Pathology of Schwinger boson mean field theory for Heisenberg spin models

We have re-analyze the Schwinger boson mean field theory (SBMFT) for Heisenberg spin models on the cubic lattice. We find that the second order phase transition point for magnetic ordering previously reported corresponds to a local maximum of the free energy functional. For both ferromagnetic and antiferromagnetic Heisenberg models with spin $S \geq S_C$, where $S_C < 1/2$, the mean field transitions are first order from the magnetically long-ranged ordered phase to the completely uncorrelated phase. In addition to erroneously giving a first order transition for magnetic ordering, the mean field theory does not include a phase with finite short-range correlation, thus negating one of the prime advantages of SBMFT. The relevance of these pathologies to other situations beyond the cubic lattice is discussed.Comment: 15 pages including 6 postscript figure

Theory for spin and orbital orderings in high temperature phase in YVO3YVO_3

Motivated by the recent neutron diffraction experiment on $YVO_3$, we consider a microscopic model where each $V^{3+}$ ion is occupied by two 3d electrons of parallel spins with two fold degenerate orbital configurations. The mean field classical solutions of the spin-orbital superexchange model predicts an antiferro-orbital ordering at a higher temperature followed by a C-type antiferromagnetic spin ordering at a lower temperature. Our results are qualitatively consistent with the observed orbital phase transition at $\sim 200K$ and the spin phase transition at $\sim 114K$ in $YVO_3$.Comment: 7 pages, 3 figures and 2 tables. Accepted to be published in PR

Dynamically generated dimension reduction and crossover in a spin orbital model

We study a spin orbital model in which the spin-spin interaction couples linearly to the orbital isospin. Fluctuations drive the transition from paramagnetic state to C type ordered state into a strongly first order one, as observed in $V_2O_3$. At T=0, there is a FOCS to FOGS transition. Close to the transition point, the system shows dynamically generated dimension reduction and crossover, resulting in one or more spin reentrant transitions.Comment: Submitted to PRL. 4 pages and one figur

RAPD analysis of colchicine induced variation of the Dendrobium Serdang beauty

Variation was detected in Dendrobium Serdang Beauty V (DSB V) plantlets regenerated from protocorm like bodies (plbs) induced by various concentration levels of colchicines in the Murashige and Skoog media (MS) supplemented with 1.5 mg/L IBA. RAPD analysis detected 6 - 26% variation in the regenerants from the mother plant. The highest variation was obtained in regenerates treated with 25 mg/L colchicine, which also exhibited reduced regeneration rates from plbs and mean plantlet fresh weight. RAPD analysis also showed high polymorphism between the mutated regenerant DSB V, and 13 species of the Dendrobium genera, and 13 orchids across generas. However, despite the 26% colchicine induced variation in the regenerants, all RAPD analysis revealed that DSB V was closely related to the mother plant. Thus, the RAPD technique is favourable for variation detection as it was sensitive enough to detect variations at species level and among somaclonal variants in this study

Establishment of a plant regeneration system from callus of Dendrobium cv. Serdang Beauty

An in vitro propagation protocol was established for the Dendrobium Serdang Beauty orchid. The propagation protocol utilized calli tissues that were successfully initiated from protocorm-like bodies(PLBs) explants, while the leaf and root tip explants died. The percentage of protocorm-like bodies explants responding to calli formation was 100% in all tested levels of IAA, IBA and NAA auxintreatments. The highest amount of calli (49.59 gram) proliferated on MS medium containing 1.5 mg/L IBA. These calli successfully regenerated on media supplemented with either KIN or BAP cytokinins and combined treatments of KIN and IAA (4 mg/L) or NAA (1.5 mg/L). However, media supplemented with only 1 mg/L KIN was sufficient to produce significantly high percentage of plantlet formation (80%),high number of planlets per explant (4-5 plantlets) and high mean fresh weight per plantlet (11.128 g). These plantlets were acclimatized on all tested media and obtained satisfactory rate of plantlet survival(80-100%), mean number of leaves per plant (4-6 leaves), and mean leaf length (4 - 5 cm). Among these media, charcoal was considered the most economical and available material in the local market. Duringthe development of this protocol, substantial necrosis of calli were observed when cultures were treated with 2,4-D and BAP. It was proposed that the presence of ethylene within the cultures, which isknown to be emitted by plant growth regulators into the micro-climate of in vitro culture vessels, is the determining factor of a suitable plant growth regulator for the survival and growth of the DendrobiumSerdang Beauty calli cultures in our study

Canard Cycles and Poincar\'e Index of Non-Smooth Vector Fields on the Plane

This paper is concerned with closed orbits of non-smooth vector fields on the plane. For a subclass of non-smooth vector fields we provide necessary and sufficient conditions for the existence of canard kind solutions. By means of a regularization we prove that the canard cycles are singular orbits of singular perturbation problems which are limit periodic sets of a sequence of limit cycles. Moreover, we generalize the Poincar\'e Index for non-smooth vector fields.Comment: 20 pages, 25 figure

Robust Fixed-Order Controller Design with Common Lyapunov Strictly Positive Realness Characterization

This paper investigates the design of a robust fixed-order controller for a polytopic system with interval uncertainties, with the aim that the closed-loop stability is appropriately ensured and the performance specifications on sensitivity shaping are conformed in a specific finite frequency range. Utilizing the notion of common Lyapunov strictly positive realness (CL-SPRness), the equivalence between strictly positive realness (SPRness) and strictly bounded realness (SBRness) is elegantly established; and then the specifications on robust stability and performance are transformed into the SPRness of newly constructed systems and further characterized in the framework of linear matrix inequality (LMI) conditions. Compared with the traditional robust controller synthesis approaches, the proposed methodology here avoids the tedious yet mandatory evaluations of the specifications on all vertices of the polytopic system; only a one-time checking of matrix existence is needed exclusively, and thus the typically heavy computational burden involved (in such robust controller design problems) is considerably alleviated. Moreover, it is noteworthy that the proposed methodology additionally provides essential necessary and sufficient conditions for this robust controller design with the consideration of a prescribed finite frequency range; and therefore significantly less conservatism is attained in the system performance.Comment: 10 pages, 6 figure

COVID-SGIS: A Smart Tool for Dynamic Monitoring and Temporal Forecasting of Covid-19

Background: The global burden of the new coronavirus SARS-CoV-2 is increasing at an unprecedented rate. The current spread of Covid-19 in Brazil is problematic causing a huge public health burden to its population and national health-care service. To evaluate strategies for alleviating such problems, it is necessary to forecast the number of cases and deaths in order to aid the stakeholders in the process of making decisions against the disease. We propose a novel system for real-time forecast of the cumulative cases of Covid-19 in Brazil. / Methods: We developed the novel COVID-SGIS application for the real-time surveillance, forecast and spatial visualization of Covid-19 for Brazil. This system captures routinely reported Covid-19 information from 27 federative units from the Brazil.io database. It utilizes all Covid-19 confirmed case data that have been notified through the National Notification System, from March to May 2020. Time series ARIMA models were integrated for the forecast of cumulative number of Covid-19 cases and deaths. These include 6-days forecasts as graphical outputs for each federative unit in Brazil, separately, with its corresponding 95% CI for statistical significance. In addition, a worst and best scenarios are presented. / Results: The following federative units (out of 27) were flagged by our ARIMA models showing statistically significant increasing temporal patterns of Covid-19 cases during the specified day-to-day period: Bahia, Maranhão, Piauí, Rio Grande do Norte, Amapá, Rondônia, where their day-to-day forecasts were within the 95% CI limits. Equally, the same findings were observed for Espírito Santo, Minas Gerais, Paraná, and Santa Catarina. The overall percentage error between the forecasted values and the actual values varied between 2.56 and 6.50%. For the days when the forecasts fell outside the forecast interval, the percentage errors in relation to the worst case scenario were below 5%. / Conclusion: The proposed method for dynamic forecasting may be used to guide social policies and plan direct interventions in a cost-effective, concise, and robust manner. This novel tools can play an important role for guiding the course of action against the Covid-19 pandemic for Brazil and country neighbors in South America