Stability of the Haldane phase in anisotropic magnetic ladders

We have considered the properties of anisotropic two-leg ladder models with S=1/2 or S=1 spins on the rungs, using White's density matrix renormalization group method. We have generalized the method by taking into account the symmetries of the model in order to reduce the dimensions of the matrix to be diagonalized, thereby making possible to consider more states. The boundaries in the parameter space of the extended region, where the Haldane phase exists, are estimated.Comment: 19 pages, 5 figure

Entanglement Measures for Single- and Multi-Reference Correlation Effects

Electron correlation effects are essential for an accurate ab initio description of molecules. A quantitative a priori knowledge of the single- or multi-reference nature of electronic structures as well as of the dominant contributions to the correlation energy can facilitate the decision regarding the optimum quantum chemical method of choice. We propose concepts from quantum information theory as orbital entanglement measures that allow us to evaluate the single- and multi-reference character of any molecular structure in a given orbital basis set. By studying these measures we can detect possible artifacts of small active spaces.Comment: 14 pages, 4 figure

The phase diagram of magnetic ladders constructed from a composite-spin model

White's density matrix renormalization group ({DMRG}) method has been applied to an $S= 1/2 + 1/2$ composite-spin model, which can also be considered as a two-leg ladder model. By appropriate choices of the coupling constants this model allows not only to study how the gap is opened around the gapless integrable models, but also to interpolate continuously between models with different spin lengths. We have found indications for the existence of several different massive phases.Comment: 30 pages, 8 Postscript figure

Accurate ab initio spin densities

We present an approach for the calculation of spin density distributions for molecules that require very large active spaces for a qualitatively correct description of their electronic structure. Our approach is based on the density-matrix renormalization group (DMRG) algorithm to calculate the spin density matrix elements as basic quantity for the spatially resolved spin density distribution. The spin density matrix elements are directly determined from the second-quantized elementary operators optimized by the DMRG algorithm. As an analytic convergence criterion for the spin density distribution, we employ our recently developed sampling-reconstruction scheme [J. Chem. Phys. 2011, 134, 224101] to build an accurate complete-active-space configuration-interaction (CASCI) wave function from the optimized matrix product states. The spin density matrix elements can then also be determined as an expectation value employing the reconstructed wave function expansion. Furthermore, the explicit reconstruction of a CASCI-type wave function provides insights into chemically interesting features of the molecule under study such as the distribution of $\alpha$- and $\beta$-electrons in terms of Slater determinants, CI coefficients, and natural orbitals. The methodology is applied to an iron nitrosyl complex which we have identified as a challenging system for standard approaches [J. Chem. Theory Comput. 2011, 7, 2740].Comment: 37 pages, 13 figure

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least $4m$. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the $6.5m$ James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 years, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space Telescope Overview, 29 pages, 4 figure

A MAC Protocol for ATM over Satellite

In this paper we analyse the performance of an Adaptive Random-Reservation Medium Access Control (MAC) protocol which can support all ATM service classes while providing the required Quality of Service (QoS). Our study focuses on parameter optimisation of the multiple access schemes for ATM over a GEO satellite with on-board processing capabilities, considering various traffic mixes of Constant Bit Rate (CBR), realtime VBR (rt-VBR), non-real-time VBR (nrt-VBR) and Unspecified Bit Rate (UBR). It is shown that a TDMA access protocol combining both Random Access and Demand Assignment Multiple Access (DAMA) can achieve a high throughput and is particularly suited for a scenario with a high number of terminals with very bursty UBR traffic (e.g. web browsing). The adaptive MAC protocol was designed to allow statistical multiplexing of ATM traffic over the air interface, especially for the independent and spatially distributed terminals. It is shown that the potential user population which..

Traffic Management For ATM Local Area Networks Using A Combined Preventive/Reactive Control Mechanism

This paper investigates how the performance of a preventive control scheme using a Leaky Bucket can be improved by using a feedback control loop. The proposed scheme is adaptive to congestion within the network. The queue occupancy of a network element is used as an indication of congestion and the individual sources, whose transmission path passes through the congested switch, are informed to reduce their activity. The effects of the transmission delay and the system parameters on the system performance are examined. The results show that appropriate feedback control in ATM LANs considerably improves the cell loss performance due to congestion

Interconnection Of Broadband Islands Via Satellite - Experiments On The Race II Catalyst Project

This paper presents the performance studies of ATM via satellite based on experiments implemented by the RACE II CATALYST project R2074. The purpose of the project was to develop an ATM satellite link for the future B-ISDN services, particularly for the interconnections of the ATM testbeds which are in the form of broadband islands. Then initial ATM based B-ISDN can be introduced by interconnections of these broadband islands. The CATALYST project has developed the equipment to be able to interconnect newly developed ATM testbeds as well as the existing networks such as DQDB, FDDI and Ethernet. Experiments carried out, demonstrated the capability of the satellite ATM connections to support data, voice, video and multimedia applications. These experiments provided a real system demonstration of ATM via satellite. In the light of the experiment, this paper evaluates the performance model and the capacity of the ATM satellite equipment, and studies the relevant issues and the impact of AT..

Flexible hardware design for RSA and Elliptic Curve Cryptosystems

This paper presents a scalable hardware implementation of both commonly used public key cryptosystems, RSA and Elliptic Curve Cryptosystem (ECC) on the same platform. The introduced hardware accelerator features a design which can be varied from very small (less than 20 Kgates) targeting wireless applications, up to a very big design (more than 100 Kgates) used for network security. In latter option it can include a few dedicated large number arithmetic units each of which is a systolic array performing the Montgomery Modular Multiplication (MMM). The bound on the Montgomery parameter has been optimized to facilitate more secure ECC point operations. Furthermore, we present a new possibility for CRT scheme which is less vulnerable to side-channel attacks