Surface roughness and thermal conductivity of semiconductor nanowires: going below the Casimir limit

By explicitly considering surface roughness at the atomic level, we quantitatively show that the thermal conductivity of Si nanowires can be lower than Casimir's classical limit. However, this violation only occurs for deep surface degradation. For shallow surface roughness, the Casimir formula is shown to yield a good approximation to the phonon mean free paths and conductivity, even for nanowire diameters as thin as 2.22 nm. Our exact treatment of roughness scattering is in stark contrast with a previously proposed perturbative approach, which is found to overpredict scattering rates by an order of magnitude. The obtained results suggest that a complete theoretical understanding of some previously published experimental results is still lacking.Comment: 11 pages, 4 figure

Environmental Conditions, Fund Characteristics, and Islamic Orientation: An Analysis of Mutual Fund Performance for the MENA Region

Islamic funds are an upcoming alternative to conventional funds, aided by the increasing prominence of Islamic finance. This paper contributes to the extant literature by comparing the performance of Islamic and conventional funds during crisis and recovery periods. In contrast to most previous literature, we focus on the countries of the Middle East and North African region (MENA), which represent an appealing context to study both from a financial and socioeconomic point of view due to recent events in the area. To this end, we consider a linear model control- ling for the bias of omitting relevant benchmarks. Although this methodology is now widely accepted in the financial literature, it is less common when evaluating Islamic mutual funds, but it is particularly appropriate when the aim is to focus on markets where Shariah-compliant investments are in home territory. Our results show that the relative performance of Islamic and conventional funds must be tempered by several factors such as the (geographical) context in which the investment is made. Considering all the MENA region, Islamic funds perform, on average, slightly worse than conventional funds. However, if the analysis is restricted to GCC countries, the result is the opposite. This evidence holds for both crisis and recovery periods. In addition, the performance gap between the two types of funds either widens or shrinks when considering recovery or crisis times, reinforcing the views that Islamic funds are more stable in hazardous time

Thermal conductivity of crystalline AlN and the influence of atomic-scale defects

Aluminum nitride (AlN) plays a key role in modern power electronics and deep-ultraviolet photonics, where an understanding of its thermal properties is essential. Here we measure the thermal conductivity of crystalline AlN by the 3${\omega}$ method, finding it ranges from 674 ${\pm}$ 56 W/m/K at 100 K to 186 ${\pm}$ 7 W/m/K at 400 K, with a value of 237 ${\pm}$ 6 W/m/K at room temperature. We compare these data with analytical models and first principles calculations, taking into account atomic-scale defects (O, Si, C impurities, and Al vacancies). We find Al vacancies play the greatest role in reducing thermal conductivity because of the largest mass-difference scattering. Modeling also reveals that 10% of heat conduction is contributed by phonons with long mean free paths, over ~7 ${\mu}$m at room temperature, and 50% by phonons with MFPs over ~0.3 ${\mu}$m. Consequently, the effective thermal conductivity of AlN is strongly reduced in sub-micron thin films or devices due to phonon-boundary scattering

A stationary impulse-radar system for autonomous deployment in cold and temperate environments

L. Mingo acknowledges the SR&ED Tax Incentive Program of the CRA for offsetting some R&D costs of the sIPR. G. Flowers and D. Bigelow are grateful to NSERC, CFI, CSA, NSTP, PCSP and SFU for funding, Kluane First Nation, Yukon Government and Parks Canada for access to Yukon field sites and S. Williams, L. Goodwin, A. Pulwicki, J. Crompton,F. Beaud and Canadian astronaut D. Saint-Jacques for support and field assistance. C. Schoof and C. Rada provided time-lapse imagery and 2017 water-pressure data. A. Crawford and D. Mueller acknowledge funding from NSERC, Transport Canada, PKC/NSTP and the Garfield Weston Foundation, as well as support from Arctic Net, the CCGS Amundsen crew and pilots O. Talbot, A. Roy and G Carpentier.Stationary ice-penetrating radar (sIPR) systems can be used to monitor temporal changes in electromagnetically sensitive properties of glaciers and ice sheets. We describe a system intended for autonomous operation in remote glacial environments, and document its performance during deployments in cold and temperate settings. The design is patterned after an existing impulse radar system, with the addition of a fibre-optic link and timing module to control transmitter pulses, a micro-UPS (uninterruptable power supply) to prevent uncontrolled system shutdown and a customized satellite telemetry scheme. Various implementations of the sIPR were deployed on the Kaskawulsh Glacier near an ice-marginal lake in Yukon, Canada, for 44–77 days in summers 2014, 2015 and 2017. Pronounced perturbations to englacial radiostratigraphy were observed commensurate with lake filling and drainage, and are interpreted as changes in englacial water storage. Another sIPR was deployed in 2015–2016 on ice island PII-A-1-f, which originated from the Petermann Glacier in northwest Greenland. This system operated autonomously for almost a year during which changes in thickness of the ice column were clearly detected.Publisher PDFPeer reviewe

Shocks, Seyferts and the SNR connection: a Chandra observation of the Circinus galaxy

We analyse new Chandra observations of the nearest (D=4 Mpc) Seyfert 2 active galaxy, Circinus, and match them to pre-existing radio, infrared and optical data to study the kpc-scale emission. The proximity of Circinus allows us to observe in striking detail the structure of the radio lobes, revealing for the first time edge-brightened emission both in X-rays and radio. After considering various other possible scenarios, we show that this extended emission in Circinus is most likely caused by a jet-driven outflow, which is driving shells of strongly shocked gas into the halo of the host galaxy. In this context, we estimate Mach numbers M=2.7-3.6 and M=2.8-5.3 for the W and E shells respectively. We derive temperatures of 0.74 (+0.06, -0.05) keV and 0.8-1.8 keV for the W and E shells, and an expansion velocity of ~900-950 km/s. We estimate that the total energy (thermal and kinetic) involved in creating both shells is ~2x10^55 erg, and their age is ~10^6 years. Comparing these results with those we previously obtained for Centaurus A, NGC 3801 and Mrk 6, we show that these parameters scale approximately with the radio power of the parent AGN. The spatial coincidence between the X-ray and edge-brightened radio emission in Circinus resembles the morphology of some SNR shocks. This parallel has been expected for AGN, but has never been observed before. We investigate what underlying mechanisms both types of systems may have in common, arguing that, in Circinus, the edge-brightening in the shells may be accounted for by a B field enhancement caused by shock compression, but do not preclude some local particle acceleration. These results can be extrapolated to other low-power systems, particularly those with late type hosts.Comment: 13 pages, 9 figures, and 5 tables. Accepted for publication in Ap

Experimental Evaluation of Transmitted Signal Distortion Caused by Power Allocation in Inter-Cell Interference Coordination Techniques for LTE/LTE-A and 5G Systems

Error vector magnitude (EVM) and out-of-band emissions are key metrics for evaluating in-band and out-band distortions introduced by all potential non-idealities in the transmitters of wireless systems. As EVM is a measure of the quality of the modulated signal/symbols, LTE/LTE-A and 5G systems specify mandatory EVM requirements in transmission for each modulation scheme. This paper analyzes the influence of the mandatory satisfaction of EVM requirements on the design of radio resource management strategies (RRM) (link adaptation, inter-cell interference coordination), specifically in the downlink (DL). EVM depends on the non-idealities of the transmitter implementations, on the allocated power variations between the subcarriers and on the selected modulations. In the DL of LTE, link adaptation is usually executed by adaptive modulation and coding (AMC) instead of power control, but some flexibility in power allocation remains being used. LTE specifies some limits in the power dynamic ranges depending on the allocated modulation, which ensures the satisfaction of EVM requirements. However, the required recommendations concerning the allowed power dynamic range when inter-cell interference coordination (ICIC) and enhanced ICIC (eICIC) mechanisms (through power coordination) are out of specification, even though the EVM performance should be known to obtain the maximum benefit of these strategies. We perform an experimental characterization of the EVM in the DL under real and widely known ICIC implementation schemes. These studies demonstrate that an accurate analysis of EVM is required. It allows a better adjustment of the design parameters of these strategies, and also allows the redefinition of the main criteria to be considered in the implementation of the scheduler/link adaptation concerning the allocable modulation coding scheme (MCS) in each resource block. © 2013 IEEE

A modular approach for remote operation of humanoid robots in search and rescue scenarios

In the present work we have designed and implemented a modular, robust and user-friendly Pilot Interface meant to control humanoid robots in rescue scenarios during dangerous missions. We follow the common approach where the robot is semi-autonomous and it is remotely controlled by a human operator. In our implementation, YARP is used both as a communication channel for low-level hardware components and as an interconnecting framework between control modules. The interface features the capability to receive the status of these modules continuously and request actions when required. In addition, ROS is used to retrieve data from different types of sensors and to display relevant information of the robot status such as joint positions, velocities and torques, force/torque measurements and inertial data. Furthermore the operator is immersed into a 3D reconstruction of the environment and is enabled to manipulate 3D virtual objects. The Pilot Interface allows the operator to control the robot at three different levels. The high-level control deals with human-like actions which involve the whole robot’s actuation and perception. For instance, we successfully teleoperated IIT’s COmpliant huMANoid (COMAN) platform to execute complex navigation tasks through the composition of elementary walking commands (e.g.[walk_forward, 1m]). The mid-level control generates tasks in cartesian space, based on the position and orientation of objects of interest (i.e. valve, door handle) w.r.t. a reference frame on the robot. The low level control operates in joint space and is meant as a last resort tool to perform fine adjustments (e.g. release a trapped limb). Finally, our Pilot Interface is adaptable to different tasks, strategies and pilot’s needs, thanks to a modular architecture of the system which enables to add/remove single front-end components (e.g. GUI widgets) as well as back-end control modules on the fly

Surface Screening Charge and Effective Charge

The charge on an atom at a metallic surface in an electric field is defined as the field-derivative of the force on the atom, and this is consistent with definitions of effective charge and screening charge. This charge can be found from the shift in the potential outside the surface when the atoms are moved. This is used to study forces and screening on surface atoms of Ag(001) c$(2\times 2)$ -- Xe as a function of external field. It is found that at low positive (outward) fields, the Xe with a negative effective charge of -0.093 $|{e}|$ is pushed into the surface. At a field of 2.3 V \AA$^{-1}$ the charge changes sign, and for fields greater than 4.1 V \AA$^{-1}$ the Xe experiences an outward force. Field desorption and the Eigler switch are discussed in terms of these results.Comment: 4 pages, 1 figure, RevTex (accepted by PRL

Traffic monitoring for assuring quality of advanced services in future internet

The final publication is available at Springer via http://dx.doi.org/10.1007/978-3-642-21560-5_16Services based on packet switched networks are becoming dominant in telecommunication business and both operators and service providers must evolve in order to guarantee the required quality. Increasing bandwidth is no longer a viable solution because of the business erosion for network operators which cannot expect revenues due to the large investments required to satisfy new applications demand of bandwidth. This paper presents devices and a specific architecture of services monitoring platform that allows network operators and service providers to analyze the perceived quality of service and check their service level agreements. Thus, a cost-effective service management, based on direct IP traffic measuring, can be supported on integrated monitoring systems to provide network-centric mechanisms for differentiated quality of service, security and other advanced services.This work has been partially developed in the framework of the Celtic and EUREKA initiative IPNQSIS (IP Network Monitoring for Quality of Service Intelligent Support)