1,919 research outputs found
Schwinger-Boson Mean-Field Theory of Mixed-Spin Antiferromagnet
The Schwinger-boson mean-field theory is used to study the three-dimensional
antiferromagnetic ordering and excitations in compounds , a large
family of quasi-one-dimensional mixed-spin antiferromagnet. To investigate
magnetic properties of these compounds, we introduce a three-dimensional
mixed-spin antiferromagnetic Heisenberg model based on experimental results for
the crystal structure of . This model can explain the experimental
discovery of coexistence of Haldane gap and antiferromagnetic long-range order
below N\'{e}el temperature. Properties such as the low-lying excitations,
magnetizations of and rare-earth ions, N\'{e}el temperatures of different
compounds, and the behavior of Haldane gap below the N\'{e}el temperature are
investigated within this model, and the results are in good agreement with
neutron scattering experiments.Comment: 12 pages, 6 figure
Smaller sample sizes for phase II trials based on exact tests with actual error rates by trading-off their nominal levels of significance and power
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Complexes of stationary domain walls in the resonantly forced Ginsburg-Landau equation
The parametrically driven Ginsburg-Landau equation has well-known stationary
solutions -- the so-called Bloch and Neel, or Ising, walls. In this paper, we
construct an explicit stationary solution describing a bound state of two
walls. We also demonstrate that stationary complexes of more than two walls do
not exist.Comment: 10 pages, 2 figures, to appear in Physical Review
A strong-coupling expansion for the Hubbard model
We reconsider the strong-coupling expansion for the Hubbard model recently
introduced by Sarker and Pairault {\it et al.} By introducing slave particles
that act as projection operators onto the empty, singly occupied and doubly
occupied atomic states, the perturbation theory around the atomic limit
distinguishes between processes that do conserve or do not conserve the total
number of doubly occupied sites. This allows for a systematic expansion
that does not break down at low temperature ( being the intersite hopping
amplitude and the local Coulomb repulsion). The fermionic field becomes a
two-component field, which reflects the presence of the two Hubbard bands. The
single-particle propagator is naturally expressed as a function of a matrix self-energy. Furthermore, by introducing a time- and
space-fluctuating spin-quantization axis in the functional integral, we can
expand around a ``non-degenerate'' ground-state where each singly occupied site
has a well defined spin direction (which may fluctuate in time). This formalism
is used to derive the effective action of charge carriers in the lower Hubbard
band to first order in . We recover the action of the t-J model in the
spin-hole coherent-state path integral. We also compare our results with those
previously obtained by studying fluctuations around the large- Hartree-Fock
saddle point.Comment: 20 pages RevTex, 3 figure
Numerical Simulation of the Performance Characteristics of the Hybrid Closed Circuit Cooling Tower
The performance characteristics of the Hybrid Closed Circuit Cooling Tower (HCCCT) have been investigated applying computational fluid dynamics (CFD). Widely reported CFD techniques are applied to simulate the air-water two phase flow inside the HCCCT. The pressure drop and the cooling capacity were investigated from several perspectives. Three different transverse pitches were tested and found that a pitch of 45 mm had lower pressure drop. The CFD simulation indicated that when air is supplied from the side wall of the HCCCT, the pressure drop can be over predicted and the cooling capacity can be under predicted mainly due to the non-uniform air flow distribution across the coil bank. The cooling capacity in wet mode have been calculated with respect to wet-bulb temperature (WBT) and cooling water to air mass flow rates for different spray water volume flow rates and the results were compared to the experimental measurement and found to conform well for the air supply from the bottom end. The differences of the cooling capacity and pressure drop in between the CFD simulation and experimental measurement in hybrid mode were less than 5 % and 7 % respectively for the uniform air flow distribution
Data describing the eco-physiological responses of Elaeagnus angustifolia grown under contrasting regime of water and fertilizer in coal-mined spoils
To improve our understanding of how coal mining areas can be re-vegetated and ecosystem function restored, we examined the potential effects of five water (W) regimes (40, 50, 60, 70 and 80% of field capacity), five nitrogen (N) (0, 24, 60, 96 and 120 mg kg‒1 soil) and five phosphorus (P) fertilizer doses (0, 36, 90, 144 and 180 mg kg‒1 soil), which control the growth and development of Elaeagnus angustifolia under adverse environmental conditions. To optimize the W-N-P application rate, three factors and five levels of central composite design along with an optimization technique named response surface methodology were utilized. Here we provide data on root-shoot biomass ratio, leaf dry matter content, stomatal conductance, chlorophyll (Chl) a, Chl b, membrane stability index and soluble protein content of E. angustifolia. The data described in this article are available in Mendeley Data, DOI: 10.17632/2vfbrdxyf2.2 [1]. These data could be used to evaluate the improvement in growth performance of E. angustifolia subjected to various regimes of W, N and P. This dataset showed that E. angustifolia grew optimally in coal-mine spoils when irrigated at 66% of field capacity and supplemented with 74.0 mg N and 36.0 mg P kg‒1 soil. This could considerably help the success of revegetation in coal-mined degraded arid areas where W is scarce. This article contains data complementary to the main research entitled "Fine-tuning of soil water and nutrient fertilizer levels for the ecological restoration of coal-mined spoils using Elaeagnus angustifolia" in the Journal of Environmental Management (Roy et al., 2020)
Temperature dependence of the resistivity in the double-exchange model
The resistivity around the ferromagnetic transition temperature in the double
exchange model is studied by the Schwinger boson approach. The spatial spin
correlation responsible for scattering of conduction electrons are taken into
account by adopting the memory function formalism. Although the correlation
shows a peak lower than the transition temperature, the resistivity in the
ferromagnetic state monotonically increases with increasing temperature due to
a variation of the electronic state of the conduction electron. In the
paramagnetic state, the resistivity is dominated by the short range correlation
of scattering and is almost independent of the temperature. It is attributed to
a cancellation between the nearest-neighbor spin correlation, the fermion
bandwidth, and the fermion kinetic energy. This result implies the importance
of the temperature dependence of the electronic states of the conduction
electron as well as the localized spin states in both ferromagnetic and
paramagnetic phases.Comment: RevTex, 4 pages, 4 PostScript figures, To appear in Phys. Rev.
Conservation tillage and residue management improve soil health and crop productivity-Evidence from a rice-maize cropping system in Bangladesh.
The rice-maize (R-M) system is rapidly expanding in Bangladesh due to its greater suitability for diverse soil types and environments. The present conventional method of cultivating puddled transplanted rice and maize is input-intensive, decreases soil health through intense ploughing, and ultimately reduces farm profitability. There is a need to investigate alternatives. Accordingly, we conducted a replicated 2-year (2020–2021) field study to investigate the effects of conservation agriculture (CA) based tillage and crop establishment (TCE) techniques and residue management practices on the physical, chemical, and biological properties of soil along with crop productivity and the profitability of rice-maize systems in the sandy loam soil of Northwest Bangladesh. Two TCE techniques Puddled transplanted rice (PTR) followed by Conventional tillage maize (CTM) and strip tillage direct-seeded rice (STDSR) followed by strip-tilled maize (STM) were assigned to the main plots and different percentages of crop residue retention (0, 25, and 50% by height) were allocated to the subplots. Results showed that a reduction in bulk density (BD), soil penetration resistance (SPR), and increased soil porosity were associated with STDSR/STM-based scenarios (strip tillage coupled with 25 and 50% residue retention). The soil organic carbon (SOC) fractions, such as dissolved organic C (DOC), light and heavy particulate organic matter C (POM-C), MAOM, and microbial biomass C (MBC) levels in the 0–10 cm layer under ST based treatments were 95, 8, 6, 2 and 45% greater, respectively, compared to CT with no residue treatment. When compared to the CT treatment, the DOC, light POM-C, heavy POM-C, and MAOM in the 10–20 cm layer with ST treatment were 8, 34, 25, 4 and 37% higher, respectively. Residue retention in ST increased average rice, maize, and system yields by 9.2, 14.0, and 14.12%, respectively, when compared to CT. The system gross margin and benefit-cost ratio (BCR) were 1,696 ha−1 and 2.15 under strip-tillage practices. Thus, our study suggests that CA could be an appropriate practice for sustaining soil fertility and crop yield under R-M systems in light-textured soils or other similar soils in Banglades
Low Temperature Magnetic Properties of the Double Exchange Model
We study the {\it ferromagnetic} (FM) Kondo lattice model in the strong
coupling limit (double exchange (DE) model). The DE mechanism proposed by Zener
to explain ferromagnetism has unexpected properties when there is more than one
itinerant electron. We find that, in general, the many-body ground state of the
DE model is {\it not} globally FM ordered (except for special filled-shell
cases). Also, the low energy excitations of this model are distinct from spin
wave excitations in usual Heisenberg ferromagnets, which will result in unusual
dynamic magnetic properties.Comment: 5 pages, RevTeX, 5 Postscript figures include
Strong-Coupling Expansion for the Hubbard Model
A strong-coupling expansion for models of correlated electrons in any
dimension is presented. The method is applied to the Hubbard model in
dimensions and compared with numerical results in . Third order expansion
of the Green function suffices to exhibit both the Mott metal-insulator
transition and a low-temperature regime where antiferromagnetic correlations
are strong. It is predicted that some of the weak photoemission signals
observed in one-dimensional systems such as should become stronger as
temperature increases away from the spin-charge separated state.Comment: 4 pages, RevTex, 3 epsf figures include
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