Tetratic Order in the Phase Behavior of a Hard-Rectangle System

Previous Monte Carlo investigations by Wojciechowski \emph{et al.} have found two unusual phases in two-dimensional systems of anisotropic hard particles: a tetratic phase of four-fold symmetry for hard squares [Comp. Methods in Science and Tech., 10: 235-255, 2004], and a nonperiodic degenerate solid phase for hard-disk dimers [Phys. Rev. Lett., 66: 3168-3171, 1991]. In this work, we study a system of hard rectangles of aspect ratio two, i.e., hard-square dimers (or dominos), and demonstrate that it exhibits a solid phase with both of these unusual properties. The solid shows tetratic, but not nematic, order, and it is nonperiodic having the structure of a random tiling of the square lattice with dominos. We obtain similar results with both a classical Monte Carlo method using true rectangles and a novel molecular dynamics algorithm employing rectangles with rounded corners. It is remarkable that such simple convex two-dimensional shapes can produce such rich phase behavior. Although we have not performed exact free-energy calculations, we expect that the random domino tiling is thermodynamically stabilized by its degeneracy entropy, well-known to be $1.79k_{B}$ per particle from previous studies of the dimer problem on the square lattice. Our observations are consistent with a KTHNY two-stage phase transition scenario with two continuous phase transitions, the first from isotropic to tetratic liquid, and the second from tetratic liquid to solid.Comment: Submitted for publicatio

Trade-offs in System of Systems Acquisition

Large organisations tend to have multiple organisational goals. Example goals for organisations that perform search and rescue might be being able to search large areas quickly, and to provide, for the speedy recovery of survivors. To satisfy these goals, organisations will acquire different resources such as new systems, training programmes, infrastructure and processes. These different resources when combined to meet the same organisational goals, can be considered as a System of Systems (SoS). Organisational goals can be satisfied by completely different resource combinations with each resource combination satisfying the individual goals to varying degrees and with different overall costs. Since organisations only have limited resources available to them, there is an incentive for organisations to find the most efficient resource combinations to satisfy their goals. This can be considered as performing trade-offs in SoS acquisition. There are several open research gaps in performing trade-offs in SoS acquisition. The first is that the resources involved are heterogeneous. How do you compare the benefits of new equipment against new training programmes or organisational structures? The second is the multi-objective nature of the problem with the different organisational goals competing for the same limited budget. The third is managing the problem through-life and maintaining the satisfaction of organisational goals as old system retire and new systems come into service. This thesis presents a model-based technique (with prototype tool support) that combines techniques from the fields of through life capability management, goal modelling, search-based software engineering and model-driven engineering. This technique addresses the three problems stated above allowing decision makers to more efficiently consider the trade-offs involved when performing SoS acquisition. The technique has been evaluated on a realistic case study and on a standard problem found in the field of search-based software engineering

Is Sacral Extension a Risk Factor for Early Proximal Junctional Kyphosis in Adult Spinal Deformity Surgery?

Study designRetrospective cohort study.PurposeTo investigate the role of sacral extension (SE) for the development of proximal junctional kyphosis (PJK) in adult spinal deformity (ASD) surgery.Overview of literatureThe development of PJK is multifactorial and different risk factors have been identified. Of these, there is some evidence that SE also affects the development of PJK, but data are insufficient.MethodsUsing a combined database comprising two propensity-matched groups of fusions following ASD surgery, one with fixation to S1 or S1 and the ilium (SE) and one without SE but with a lower instrumented vertebra of L5 or higher (lumbar fixation, LF), PJK and the role of further parameters were analyzed. The propensity-matched variables included age, the upper-most instrumented vertebra (UIV), preoperative sagittal alignment, and the baseline to one year change of the sagittal alignment.ResultsPropensity matching led to two groups of 89 patients each. The UIV, pelvic incidence minus lumbar lordosis, sagittal vertical axis, pelvic tilt, age, and body mass index were similar in both groups (p >0.05). The incidence of PJK at postoperative one year was similar for SE (30.3%) and LF (22.5%) groups (p =0.207). The PJK angle was comparable (p =0.963) with a change of -8.2° (SE) and -8.3° (LF) from the preoperative measures (p =0.954). A higher rate of PJK after SE (p =0.026) was found only in the subgroup of patients with UIV levels between T9 and T12.ConclusionsInstrumentation to the sacrum with or without iliac extension did not increase the overall risk of PJK. However, an increased risk for PJK was found after SE with UIV levels between T9 and T12

The national assessment of educational progress

The purpose of this paper is to provide the reader with a knowledge of the National Assessment of Educational Progress (NAEP) project. The first section of the paper provides an overview and general discussion of National Assessment; the second section presents some of the operating issues related to the project and its purposes; and the third section provides a short history of the project and its governance.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23657/1/0000625.pd

The Ursinus Weekly, February 28, 1949

Ruby staff sought; Junior class hopes to better yearbook • Sophs choose Woody Leigh for hop • IRC group travels to Norristown High, hears commentator • Pancoast to speak to PAC commission on Congress topic • Hoopsters, Maulers to finish seasons; coeds to continue • Curtain Club groups present entertaining bevy of events • Parking offenders to pay fines • Pancoasts head Red Cross drive • Juniors to hold television show featuring music, food, variety • Do you favor the North Atlantic Pact? • Radio station sees hope for wider use of talent • Belles lose second as Chestnut Hillers take 35-22 victory • Both sexes reach stride in campus court strife • Jaffe gets award from Philly scribes • Haverford to meet bruins in opener of baseball season • Bruin matmen take two as Drexel and CCNY fall • Coeds trip Temple; last period surge sets total at 43-39 • Grizzlies fail to win in three court battles • Cadets retain lead in MA court race • Messiah group heard on WNARhttps://digitalcommons.ursinus.edu/weekly/1609/thumbnail.jp

Lattice Effects in Crystal Evaporation

We study the dynamics of a stepped crystal surface during evaporation, using the classical model of Burton, Cabrera and Frank, in which the dynamics of the surface is represented as a motion of parallel, monoatomic steps. The validity of the continuum approximation treated by Frank is checked against numerical calculations and simple, qualitative arguments. The continuum approximation is found to suffer from limitations related, in particular, to the existence of angular points. These limitations are often related to an adatom detachment rate of adatoms which is higher on the lower side of each step than on the upper side ("Schwoebel effect").Comment: DRFMC/SPSMS/MDN, Centre d'Etudes Nucleaires de Grenoble, 25 pages, LaTex, revtex style. 8 Figures, available upon request, report# UBFF30119

On equilibrium shapes of charged flat drops

Equilibrium shapes of two-dimensional charged, perfectly conducting liquid drops are governed by a geometric variational problem that involves a perimeter term modeling line tension and a capacitary term modeling Coulombic repulsion. Here we give a complete explicit solution to this variational problem. Namely, we show that at fixed total charge a ball of a particular radius is the unique global minimizer among all sufficiently regular sets in the plane. For sets whose area is also fixed, we show that balls are the only minimizers if the charge is less than or equal to a critical charge, while for larger charge minimizers do not exist. Analogous results hold for drops whose potential, rather than charge, is fixed