arXiv subjects

A. F. Isakovic

Publications and source records attributed to A. F. Isakovic.

11 recordsLinked to original sources

arXiv2021

Ni3Si2 Nanowires for Efficient Electron Field Emission and Limitations of the Fowler-Nordheim Model

The paper reports on top-down nanofabricated Ni3Si2 nanowires and tests of their electron field emission capabilities. The results include low turn on electric field, moderate work function, and the field enhancement factor, customizable through nanofabrication. The paper also reports on the issues ahead in the field of nanowires-based electron mission, as there are quantitative limitations of the applicability of the Fowler-Nordheim model, which will become increasingly apparent as we continue to optimize field emission of electrons. To this end, we suggest adding the studies of surface-to-volume ratio effects of the nanowires as another standard for comparison, in order to lead to the input form of the density of states as quantum effects becoming more prominent.

cond-mat.mes-hall↗

arXiv2021

Search and Rescue in a Maze-like Environment with Ant and Dijkstra Algorithms

With the growing reliability of modern Ad Hoc Networks, it is encouraging to analyze potential involvement of autonomous Ad Hoc agents in critical situations where human involvement could be perilous. One such critical scenario is the Search and Rescue effort in the event of a disaster where timely discovery and help deployment is of utmost importance. This paper demonstrates the applicability of a bio-inspired technique, namely Ant Algorithms (AA), in optimizing the search time for a near optimal path to a trapped victim, followed by the application of Dijkstra's algorithm in the rescue phase. The inherent exploratory nature of AA is put to use for a faster mapping and coverage of the unknown search space. Four different AA are implemented, with different effects of the pheromone in play. An inverted AA, with repulsive pheromones, was found to be the best fit for this particular application. After considerable exploration, upon discovery of the victim, the autonomous agents further facilitate the rescue process by forming a relay network, using the already deployed resources. Hence, the paper discusses a detailed decision making model of the swarm, segmented into two primary phases, responsible for the search and rescue respectively. Different aspects of the performance of the agent swarm are analyzed, as a function of the spatial dimensions, the complexity of the search space, the deployed search group size, and the signal permeability of the obstacles in the area.

cs.MA↗

arXiv2021

Spin Relaxation in Quasi-1D GaAs Mesowires: Control via Electric Field and Aspect Ratio

We report on the measurements of spin relaxation in GaAs quasi-one-dimensional mesowires, relying on spin noise spectroscopy, thus adding to the existing body of spin relaxation studies in bulk, two-dimensional and zero dimensional systems. In addition to temperature and magnetic field dependence, we modify the spin relaxation time via applied electric field and aspect ratio of the mesowires, suggesting that scalable spintronics devices with controllable spin relaxation are achievable. Overall, we observed higher spin-relaxation time in mesowires compared to bulk with a spin noise exhibiting Dyakonov-Perel scattering and other scattering behavior. Spectral spin noise data are interpreted in part via Glazov-Sherman model, where both, diffusive and ballistic spin relaxation are accounted for.

cond-mat.mes-hall↗

arXiv2021

Temporal Graphs and Temporal Network Characteristics for Bio-Inspired Networks During Optimization

Temporal network analysis and time evolution of network characteristics are powerful tools in describing the changing topology of dynamic networks. This paper uses such approaches to better visualize and provide analytical measures for the changes in performance that we observed in Voronoi-type spatial coverage, particularly for the example of time evolving networks with a changing number of wireless sensors being deployed. Specifically, our analysis focuses on the role different combinations of impenetrable obstacles and environmental noise play in connectivity and overall network structure. It is shown how the use of (i) temporal network graphs, and (ii) network centrality and regularity measures illustrate the differences between various options developed for the balancing act of energy and time efficiency in network coverage. Lastly, we compare the outcome of these measures with the less abstract classification variables, such as percent area covered, and cumulative distance travelled.

cs.MA↗

arXiv2020

Coverage and Energy Analysis of Mobile Sensor Nodes in Obstructed Noisy Indoor Environment: A Voronoi Approach

The rapid deployment of wireless sensor network (WSN) poses the challenge of finding optimal locations for the network nodes, especially so in (i) unknown and (ii) obstacle-rich environments. This paper addresses this challenge with BISON (Bio-Inspired Self-Organizing Network), a variant of the Voronoi algorithm. In line with the scenario challenges, BISON nodes are restricted to (i) locally sensed as well as (ii) noisy information on the basis of which they move, avoid obstacles and connect with neighboring nodes. Performance is measured as (i) the percentage of area covered, (ii) the total distance traveled by the nodes, (iii) the cumulative energy consumption and (iv) the uniformity of nodes distribution. Obstacle constellations and noise levels are studied systematically and a collision-free recovery strategy for failing nodes is proposed. Results obtained from extensive simulations show the algorithm outperforming previously reported approaches in both, convergence speed, as well as deployment cost.

cs.NI↗

arXiv2014

Secondary implementation of interactive engagement teaching techniques: Choices and challenges in a Gulf Arab context

We report on a "Collaborative Workshop Physics" instructional strategy to deliver the first IE calculus-based physics course at Khalifa University, UAE. To these authors' knowledge, this is the first such course on the Arabian Peninsula using PER-based instruction. A brief history of general university and STEM teaching in the UAE is given. We present this secondary implementation (SI) as a case study of a novel context and use it to determine if PER-based instruction can be successfully implemented far from the cultural context of the primary developer and, if so, how might such SIs differ from SIs within the US. With these questions in view, a pre-reform baseline of MPEX, FCI, course exam and English language proficiency data are used to design a hybrid implementation of Cooperative Group Problem Solving. We find that for students with high English proficiency, normalized gain on FCI improves from = 0.16+/-0.10 pre- to = 0.47+/-0.08 post-reform, indicating successful SI. We also find that is strongly modulated by language proficiency and discuss likely causes. Regardless of language skill, problem-solving skill is also improved and course DFW rates drop from 50% to 24%. In particular, we find evidence in post-reform student interviews that prior classroom experiences, and not broader cultural expectations about education, are the more significant cause of expectations at odds with the classroom norms of well-functioning PER-based instruction. This result is evidence that PER-based innovations can be implemented across great changes in cultural context, provided that the method is thoughtfully adapted in anticipation of context and culture-specific student expectations. This case study should be valuable for future reforms at other institutions, both in the Gulf Region and developing world, facing similar challenges involving SI of PER-based instruction outside the US.

physics.ed-ph↗

arXiv2009

A Virtual Young's Double Slit Experiment for Hard X-ray Photons

We have implemented a virtual Young's double slit experiment for hard X-ray photons with micro-fabricated bi-prisms. We observe fringe patterns with a scintillator, and quantify interferograms by detecting X-ray fluorescence from a scanned 30nm Cr metal film. The observed intensities are best modeled with a near-field, Fresnel analysis. The maximum fringe number in the overlap region is proportional to the ratio of real to imaginary parts refractive index of the prism material. The horizontal and vertical transverse coherence lengths at beamline APS 8-ID are measured.

cond-mat.mes-hall↗

arXiv2005

Electron Spin Dynamics and Hyperfine Interactions in Fe/Al_0.1Ga_0.9As/GaAs Spin Injection Heterostructures

We have studied hyperfine interactions between spin-polarized electrons and lattice nuclei in Al_0.1Ga_0.9As/GaAs quantum well (QW) heterostructures. The spin-polarized electrons are electrically injected into the semiconductor heterostructure from a metallic ferromagnet across a Schottky tunnel barrier. The spin-polarized electron current dynamically polarizes the nuclei in the QW, and the polarized nuclei in turn alter the electron spin dynamics. The steady-state electron spin is detected via the circular polarization of the emitted electroluminescence. The nuclear polarization and electron spin dynamics are accurately modeled using the formalism of optical orientation in GaAs. The nuclear spin polarization in the QW is found to depend strongly on the electron spin polarization in the QW, but only weakly on the electron density in the QW. We are able to observe nuclear magnetic resonance (NMR) at low applied magnetic fields on the order of a few hundred Oe by electrically modulating the spin injected into the QW. The electrically driven NMR demonstrates explicitly the existence of a Knight field felt by the nuclei due to the electron spin.

cond-mat.mtrl-sci↗

arXiv2003

Nuclear Magnetic Resonance in a Ferromagnet-Semiconductor Heterostructure

We report the observation of nuclear magnetic resonance (NMR) in a ferromagnet-semiconductor heterostructure in the presence of a spin-polarized current. Spin-polarized electrons injected from a metallic ferromagnet generate a large nuclear spin population in a GaAs quantum well by dynamic polarization. The characteristic time for the polarization process is approximately 20 sec, and the nuclear polarization can persist for several minutes after the current is turned off. Resonant depolarization is observed in the presence of an AC magnetic field or when the injection current is modulated at the NMR frequency.

cond-mat↗

arXiv2003

Dynamical Nuclear Polarization by Electrical Spin Injection in Ferromagnet-Semiconductor Heterostructures

Electrical spin injection from Fe into Al$_x$Ga$_{1-x}$As quantum well heterostructures is demonstrated in small (< 500 Oe) in-plane magnetic fields. The measurement is sensitive only to the component of the spin that precesses about the internal magnetic field in the semiconductor. This field is much larger than the applied field and depends strongly on the injection current density. Details of the observed hysteresis in the spin injection signal are reproduced in a model that incorporates the magnetocrystalline anisotropy of the epitaxial Fe film, spin relaxation in the semiconductor, and the dynamical polarization of nuclei by the injected spins.

cond-mat↗

arXiv2001

Optical Pumping in Ferromagnet-Semiconductor Heterostructures: Magneto-optics and Spin Transport

Epitaxial ferromagnetic metal - semiconductor heterostructures are investigated using polarization-dependent electroabsorption measurements on GaAs p-type and n-type Schottky diodes with embedded In1-xGaxAs quantum wells. We have conducted studies as a function of photon energy, bias voltage, magnetic field, and excitation geometry. For optical pumping with circularly polarized light at energies above the band edge of GaAs, photocurrents with spin polarizations on the order of 1 % flow from the semiconductor to the ferromagnet under reverse bias. For optical pumping at normal incidence, this polarization may be enhanced significantly by resonant excitation at the quantum well ground-state. Measurements in a side-pumping geometry, in which the ferromagnet can be saturated in very low magnetic fields, show hysteresis that is also consistent with spin-dependent transport. Magneto-optical effects that influence these measurements are discussed.

cond-mat↗