Magnetic degeneracy and hidden metallicity of the spin density wave state in ferropnictides

We analyze spin density wave (SDW) order in iron-based superconductors and electronic structure in the SDW phase. We consider an itinerant model for Fe-pnictides with two hole bands centered at $(0,0)$ and two electron bands centered at $(0,\pi)$ and $(\pi,0)$ in the unfolded BZ. A SDW order in such a model is generally a combination of two components with momenta $(0,\pi)$ and $(\pi,0)$, both yield $(\pi,\pi)$ order in the folded zone. Neutron experiments, however, indicate that only one component is present. We show that $(0,\pi)$ or $(\pi,0)$ order is selected if we assume that only one hole band is involved in the SDW mixing with electron bands. A SDW order in such 3-band model is highly degenerate for a perfect nesting and hole-electron interaction only, but we show that ellipticity of electron pockets and interactions between electron bands break the degeneracy and favor the desired $(0,\pi)$ or $(\pi,0)$ order. We further show that stripe-ordered system remains a metal for arbitrary coupling. We analyze electronic structure for parameters relevant to the pnictides and argue that the resulting electronic structure is in good agreement with ARPES experiments. We discuss the differences between our model and $J_1-J_2$ model of localized spins.Comment: reference list updated, typos are correcte

Interplay between magnetism and superconductivity in Fe-pnictides

We consider phase transitions and potential co-existence of spin-density-wave (SDW) magnetic order and extended s-wave ($s^+$) superconducting order within a two-band itinerant model of iron pnictides, in which SDW magnetism and $s^+$ superconductivity are competing orders. We show that depending on parameters, the transition between these two states is either first order, or involves an intermediate phase in which the two orders co-exist. We demonstrate that such co-existence is possible when SDW order is incommensurate.Comment: 5 pages, 3 figure

Superconductivity and spin-density-waves in multi-band metals

We present a detailed description of two-band quasi-2D metals with s-wave superconducting (SC) and antiferromagnetic spin-density wave (SDW) correlations. We present a general approach and use it to investigate the influence of the difference between the shapes and the areas of the two Fermi surfaces on the phase diagram. In particular, we determine the conditions for the co-existence of SC and SDW orders at different temperatures and dopings. We argue that a conventional s-wave SC order co-exists with SDW order only at very low $T$ and in a very tiny range of parameters. An extended s-wave superconductivity, for which SC gap changes sign between the two bands, co-exists with antiferromagnetic SDW over a much wider range of parameters and temperatures, but even for this SC order the regions of SDW and SC can still be separated by a first order transition. We show that the co-existence range becomes larger if SDW order is incommensurate. We apply our results to iron-based pnictide materials, in some of which co-existence of SDW and SC orders has been detected.Comment: 18 figures, 22 pages, published version with minor correction

Challenges of enterprise resource planning (ERP) implementation in agriculture

The underlying assumption of the study is that ERP systems can crucially facilitate information exchange; yet, the agricultural sector is slow in their adoption due to different reasons, including a shortage of skilled personnel as well as a lack of knowledge about ERP capabilities among top managers and key employees. The study intends to identify challenges and prospects for ERP implementation in agriculture. The applied methods include the analysis of WoS publications and questionnaire surveys of executives of 55 companies operating in the Middle Urals’ agricultural sector. ERP systems can be defined as comprehensive software solutions aimed to integrate business and management processes through a holistic approach and a single information system. According to expert estimates, in today’s Russia the projects related to the agro-industrial sector account for 1-2% to 10-15% of the projects from the leading ERP vendors, including 1C, Bars Group, and Navigator-Agro. ERP systems in agriculture help improve business performance, reduce and monitor costs. These systems are effective in decision-making and can serve as the basis for precision agriculture. The main barriers are poor personnel skills and competencies, shortage of funds for ERP adoption, poorly developed or absent infrastructure, difficulties of fitting and adapting of ERP systems to agricultural business. In addition, agricultural business owners show no confidence in high-tech solutions and poor knowledge of the above systems. Other problems include operation complexity and insufficient government support in ERP implementation. The results of the study can be used by government authorities in their programs for innovative development and technical upgrading of the agriculture industry. © 2020 by author(s) and VsI Entrepreneurship and Sustainability Center.Russian Foundation for Basic Research, RFBR: 19-41-000001, 19-41-000001, 19-41-000001, 19-41-000001Russian Foundation for Basic Research, RFBR: 19-41-000001, 19-41-000001, 19-41-000001, 19-41-000001The reported study was funded by RFBR and Sverdlovsk region, project number 19-41-000001, Russian FederationThe reported study was funded by RFBR and Sverdlovsk region, project number 19-41-000001, Russian Federatio

On the angular distribution of extensive air showers

Angular distributions of extensive air showers with different number of charged particles in the range 2.5x10^5--4x10^7 are derived using the experimental data obtained with the EAS MSU array. Possible approximations of the obtained distributions with different empiric functions available in literature, are analysed. It is shown that the exponential function provides the best approximation of the angular distributions in the sense of the chi-squared criterion.Comment: 5 pages including 1 figur

Lateral distribution of high energy muons in EAS of sizes Ne approximately equals 10(5) and Ne approximately equals 10(6)

Muon energy spectra and muon lateral distribution in EAS were investigated with the underground magnetic spectrometer working as a part of the extensive air showers (EAS) array. For every registered muon the data on EAS are analyzed and the following EAS parameters are obtained, size N sub e, distance r from the shower axis to muon, age parameter s. The number of muons with energy over some threshold E associated to EAS of fixed parameters are measured, I sub reg. To obtain traditional characteristics, muon flux densities as a function of the distance r and muon energy E, muon lateral distribution and energy spectra are discussed for hadron-nucleus interaction model and composition of primary cosmic rays