Labeling Diversity for 2x2 WLAN Coded-Cooperative Networks

Labelling diversity is an efficient technique recently proposed in the literature and aims to improve the bit error rate(BER) performance of wireless local area network (WLAN) systems with two transmit and two receive antennas without increasing the transmit power and bandwidth requirements. In this paper, we employ labelling diversity with different space-time channel codes such as convolutional, turbo and low density parity check (LDPC) for both point-to-point and coded-cooperative communication scenarios. Joint iterative decoding schemes for distributed turbo and LDPC codes are also presented. BER performance bounds at an error floor (EF) region are derived and verified with the help of numerical simulations for both cooperative and non-cooperative schemes. Numerical simulations show that the coded-cooperative schemes with labelling diversity achieve better BER performances and use of labelling diversity at the source node significantly lowers relay outage probability and hence the overall BER performance of the coded-cooperative scheme is improved manifolds

Interplay between Kitaev interaction and single ion anisotropy in ferromagnetic CrI3_3 and CrGeTe3_3 monolayers

Magnetic anisotropy is crucially important for the stabilization of two-dimensional (2D) magnetism, which is rare in nature but highly desirable in spintronics and for advancing fundamental knowledge. Recent works on CrI$_3$ and CrGeTe$_3$ monolayers not only led to observations of the long-time-sought 2D ferromagnetism, but also revealed distinct magnetic anisotropy in the two systems, namely Ising behavior for CrI$_3$ versus Heisenberg behavior for CrGeTe$_3$. Such magnetic difference strongly contrasts with structural and electronic similarities of these two materials, and understanding it at a microscopic scale should be of large benefits. Here, first-principles calculations are performed and analyzed to develop a simple Hamiltonian, to investigate magnetic anisotropy of CrI$_3$ and CrGeTe$_3$ monolayers. The anisotropic exchange coupling in both systems is surprisingly determined to be of Kitaev-type. Moreover, the interplay between this Kitaev interaction and single ion anisotropy (SIA) is found to naturally explain the different magnetic behaviors of CrI$_3$ and CrGeTe$_3$. Finally, both the Kitaev interaction and SIA are further found to be induced by spin-orbit coupling of the heavy ligands (I of CrI$_3$ or Te of CrGeTe$_3$) rather than the commonly believed 3d magnetic Cr ions

Evaluating Gilbert Damping in Magnetic Insulators from First Principles

Magnetic damping has a significant impact on the performance of various magnetic and spintronic devices, making it a long-standing focus of research. The strength of magnetic damping is usually quantified by the Gilbert damping constant in the Landau-Lifshitz-Gilbert equation. Here we propose a first-principles based approach to evaluate the Gilbert damping constant contributed by spin-lattice coupling in magnetic insulators. The approach involves effective Hamiltonian models and spin-lattice dynamics simulations. As a case study, we applied our method to Y$_3$Fe$_5$O$_{12}$, MnFe$_2$O$_4$ and Cr$_2$O$_3$. Their damping constants were calculated to be $0.8\times10^{-4}$, $0.2\times10^{-4}$, $2.2\times 10^{-4}$, respectively at a low temperature. The results for Y$_3$Fe$_5$O$_{12}$ and Cr$_2$O$_3$ are in good agreement with experimental measurements, while the discrepancy in MnFe$_2$O$_4$ can be attributed to the inhomogeneity and small band gap in real samples. The stronger damping observed in Cr$_2$O$_3$, compared to Y$_3$Fe$_5$O$_{12}$, essentially results from its stronger spin-lattice coupling. In addition, we confirmed a proportional relationship between damping constants and the temperature difference of subsystems, which had been reported in previous studies. These successful applications suggest that our approach serves as a promising candidate for estimating the Gilbert damping constant in magnetic insulators.Comment: 14 pages, 11 figure

In the Shadows of the Government: Relationship Building during Political Turnovers

We document that following a turnover of the Party Secretary or mayor of a city in China, firms (especially private firms) headquartered in that city significantly increase their “perk spending.” Both the instrumental-variable-based results and heterogeneity analysis are consistent with the interpretation that the perk spending is used to build relations with local governments. Moreover, local political turnover in a city tends to be followed by changes of Chairmen or CEOs of state-owned firms that are controlled by the local government. However, the Chairmen or CEOs who have connections with local government officials are less likely to be replaced

Triple-Well Charge Density Wave Transition Driven by Partially Occupied Ge Electronic States in Kagome FeGe

Kagome materials provides a promising platform for exploring intriguing correlated phenomena. Recently, a charge density wave (CDW) order was observed in kagome antiferromagnetic metal FeGe, sparking enormous research interests in intertwining physics. Two of the core questions are (i) what drive the CDW formation in FeGe and (ii) whether it is associated with magnetism. However, previous analysis on Fe-derived van Hove singularities and Fermi nesting can't account for the CDW phase transition process and the energy minimum pristine phase well, the microscopic origin of the CDW phase in FeGe remains elusive. Here, supported by density functional theory calculations, we reveal a triple-well CDW landscape in FeGe and demonstrate it as a consequence of the reconstruction of partially occupied Ge electronic band structure, without the need for Fe-derived van Hove singularities like in non-magnetic kagome material CsV$_3$Sb$_5$. Moreover, we emphasize that the antiferromagnetic order, intertwined with structural distortion, is crucial for stabilizing the CDW phase. Our work thus not only deepens the understanding of the CDW mechanism in FeGe, but also indicate an intertwined connection between the emergent magnetism and CDW in kagome materials.Comment: 18 pages,3 figur

Decline curve analysis for multiple-fractured horizontal wells in tight oil reservoirs

        The production of multiple-fractured horizontal wells (MFHW) in tight oil reservoirs decreases rapidly in the initial period. It then enters a stable state of the low production with a slow loss-ratio. Because of its complicated production dynamic characteristics, the conventional rate-decline analysis method is no longer applicable. We adopt the semi- analytical solution of the MFHW five-region linear flow model to get the multiple-fractured dimensionless rate-decline analysis curves and continuously calculate the loss-ratio of the decline curves. Based on the characteristics of the loss-ratio in the log-log plot, the decline curve analysis (DCA) model for tight oil MFHW is established. According to the DCA model, dimensionless production decline curves are obtained, and the sensitivity of the parameters in the model is analyzed. Finally, the DCA model is used to analyze the production rate decline of tight oil MFHW, and good matching results and production forecasts are achieved. This method provides a scientific basis for the rate-decline analysis of tight oil MFHW.Cited as: Xu, G., Yin., H., Yuan, H., Xing, C. Decline curve analysis for multiple-fractured horizontal wells in tight oil reservoirs. Advances in Geo-Energy Research, 2020, 4(3): 296-304, doi: 10.46690/ager.2020.03.0