Static Shear and Liquefaction Potential of Sand

The state parameter, which describes the initial state of sand before shearing, combining with the concepts of critical stress ratio line and phase transform line are used to assess the characteristics of sand during shearing. The state parameter of each tested sample is determined after the steady state line is constructed, and then relations of characteristics of the cyclic deformation behavior and the state parameter of sand are discussed. The value of the lowest point, Sup1 on the CSR line in the p’ – q plot is related to ec for a specific sand. Once the stress path of monotonic or cyclic tests crosses the CSR line, strain softening may quickly occur. On the contrary, the stress path bends sharply upwards and to the right along the PT line if it touches the PT line. Liquefaction resistance may decrease with increasing the value of α for sand having ψ \u3e 0. Oppositely, it may increase with increasing the value it of α for sand having ψ \u3c 0. In the case of sand having ψ ≅ 0, nearly keeps no change with increasing the value of α

Deformation of Sand Under Cyclic Simple Shear Loading

The deformation behavior of sand under simple shear loading was studied. The relation between the shear stress ratio and fabric factor, and the relationship between the fabric and dilatancy rate of a sand were developed. Thus the stress-dilatancy relation for sand was derived. Adopting the concept that the volumetric changes of the sample induced by the cyclic shear stress were compensated by the volumetric changes due to vertical stress changes in the constant volume simple shear tests, the relation of shear stress ratio and vertical stress change was derived

Einstein Static Universe in Exponential f(T)f(T) Gravity

We analyze the stability of the Einstein static closed and open universe in two types of exponential $f(T)$ gravity theories. We show that the stable solutions exist in these two models. In particular, we find that large regions of parameter space in equation of state $w=p/\rho$ for the stable universe are allowed in the $f(T)$ theories.Comment: 11 pages, 4 figures, published version with references update

Public Pensions and Capital Accumulation: The Case of Brazil

We use an OLG model to study the effects of the generous public sector pension system in Brazil. In our model there are two types of workers, one working in the private sector, the other working in the public sector. Public workers produce infrastructure or education services. We find that reducing generosity of the public sector pensions has large effects on capital accumulation and steady state income.pension reform, capital accumulation

Public Sector Pension Policies and Capital Accumulation in Emerging Economies

In many emerging economies pension programs of public sector workers are more generous than pension programs of private sector workers. In this paper we investigate public pension reforms that improve efficiency and welfare by reallocating government resources from non-productive public pensions to productive public education and infrastructure investments. We argue that the opportunity costs of running generous public pension schemes for civil servants are potentially large in emerging economies that often suffer from low public investments in education and infrastructure. In addition, we quantitfy the savings distortions as well as the tax distortions from running a generous public pension program. Calculating transitions to the post-reform steady state, we find that welfare losses for the generation born before the reform are offset by welfare gains by the generations born after the reform.Social Security Reform; Generous Public Sector Pensions; Capital Accumulation; Public Education and Infrastructure Investments

Multi-user lattice coding for the multiple-access relay channel

This paper considers the multi-antenna multiple access relay channel (MARC), in which multiple users transmit messages to a common destination with the assistance of a relay. In a variety of MARC settings, the dynamic decode and forward (DDF) protocol is very useful due to its outstanding rate performance. However, the lack of good structured codebooks so far hinders practical applications of DDF for MARC. In this work, two classes of structured MARC codes are proposed: 1) one-to-one relay-mapper aided multiuser lattice coding (O-MLC), and 2) modulo-sum relay-mapper aided multiuser lattice coding (MS-MLC). The former enjoys better rate performance, while the latter provides more flexibility to tradeoff between the complexity of the relay mapper and the rate performance. It is shown that, in order to approach the rate performance achievable by an unstructured codebook with maximum-likelihood decoding, it is crucial to use a new K-stage coset decoder for structured O-MLC, instead of the one-stage decoder proposed in previous works. However, if O-MLC is decoded with the one-stage decoder only, it can still achieve the optimal DDF diversity-multiplexing gain tradeoff in the high signal-to-noise ratio regime. As for MS-MLC, its rate performance can approach that of the O-MLC by increasing the complexity of the modulo-sum relay-mapper. Finally, for practical implementations of both O-MLC and MS-MLC, practical short length lattice codes with linear mappers are designed, which facilitate efficient lattice decoding. Simulation results show that the proposed coding schemes outperform existing schemes in terms of outage probabilities in a variety of channel settings.Comment: 32 pages, 5 figure

Filter and nested-lattice code design for fading MIMO channels with side-information

Linear-assignment Gel'fand-Pinsker coding (LA-GPC) is a coding technique for channels with interference known only at the transmitter, where the known interference is treated as side-information (SI). As a special case of LA-GPC, dirty paper coding has been shown to be able to achieve the optimal interference-free rate for interference channels with perfect channel state information at the transmitter (CSIT). In the cases where only the channel distribution information at the transmitter (CDIT) is available, LA-GPC also has good (sometimes optimal) performance in a variety of fast and slow fading SI channels. In this paper, we design the filters in nested-lattice based coding to make it achieve the same rate performance as LA-GPC in multiple-input multiple-output (MIMO) channels. Compared with the random Gaussian codebooks used in previous works, our resultant coding schemes have an algebraic structure and can be implemented in practical systems. A simulation in a slow-fading channel is also provided, and near interference-free error performance is obtained. The proposed coding schemes can serve as the fundamental building blocks to achieve the promised rate performance of MIMO Gaussian broadcast channels with CDIT or perfect CSITComment: submitted to IEEE Transactions on Communications, Feb, 200

Ultrafast Carrier Dynamics at p-n Junction of Cu(In,Ga)Se2- Based Solar Cells Measured by Optical Pump Terahertz Probe Spectroscopy

Among other materials, the p-type Cu(In,Ga)Se2 (CIGS) alloy has attracted attention as the most efficient absorber in thin-film solar cells. The typical CIGS layer is deposited with a polycrystalline structure containing an amount of native defect states, which serve as carrier traps and recombination centers. These defect states in the CIGS layer can be easily changed after deposition of an n-type buffer layer, due to the formation of p-n junctions. To understand the influence of the p-n junction on these defect states, the behavior of photoexcited carriers, from the CIGS absorber to the buffer layer, is considered to be an important issue and is closely related to solar cell performance. In this study, we performed experiments to investigate the ultrafast carrier dynamics of CIGS-based solar cells, using optical pump terahertz (THz) probe (OPTP) spectroscopy, and demonstrated the correlation between solar cell performance and the behavior of photoexcited carrier dynamics