Exact time correlation functions for N classical Heisenberg spins in the `squashed' equivalent neighbor model

We present exact integral representations of the time-dependent spin-spin correlation functions for the classical Heisenberg N-spin `squashed' equivalent neighbor model, in which one spin is coupled via the Heisenberg exchange interaction with strength $J_1$ to the other N-1 spins, each of which is coupled via the Heisenberg exchange coupling with strength $J_2$ to the remaining N-2 spins. At low temperature T we find that the N spins oscillate in four modes, one of which is a central peak for a semi-infinite range of the values of the exchange coupling ratio. For the N=4 case of four spins on a squashed tetrahedron, detailed numerical evaluations of these results are presented. As $T\to\infty$, we calculate exactly the long-time asymptotic behavior of the correlation functions for arbitrary N, and compare our results with those obtained for three spins on an isosceles triangle.Comment: 9 pages, 8 figures, submitted to Phys. Rev.

Dark-Bright Soliton Bound States in a Microresonator

The recent discovery of dissipative Kerr solitons in microresonators has facilitated the development of fully coherent, chip-scale frequency combs. In addition, dark soliton pulses have been observed in microresonators in the normal dispersion regime. Here, we report bound states of mutually trapped dark-bright soliton pairs in a microresonator. The soliton pairs are generated seeding two modes with opposite dispersion but with similar group velocities. One laser operating in the anomalous dispersion regime generates a bright soliton microcomb, while the other laser in the normal dispersion regime creates a dark soliton via Kerr-induced cross-phase modulation with the bright soliton. Numerical simulations agree well with experimental results and reveal a novel mechanism to generate dark soliton pulses. The trapping of dark and bright solitons can lead to light states with the intriguing property of constant output power while spectrally resembling a frequency comb. These results can be of interest for telecommunication systems, frequency comb applications, ultrafast optics and soliton states in atomic physics

Clinical comparison of different cardiovascular risk scores for cardiovascular risk prediction in Indian patients

Background: Cardiovascular diseases (CVD) are the main cause of mortality and disability in India. Early and sustained exposure to behavioral risk factors leads to development of CVD. The present study was conducted to compare different cardiovascular calculators for CVD risk assessment models in young Indian patients presenting with myocardial infarction.Methods: This study included 85 patients with myocardial infarction (MI). Their predicted 10-year risk of CVD was calculated using three clinically most relevant risk assessment models viz. Framingham Risk score (RiskFRS), American College of Cardiology/American Heart Association (RiskACC/AHA) and the 3rd Joint British Societies risk calculator (RiskJBS).Results: RiskFRS recognized the highest number of patients (15.4%) at high CVD risk while RiskACC/AHA and RiskJBS calculators provided inferior risk assessment but statistically significant relationship. RiskFRS and RiskACC/AHA (Pearson's r 0.870, p<0.001).Conclusions: RiskFRS seems to be as most useful CVD risk assessment model in young Indian patients. RiskFRS is likely to identify the number of patients at ‘high-risk’ as compared to RiskJBS and RiskACC/AHA

Creating Udyog Sahayak Enterprises Network (USENET) for Employment Generation and Scale-up in the MSME Sector

We cannot achieve the demographic dividend unless we create jobs for millions of youth. There are multiple forces – thirteen of them identified in this report - working against employment growth in India from both supply side and the demand side. It is proven that large enterprises create very few jobs. Even these few jobs are created for educated youth. Only Micro and Small Enterprises (MSEs) can create large number of jobs across India for workers with wide range of skills and education. Out of a total of just over 63 million enterprises in the MSME sector, 62 million are informal micro and very small enterprises (MSEs). Of these, single worker firms (own-account) are 40 million, firms with 2 to 5 workers are 22 million and firms with more than 5 but less than 10 are estimated at 1 million. 107.6 million workers (97% of all employment in the MSME sector) are in this MSE segment. These MSEs, surprisingly, are more rural than urban - 51% are in rural India. Women-owned firms accounted for 20 % of all enterprises, 16 % of all workers and 9 % of aggregate value-added. There is a huge potential to grow these women owned enterprises. MSEs are the most important segment of the Indian Economy if we want to achieve GDP and Employment growth. Moreover, many of these enterprises are closely aligned with households, particularly in location and ownership. These enterprises play a pivotal role in linking the formal and the informal sector through value chains. We don’t need more micro-enterprises or more single-person firms. Instead, we need to scale-up these existing MSEs. How can we do this? By creating a support system which will make these MSEs go digital, grow in scale and increase their productivity. This will automatically create jobs The Udyog Sahayak Enterprise Network (USENET) proposed in this report is that support system. Why a support system? Because it is not possible to educate millions of the MSE entrepreneurs to become digital experts. Why now? Because India and the world are going digital. In the digital marketplace small can compete better and grow

Interplay of Polarization and Time-Reversal Symmetry Breaking in Synchronously Pumped Ring Resonators

Optically induced breaking of symmetries plays an important role in nonlinear photonics, with applications ranging from optical switching in integrated photonic circuits to soliton generation in ring lasers. In this work we study for the first time the interplay of two types of spontaneous symmetry breaking that can occur simultaneously in optical ring resonators. Specifically we investigate a ring resonator (e.g. a fiber loop resonator or whispering gallery microresonator) that is synchronously pumped with short pulses of light. In this system we numerically study the interplay and transition between regimes of temporal symmetry breaking (in which pulses in the resonator either run ahead or behind the seed pulses) and polarization symmetry breaking (in which the resonator spontaneously generates elliptically polarized light out of linearly polarized seed pulses). We find ranges of pump parameters for which each symmetry breaking can be independently observed, but also a regime in which a dynamical interplay takes place. Besides the fundamentally interesting physics of the interplay of different types of symmetry breaking, our work contributes to a better understanding of the nonlinear dynamics of optical ring cavities which are of interest for future applications including all-optical logic gates, synchronously pumped optical frequency comb generation, and resonator-based sensor technologies

Fragmentation by major dams and implications for the future viability of platypus populations

The evolutionarily unique platypus (Ornithorhynchus anatinus) has experienced major declines and extinctions from a range of historical and recent interacting human-mediated threats. Although spending most of their time in the water, platypuses can move over land. Nevertheless, uncertainties remain whether dams are barriers to movement, thus limiting gene flow and dispersal, essential to evolution and ecology. Here we examined disruption of gene flow between platypus groups below and above five major dams, matched to four adjacent rivers without major dams. Genetic differentiation (FST) across dams was 4- to 20-fold higher than along similar stretches of adjacent undammed rivers; FST across dams was similar to differentiation between adjacent river systems. This indicates that major dams represent major barriers for platypus movements. Furthermore, FST between groups was correlated with the year in which the dam was built, increasing by 0.011 every generation, reflecting the effects of these barriers on platypus genetics. This study provides evidence of gene flow restriction, which jeopardises the long-term viability of platypus populations when groups are fragmented by major dams. Mitigation strategies, such as building of by-pass structures and translocation between upstream and downstream of the dam, should be considered in conservation and management planning

A Kerr Polarization Controller

Kerr-effect-induced changes of the polarization state of light are well known in pulsed laser systems. An example is nonlinear polarization rotation, which is critical to the operation of many types of mode-locked lasers. Here, we demonstrate that the Kerr effect in a high-finesse Fabry-Pérot resonator can be utilized to control the polarization of a continuous wave laser. It is shown that a linearly-polarized input field is converted into a left- or right-circularly-polarized field, controlled via the optical power. The observations are explained by Kerr-nonlinearity induced symmetry breaking, which splits the resonance frequencies of degenerate modes with opposite polarization handedness in an otherwise symmetric resonator. The all-optical polarization control is demonstrated at threshold powers down to 7 mW. The physical principle of such Kerr effect-based polarization controllers is generic to high-Q Kerr-nonlinear resonators and could also be implemented in photonic integrated circuits. Beyond polarization control, the spontaneous symmetry breaking of polarization states could be used for polarization filters or highly sensitive polarization sensors when operated close to the symmetry-breaking point

Coherent suppression of backscattering in optical microresonators

As light propagates along a waveguide, a fraction of the field can be reflected by Rayleigh scatterers. In high quality-factor whispering-gallery-mode microresonators, this intrinsic backscattering is primarily caused by either surface or bulk material imperfections. For several types of microresonator-based experiments and applications, minimal backscattering in the cavity is of critical importance, and thus the ability to suppress the backscattering is essential. We demonstrate that introducing an additional scatterer in the near-field of a high-quality-factor microresonator can coherently suppress the amount of backscattering in a microresonator by more than 30 dB. The method relies on controlling the scatterer's position in order for the intrinsic and scatterer-induced backpropagating fields to destructively interfere. This technique is useful in microresonator applications where backscattering is currently limiting the performance of devices, such as ring-laser gyroscopes and dual frequency combs that both suffer from injection locking. Moreover, these findings are of interest for integrated photonic circuits in which backreflections could negatively impact the stability of laser sources or other components

Performance of Wind Pump Prototype

A wind pump prototype with 3.6 m rotor diameter, 19 m hub height above ground and 0.22 mm reciprocating pump stroke has been developed at the Department of Mechanical Engineering, Mekelle University. The prototype was designed and manufactured locally. Theoretical model based on combined efficiency of the rotor and the reciprocating pump was used to estimate the performance of the wind pump. One year wind speed data collected at 10 m height was extrapolated to the wind pump hub height using wind shear coefficient. The model assumed balanced rotor power and reciprocating pump, hence did not consider the effect of pump size. The theoretical model estimated the average daily discharge to be around 50 m3 and 30 m3 at 8 m and 12 m head, respectively. The prototype was tested with the same pump stroke but two different size pumps at two different heads. The pumps were with internal diameter of 55 and 70 mm and the test heads were at 8 and 12 m. Measurement of the flow rate, rotational speed and wind speed were made every 10 minutes during the test period. The data collected were analyzed to find the performance of the wind pump at the two test heads and two pump sizes. The flow rate data was plotted against binned wind speed data to determine the linear fit function. The linear fit function was then used to estimate the flow rate at any wind speed. With the 55 mm pump the measured average daily discharge was 20 and 19 m3 at 8 m and 12 m head, respectively. With the 70 mm pump the measured average daily discharge was 41 m3 and 30 m3 at 8 m and 12 m head, respectively.Keywords: Wind pump, Windmill, Performance testing, Pump efficiency, Pump discharge, Ethiopia