Schwinger-Boson Mean-Field Theory of Mixed-Spin Antiferromagnet L2BaNiO5L_2BaNiO_5

The Schwinger-boson mean-field theory is used to study the three-dimensional antiferromagnetic ordering and excitations in compounds $L_2BaNiO_5$, 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 $L_2BaNiO_5$. 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 $Ni$ 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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spl(2,1) dynamical supersymmetry and suppression of ferromagnetism in flat band double-exchange models

The low energy spectrum of the ferromagnetic Kondo lattice model on a N-site complete graph extended with on-site repulsion is obtained from the underlying spl(2,1) algebra properties in the strong coupling limit. The ferromagnetic ground state is realized for 1 and N+1 electrons only. We identify the large density of states to be responsible for the suppression of the ferromagnetic state and argue that a similar situation is encountered in the Kagome, pyrochlore, and other lattices with flat bands in their one-particle density of states.Comment: 7 pages, 1 figur

Hydrogen absorption properties of amorphous (Ni0.6Nb0.4−yTay)100−xZrx membranes

Ni based amorphous materials have great potential as hydrogen purification membranes. In the present work the melt spun (Ni0.6Nb0.4−yTay)100−xZrx with y=0, 0.1 and x=20, 30 was studied. The result of X-ray diffraction spectra of the ribbons showed an amorphous nature of the alloys. Heating these ribbons below T < 400 °C, even in a hydrogen atmosphere (1−10 bar), the amorphous structure was retained. The crystallization process was characterized by differential thermal analysis and the activation energy of such process was obtained. The hydrogen absorption properties of the samples in their amorphous state were studied by the volumetric method, and the results showed that the addition of Ta did not significantly influence the absorption properties, a clear change of the hydrogen solubility was observed with the variation of the Zr content. The values of the hydrogenation enthalpy changed from ~37 kJ/mol for x=30 to ~9 kJ/mol for x=20. The analysis of the volumetric data provides the indications about the hydrogen occupation sites during hydrogenation, suggesting that at the beginning of the absorption process the deepest energy levels are occupied, while only shallower energy levels are available at higher hydrogen content, with the available interstitial sites forming a continuum of energy levels

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 $t/U$ expansion that does not break down at low temperature ($t$ being the intersite hopping amplitude and $U$ 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 $2 \times 2$ 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 $t/U$. 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-$U$ Hartree-Fock saddle point.Comment: 20 pages RevTex, 3 figure

PyDDRBG: A Python framework for benchmarking and evaluating static and dynamic multimodal optimization methods

PyDDRBG is a Python framework for generating tunable test problems for static and dynamic multimodal optimization. It allows for quick and simple generation of a set of predefined problems for non-experienced users, as well as highly customized problems for more experienced users. It easily integrates with an arbitrary optimization method. It can calculate the optimization performance when measured according to the robust mean peak ratio. PyDDRBG is expected to advance the fields of static and dynamic multimodal optimization by providing a common platform to facilitate the numerical analysis, evaluation, and comparison in these fields

Climate change-driven losses in ecosystem services of coastal wetlands: A case study in the West coast of Bangladesh

© 2018 The Authors Climate change is globally recognized as one of the key drivers of degradation of coastal wetland ecosystems, causing considerable alteration of services provided by these habitats. Quantifying the physical impacts of climate change on these services is therefore of utmost importance. Yet, practical work in this field is fragmented and scarce in current literature, especially in developing countries which are likely to suffer most from the adverse climate change impacts. Using a coherent scenario-based approach that combines assessment of physical impacts with economic valuation techniques, here we quantify potential climate change driven losses in the value of wetland ecosystems services due to relative sea-level rise (RSLR)-induced inundation in the vulnerable Western coastal area of Bangladesh in 2100. The results show a small inundation area in 2100 under the three IPCC climate scenarios of RCP2.6 (with 0.25 m of RSLR), RCP6.0 (with 1.18 m of RSLR), and RCP8.5 (with 1.77 m of RSLR) for the coastal wetland ecosystems including the Sundarbans mangrove forest, neritic system and aquaculture ponds. In all scenarios, RSLR will drive a loss in the total value of ecosystem services such as provision of raw materials, and food provision, ranging from US$0–1 million to US$ 16.5–20 million, respectively. The outcomes of this study reveal that RSLR-induced inundation on its own, is unlikely to be a major threat to the wetland ecosystems in Western coast of Bangladesh. This would suggest that other climate change impacts such as coastal erosion, increase in frequency of cyclone events, and sea temperature rise might be the likely primary drivers of change in the value of wetland ecosystems services in this area

Static and Dynamic Multimodal Optimization by Improved Covariance Matrix Self-Adaptation Evolution Strategy with Repelling Subpopulations

The covariance matrix self-adaptation evolution strategy with repelling subpopulations (RS-CMSA-ES) is one of the most successful multimodal optimization (MMO) methods currently available. However, some of its components may become inefficient in certain situations. This study introduces the second variant of this method, called RS-CMSA-ESII. It improves the adaptation schemes for the normalized taboo distances of the archived solutions and the covariance matrix of the subpopulation, the termination criteria for the subpopulations, and the way in which the infeasible solutions are treated. It also improves the time complexity of RS-CMSA-ES by updating the initialization procedure of a subpopulation and developing a more accurate metric for determining critical taboo regions. The effects of these modifications are illustrated by designing controlled numerical simulations. RS-CMSA-ESII is then compared with the most successful and recent niching methods for MMO on a widely adopted test suite. The results obtained reveal the superiority of RS-CMSA-ESII over these methods, including the winners of the competition on niching methods for MMO in previous years. Besides, this study extends RS-CMSA-ESII to dynamic MMO and compares it with a few recently proposed methods on the modified moving peak benchmark functions

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

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.