Phase transitions in the Shastry-Sutherland lattice

Two recently developed theoretical approaches are applied to the Shastry-Sutherland lattice, varying the ratio $J'/J$ between the couplings on the square lattice and on the oblique bonds. A self-consistent perturbation, starting from either Ising or plaquette bond singlets, supports the existence of an intermediate phase between the dimer phase and the Ising phase. This existence is confirmed by the results of a renormalized excitonic method. This method, which satisfactorily reproduces the singlet triplet gap in the dimer phase, confirms the existence of a gapped phase in the interval $0.66<J'/J<0.86$Comment: Submited for publication in Phys. Rev.

Design of a Dual-Band Sectoral Antenna for Hiperlan2 Application Using Double Layers of Metallic Electromagnetic Band Gap (M-EBG) Materials as a Superstrate

A novel design of a sectoral antenna that utilizes a double layer Metallic Electromagnetic Band Gap (M-EBG) as a superstrate for dual band directivity enhancement is presented in this paper. We obtain the different operating frequencies by adjusting the distance of the lower M-EBG layer from printed patch antenna and also the height between upper and lower M-EBG layers. This antenna operates according to a sectoral radiation pattern form presenting a half power beamwidth of at least 60∘. The proposed structure presents more than 17 dB directivity enhancement at 5.25 GHz and 5.65 GHz as compared to those of a patch antenna with 9 dB directivity. The principle is explained and applied to a Hiperlan2 antenna

Measurement of Nonlinear Receptivity to Surface Irregularities

Acoustic receptivity is the process by which acoustic disturbances are internalized into the shear layer to generate instability waves. Experiments have shown that, when tuned to the eigenvalue modes, the amplitude of the resulting T-S waves scales with the acoustic field intensity. When a surface irregularity is present, the characteristic wall wavenumber forces a spatial mode onto the near-wall mean velocity field, thus providing modal length scales comparable to those of T-S waves. In this experiment an attempt was made to increase the acoustic receptivity by exciting a difference mode via a quadratic interaction between two larger-wavenumber, forced modes. The difference mode is tuned to the dominant T-S eigenmode wavenumber. As expected, an increased receptivity corresponding to the difference mode was measured downstream of branch I, suggesting the presence of the nonlinearity

SiGe Raman spectra vs. local clustering/anticlustering : Percolation scheme and ab initio calculations

We formalize within the percolation scheme, that operates along the linear chain approximation, namely at one dimension (1D), an intrinsic ability behind Raman scattering to achieve a quantitative insight into local clustering or anticlustering in an alloy, using SiGe as a case study. For doing so, we derive general expressions of the individual fractions of the six SiGe percolation-type oscillators [1(Ge-Ge), 3(Si-Ge), 2(Si-Si)], which monitor directly the Raman intensities, via a relevant order parameter k. This is introduced by adapting to the 1D oscillators of the SiGe diamond version of the 1D percolation scheme, namely along a fully consistent 1D treatment, the approach originally used by Verleur and Barker for the three-dimensional (3D) oscillators of their 1D cluster scheme applying to zincblende alloys [H.W. Verleur and A.S. Barker, Phys. Rev. 149, 715 (1966)], a somehow problematic one in fact, due to its 3D vs. 1D ambivalence. Predictive k-dependent intensity interplays between the SiGe (50 at.%Si) Raman lines are confronted with existing experimental data and with ab initio Raman spectra obtained by using large (32 atom) disordered supercells matching the required k values, with special attention to the Si-Ge triplet and to the Si-Si doublet, respectively.Comment: 20 pages, 6 figure

High Injection Effects on Solar Cell Performances

Experiments are performed on solar cells under concentrated sunlight in order to explore fundamental physical processes with high injection conditions. Saturation effects are observed on the cell open circuit voltage and on the extracted values of the recombination current. A large decrease of the initial decay of the transient voltage have been measured. High injection effects are shown to be correlated with the increase of recombination current in the space charge region together with an increase of the emitterbase coupling

Robust and Skew-resistant Parallel Joins in Shared-Nothing Systems

The performance of joins in parallel database management systems is critical for data intensive operations such as querying. Since data skew is common in many applications, poorly engineered join operations result in load imbalance and performance bottlenecks. State-of-the-art methods designed to handle this problem offer significant improvements over naive implementations. However, performance could be further improved by removing the dependency on global skew knowledge and broadcasting. In this paper, we propose PRPQ (partial redistribution & partial query), an efficient and robust join algorithm for processing large-scale joins over distributed systems. We present the detailed implementation and a quantitative evaluation of our method. The experimental results demonstrate that the proposed PRPQ algorithm is indeed robust and scalable under a wide range of skew conditions. Specifically, compared to the state-of-art PRPD method, we achieve 16% - 167% performance improvement and 24% - 54% less network communication under different join workloads

Signal level comparison between TerraSAR-X and COSMO-SkyMed SAR Sensors

International audienceSoil and vegetation biophysical parameter retrieval using synthetic-aperture-radar images requires radiometrically well-calibrated sensors. In this letter, a comparison of signal levels between TerraSAR-X (TSX) and the COSMO-SkyMed (CSK) constellation (CSK1, CSK2, CSK3, and CSK4) was carried out in order to analyze the ability to use jointly all current X-band sensors. The analysis of the X-band signal over forest stands showed a stable signal (variation lower than 1 dB) over time for each of the studied sensors, but a significant difference was observed between the different X-band sensors. Differences between radar signals were higher in HH than in HV polarization. TSX and CSK4 showed similar backscatter signals, with signal level differences of 0.6 dB in HH and 1.4 dB in HV. The CSK3 signal was observed to be lower than those from TSX and CSK4 by about 2.1 dB and 1.5 dB in HH against 3.2 dB and 1.8 dB in HV, respectively. Moreover, CSK2 and CSK1 which showed slightly different backscatter signals (within 1.1 dB in HH and 1.9 dB in HV) had signal levels lower than those obtained from TSX (2.2-3.3 dB in HH and 3.2-5.1 dB in HV for about 29° incidence angle). These results show that it is currently difficult to use jointly the available X-band satellites (CSK and TSX) for estimating the biophysical parameters of soil or vegetation. This is due to the significant difference in the radar signal level between some of the analyzed satellites, which will cause a high overor underestimation of biophysical parameters

A risk-oriented degradation model for maintenance of reinforced concrete structure subjected to cracking

This article is within the context of decision models aimed for maintenance of structures and infrastructures in civil engineering. The contribution relies on the construction of a degradation model oriented toward risk analysis. The proposed model can be defined as a meta-model in the sense that it is based on observations while incorporating key features from the degradation process necessary for the maintenance decision. We propose to stimulate the construction of the degradation model based on the crack propagation of a submerged reinforced concrete structure subject to chloride-induced corrosion. Furthermore, a set of numerical illustrations is performed to demonstrate the advantages and applicability of the proposed approach in risk management and maintenance contexts