by the DFT calculations. The
interchain interactions are two orders of magnitude weaker than the
nearest-neighbour intrachain interaction = 8.7~meV. The DFT calculations
reveal that the dominant intrachain nearest-neighbor interaction occurs via
nontrivial extended superexchange pathways Ni--O--V--O--Ni involving the empty
orbital of V ions. The present single crystal study also allows us to
correctly position SrNiVO in the theoretical - phase
diagram [T. Sakai and M. Takahashi, Phys. Rev. B 42, 4537 (1990)] showing where
it lies within the spin-liquid phase.Comment: 12 pages, 12 figures, 3 tables PRB (accepted). in Phys. Rev. B (2015
Experimental studies of the cross-excitation instability in a relativistic backward wave oscillator
Our group first reported the operation of a relativistic backward wave oscillator (BWO) in the so-called âcross-excitationâ regime in 1998. This instability, whose general properties were predicted earlier through numerical studies, was a consequence of using a particularly shallow rippled-wall waveguide (slow wave structure â SWS) that was installed in the experiment to diagnose pulse shortening in a long pulse electron beam-driven high power microwave (HPM) source. This particular SWS was required to accommodate laser interferometry measurements during the course of microwave generation. Since those early experiments we have further studied this regime in greater detail using two different SWS lengths. We have invoked time-frequency analysis, the smoothed-pseudo Wigner-Ville distribution in particular, to interpret the heterodyned signals of the radiated power measurements. These recent results are consistent with earlier theoretical predictions for the onset, voltage scaling, and general behavior for this instability. This paper presents data for a relativistic BWO operating in the single frequency regime for two axial modes, operating in the cross-excitation regime, and discusses the interpretation of the data, as well as the methodology used for its analysis. Although operation in the cross-excitation regime is typically avoided due to its poorer efficiency, we discuss how it may be exploited in HPM effects studies
From Newton's Laws to the Wheeler-DeWitt Equation
This is a pedagogical paper which explains some ideas in cosmology at a level
accessible to undergraduate students. It does not use general relativity, but
uses the ideas of Newtonian cosmology worked out by Milne and McCrea. The
cosmological constant is also introduced within a Newtonian framework.
Following standard quantization procedures the Wheeler-DeWitt equation in the
minisuperspace approximation is derived for empty and non-empty universes.Comment: 13 pages, 1 figur
Asymmetric Thermal Lineshape Broadening in a Gapped 3-Dimensional Antiferromagnet - Evidence for Strong Correlations at Finite Temperature
It is widely believed that magnetic excitations become increasingly
incoherent as temperature is raised due to random collisions which limit their
lifetime. This picture is based on spin-wave calculations for gapless magnets
in 2 and 3 dimensions and is observed experimentally as a symmetric Lorentzian
broadening in energy. Here, we investigate a three-dimensional dimer
antiferromagnet and find unexpectedly that the broadening is asymmetric -
indicating that far from thermal decoherence, the excitations behave
collectively like a strongly correlated gas. This result suggests that a
temperature activated coherent state of quasi-particles is not confined to
special cases like the highly dimerized spin-1/2 chain but is found generally
in dimerized antiferromagnets of all dimensionalities and perhaps gapped
magnets in general
Magnetic excitations in the S = 1/2 antiferromagnetic-ferromagnetic chain compound BaCu2V2O8 at zero and finite temperature
Unlike most quantum systems which rapidly become incoherent as temperature is
raised, strong correlations persist at elevated temperatures in dimer
magnets, as revealed by the unusual asymmetric lineshape of their excitations
at finite temperatures. Here we quantitatively explore and parameterize the
strongly correlated magnetic excitations at finite temperatures using the high
resolution inelastic neutron scattering on the model compound
BaCuVO which we show to be an alternating
antiferromagnetic-ferromagnetic spin chain. Comparison to state of the
art computational techniques shows excellent agreement over a wide temperature
range. Our findings hence demonstrate the possibility to quantitatively predict
coherent behavior at elevated temperatures in quantum magnets.Comment: 5 pages + 6 pages supplement; problems with list of references are
fixe
Raman effect in AlGaAs waveguides for subpicosecond pulses
The Raman effect in semiconductor waveguides below halfâgap is studied both experimentally and numerically. We report the depolarized Raman gain spectra up to 300 cmâ1 in Al0.24Ga0.76As at pump wavelengths of 0.515 and 1.55 ÎŒm from the measurement of the absolute Raman scattering cross sections using GaAs as a reference scatterer. In addition, the coupled propagation equations for the AlGaAs waveguides are modified to include the Raman effect. By solving the coupled propagation equations numerically, we verify that the energy transfer between two orthogonally polarized pulses demonstrated in previous pumpâprobe experiments [M. N. Islam et al., J. Appl. Phys. 71, 1927 (1992)] is caused by Raman effect. We also show numerically that the Raman effect induces spectral distortions on the pulses, and the energy transfer is inversely proportional to the pulse widths. The energy transfer results in a severe crossâtalk problem for subâpicosecond pulses in AlGaAs waveguides. For example, the energy exchange is about 30% for 300 fs pulses under Ï phase shift conditions. Therefore, the Raman effect limits the performance of semiconductor waveguides in optical switching applications for subâpicosecond pulses. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71265/2/JAPIAU-78-4-2198-1.pd
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