Integral Equation Analysis of Plane Wave Scattering by Coplanar Graphene-Strip Gratings in the THz Range

The plane wave scattering and absorption by finite and infinite gratings of free-space standing infinitely long graphene strips are studied in the THz range. A novel numerical approach, based on graphene surface impedance, hyper-singular integral equations, and the Nystrom method, is proposed. This technique guarantees fast convergence and controlled accuracy of computations. Reflectance, transmittance, and absorbance are carefully studied as a function of graphene and grating parameters, revealing the presence of surface plasmon resonances. Specifically, larger graphene relaxation times increases the number of resonances in the THz range, leading to higher wave transmittance due to the reduced losses; on the other hand an increase of graphene chemical potential up-shifts the frequency of plasmon resonances. It is also shown that a relatively low number of graphene strips (>10) are able to reproduce Rayleigh anomalies. These features make graphene strips good candidates for many applications, including tunable absorbers and frequency selective surfaces.Comment: 11 pages, 26 figure

Incompressible viscous flow near the leading edge of a flat plate admitting slip

The shear stress at the leading edge, calculated on basis of the Navier-Stokes equations and the no-slip boundary condition, approaches infinity. However, taking into account the mean free path of the molecules, which implies admitting a certain slip, the shear stress becomes inversely proportional to the square root of the Knudsen number κ if κ→0. κ is defined as the ratio between the mean free path and the viscous length. The new boundary condition modifies the shear stress only within the Knudsen region of which the size is of the order of 3 to 4 times the mean free path.

Developing Leadership Dispositions for Preparing Urban School Leaders in Chronically Low-Performing Schools

This study originated as an institutionally sponsored research residency conducted using utilization-focused evaluation (Patton, 2008) to investigate the nature of dispositional changes in candidates pursuing National Louis University’s M.Ed. and Ed.S. degrees leading to educational leadership (EDL) state certification in one large urban school district in a Southern state. The EDL program organized learning objectives intending to develop a specific knowledge base, an operational competency set, and, as this study’s focus, leadership dispositions related to preparing assistant principals and principals to lead effectively in chronically low-performing (CLP) schools. The inquiry cross-analyzed data at the intersection of the graduate program’s disposition-related learning objectives with 13 leadership dispositions identified in The Haberman Educational Foundation Star Urban Administrator Pre-Screener. EDL program faculty administered the pre-screener to EDL program candidates twice, yielding 187 matched pairs in pre-program and post-program administrations. Statistical analyses yielded a significant difference (improvement) in overall Haberman scores at the .01 alpha level, as well as an effect size considered (Cohen, 1969) to be a medium effect size. Faculty triangulated the data with interviews of alumni and faculty with experience in leading CLP schools, who affirmed that focus on leadership dispositions serves an important developmental role in an EDL program. The study appears to validate the measurable presence and dynamic changes in EDL candidate dispositions as an element of a graduate program focused on developing effective leaders of CLP schools

Filtros para un cabezal de comunicaciones entre vehículos

In this paper two reconfigurable bandpass filters able to switch between WiFi and UMTS for transmit and receive band standards are presented. The filters are designed in such a way that center frequency and bandwidth specifications are precisely met by defining two switchable filter topologies. Design specifications require two center frequency states, one at 2.440 GHz with an 80 MHz bandwidth and a second center frequency state at 1.955 GHz with a 140 MHz bandwidth for the WiFi and UMTS transmit bands. The second filter is able to have one center frequency at 2.440 GHz with an 80 MHz bandwidth and a second center frequency state at 2.165 GHz with a 110 MHz bandwidth for the WiFi and UMTS receive bands, respectively. Filter simulations were performed to match the required filters specifications. Measured results on the transmit filter show a very good agreement with the simulations where a 2.428 GHz center frequency with a 71 MHz bandwidth was obtained for the WiFi state, and a 1.939 GHz center frequency with a 144 MHz bandwidth was obtained for the UMTS filter state. Simulated results on the receive filter show a very good agreement with the specified parameters. The filters specifications were successfully matched with the proposed filter topologies.Postprint (published version

A Matrix Hyperbolic Cosine Algorithm and Applications

In this paper, we generalize Spencer's hyperbolic cosine algorithm to the matrix-valued setting. We apply the proposed algorithm to several problems by analyzing its computational efficiency under two special cases of matrices; one in which the matrices have a group structure and an other in which they have rank-one. As an application of the former case, we present a deterministic algorithm that, given the multiplication table of a finite group of size $n$, it constructs an expanding Cayley graph of logarithmic degree in near-optimal O(n^2 log^3 n) time. For the latter case, we present a fast deterministic algorithm for spectral sparsification of positive semi-definite matrices, which implies an improved deterministic algorithm for spectral graph sparsification of dense graphs. In addition, we give an elementary connection between spectral sparsification of positive semi-definite matrices and element-wise matrix sparsification. As a consequence, we obtain improved element-wise sparsification algorithms for diagonally dominant-like matrices.Comment: 16 pages, simplified proof and corrected acknowledging of prior work in (current) Section

Comprehensive study of the magnetic stars HD 5797 and HD 40711 with large chromium and iron overabundances

We present the results of a comprehensive study of the chemically peculiar stars HD 5797 and HD 40711. The stars have the same effective temperature, Teff = 8900 K, and a similar chemical composition with large iron (+1.5 dex) and chromium (+3 dex) overabundances compared to the Sun. The overabundance of rare-earth elements typically reaches +3 dex. We have measured the magnetic field of HD 5797. The longitudinal field component Be has been found to vary sinusoidally between -100 and +1000 G with a period of 69 days. Our estimate of the evolutionary status of the stars suggests that HD 5797 and HD 40711, old objects with an age t \approx 5 \times 108 yr, are near the end of the core hydrogen burning phase.Comment: 26 pages, 5 Encapsulated Postscript figure

Transformation cloaking and radial approximations for flexural waves in elastic plates

It is known that design of elastic cloaks is much more challenging than that of acoustic cloaks, cloaks of electromagnetic waves or scalar problems of antiplane shear. In this paper, we address fully the fourth-order problem and develop a model of a broadband invisibility cloak for channelling flexural waves in thin plates around finite inclusions. We also discuss an option to employ efficiently an elastic pre-stress and body forces to achieve such a result. An asymptotic derivation provides a rigorous link between the model in question and elastic wave propagation in thin solids. This is discussed in detail to show connection with non-symmetric formulations in vector elasticity studied in earlier work

Inclusive School Community: Why is it so Complex?

This paper addresses the question: why is it so hard for school communities to respond to diversity in learners, staff and parents in inclusive ways? The authors draw on theory and recent professional experience in Queensland, Australia, to offer four guiding principles that address traditional assumptions about learning that result in inequality of opportunity and outcomes for students. The authors suggest these principles to support the development of a more inclusive school community: (1) develop a learning community incorporating a critical friend; (2) value and collaborate with parents and the broader community; (3) engage students as citizens in school review and develop¬ment; and (4) support teachers’ critical engagement with inclusive ideals and practices. The authors describe how the principles can work in concert in a school community

Rethinking the social impacts of the arts

The paper presents a critical discussion of the current debate over the social impacts of the arts in the UK. It argues that the accepted understanding of the terms of the debate is rooted in a number of assumptions and beliefs that are rarely questioned. The paper goes on to present the interim findings of a three‐year research project, which aims to rethink the social impact of the arts, with a view to determining how these impacts might be better understood. The desirability of a historical approach is articulated, and a classification of the claims made within the Western intellectual tradition for what the arts “do” to people is presented and discussed